Tensile properties of carbon nanotubes reinforced aluminum matrix composites: A review

Journal Pre-proof Tensile properties of carbon nanotubes reinforced aluminum matrix composites: A review M. Jagannatham, Prathap Chandran, S. Sankaran, Prathap Haridoss, Niraj Nayan, Srinivasa R. Bakshi PII:

S0008-6223(20)30007-5

DOI:

https://doi.org/10.1016/j.carbon.2020.01.007

Reference:

CARBON 14941

To appear in:

Carbon

Received Date: 19 August 2019 Revised Date:

27 November 2019

Accepted Date: 2 January 2020

Please cite this article as: M. Jagannatham, P. Chandran, S. Sankaran, P. Haridoss, N. Nayan, S.R. Bakshi, Tensile properties of carbon nanotubes reinforced aluminum matrix composites: A review, Carbon (2020), doi: https://doi.org/10.1016/j.carbon.2020.01.007. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

Tensile Properties of Carbon Nanotubes Reinforced Aluminum Matrix Composites: A Review M. Jagannathama, Prathap Chandrana, S. Sankarana, Prathap Haridossa, Niraj Nayanb and Srinivasa R. Bakshia* a

Department of Metallurgical and Materials Engineering

Indian Institute of Technology Madras, Chennai, India 600036 b

Materials and Mechanical Entity

Vikram Sarabhai Space Centre, Thiruvananthapuram, India 695022 *Corresponding Author Email: [email protected]

Graphical Abstract

Tensile Properties of Carbon Nanotubes Reinforced Aluminum Matrix Composites: A Review M. Jagannathama, Prathap Chandrana, S. Sankarana, Prathap Haridossa, Niraj Nayanb and Srinivasa R. Bakshia* a

Department of Metallurgical and Materials Engineering

Indian Institute of Technology Madras, Chennai, India 600036 b

Materials and Mechanical Entity

Vikram Sarabhai Space Centre, Thiruvananthapuram, India 695022 *Corresponding Author Email: [email protected]

Abstract Carbon nanotubes (CNT) have received huge attention from the scientific community in the last two decades due to their unique structure and properties. They have been considered for potential applications in various areas of science and technology. One of the major applications of CNT is as reinforcement for fabrication of light weight high strength composite materials for use in automobile and aerospace applications. Aluminium and its alloys are natural choices for such applications due to their low density, high specific strength and modulus. In the last decade, there have been significant advances in the processing of carbon nanotube reinforced aluminium matrix (Al-CNT) composites. New understanding has emerged due to research on several aspects such as damage to CNTs during processing, interfacial phenomena, novel methods of processing for improving CNT dispersion, tensile behaviour, numerical modelling and in situ tensile testing. This review summarizes the present status of the tensile properties of pure Al-CNT and Al alloy-CNT composites. The various processing routes for fabrication of Al-CNT composites have been compared in terms of the resulting microstructure, degree of CNT dispersion, extent of interfacial reaction and its effect on the tensile properties. Factors affecting strengthening efficiency and the strengthening mechanisms in Al-CNT composites are discussed.

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List of Contents 1. Introduction 2. Tensile properties of Al-CNT composites 2.1. Young’s Modulus of Al-CNT Composites 2.2. Strength of Al-CNT composites 2.3. Numerical modelling of tensile properties of Al-CNT composite 2.4. Failure strain and toughness of Al-CNT composites 2.5. Insights from in situ tensile and nano-mechanical tests of Al-CNT composites 3. Effect of processing techniques on properties of Pure Al-CNT composites 3.1. Sintering without post processing 3.2. Conventional sintering/hot pressing followed by hot extrusion/hot rolling 3.3. Spark plasma sintering followed by hot extrusion/hot rolling 3.4. Powder deformation processing 3.5 Other Processing routes 3.6. Comparison of various processes for Al-CNT composites 4. Reinforcement of Al alloys with CNT 4.1. Al-Cu alloys 4.2. Al-Mg-Si alloys 4.3. Other alloys 5. Role of Interfacial phenomena 6. Challenges in fabrication of Al-CNT composites 7. Scope for future work 8. Conclusions

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1. Introduction Light-weight materials having high specific strength and high specific stiffness are necessary for automotive and aerospace applications in order to reduce the weight of the vehicle, improve fuel efficiency and reduce vehicle emissions [1-4]. This has been the driving force for the development of composite materials, with better physical and mechanical properties to meet these challenging requirements. The invention of Boron fibers in late 1950s led to the development of Metal Matrix Composites (MMCs) in the mid of 1960s. Dan Miracle has provided a very good account of the recent developments in this field [1]. During the early years, MMCs started with continuous fiber reinforcement and most of the research was focused on long-fiber reinforced MMCs. Aluminum and its alloys were preferred choice as the matrix due to their low density, high specific tensile properties, good corrosion resistance, high thermal and electrical conductivities, and high damping capacity [4]. Currently, Aluminum matrix composites (AMCs) are being used/considered for several applications including sporting equipment, electronic packaging and several automotive parts such as engine block liners, piston crown, piston ring, wrist pin, bearings, connecting rods, gears, wheels, drive shafts, gear box bearings, pumps and disk rotors. Al based MMCs are also appropriate candidates for other applications in the automobile industry such as pump housings, gears, valves, brackets, pulleys, clutch parts, turbo chargers, etc. Recent developments in composite materials show that a lot of interest is devoted to the fabrication of nano-composites to obtain improved properties over conventional composites. Since the primary strengthening mechanism in particle reinforced aluminum composite is the Orowan looping mechanism, strengthening is inversely proportional to inter-particle spacing which decreases with increase in number density of particles. Thus, it is possible to obtain equivalent strength with lower concentration of nanosized reinforcement. The report by Iijima on carbon nanotubes (CNT) in 1991 [5] renewed the interest in carbon based nanomaterials. The main attraction of carbon nanotubes for metal matrix composite application is their extraordinary strength. Yu et al. have measured the properties of multiwalled CNT (MWCNT) in situ using a Scanning Electron Microscope (SEM) and they have reported a maximum strength of 63 GPa [6]. However, it is noted only the annular area of the outermost layer was considered in strength calculations. The strength of carbon nanotubes have also been measured in-situ using a Transmission Electron Microscope (TEM) by Peng 3

and co-workers [7] and reported to be as high as 110 GPa. The “sword-in-sheath” type of failure mechanism was observed during the tensile tests which may have a good effect on their performance as reinforcements. The strength of most advanced high strength steels is of the order of 2 GPa while the strength of carbon fibres is known to be about 6-8 GPa. Single walled CNT (SWCNT) have been shown to have highest strength and stiffness of all known materials to mankind which is due to the strong C-C sp2 bond of graphite which translates into the axial strength of the tube. Their high strength coupled with large aspect ratio (length to diameter ratio) and low density, makes them very promising reinforcements in composites for structural applications. However, very few reports exist on use of SWCNT in metal matrix composites including Al matrix. This is mainly due to the high cost of SWCNT and the fact that reaction with metal matrix would lead to destruction of the SWCNT. Thus, almost all the researchers have used MWCNT for Al-based composites and it will be referred to as CNT hereafter. Since the individual tubes in CNT are independent of each other, sliding of tubes with respect to each other is possible, which is inhibited by cross linking between the tubes. Due to the ballistic heat transport mechanism, high thermal conductivity is observed in the axial direction (6000 W.m-1K-1 for SWCNT [8] and 3000 W.m-1K-1 for MWCNT [9]) at room temperature. Analytical modeling studies have shown that the coefficient of thermal expansion is close to zero [10]. Due to these reasons, CNT are considered as an ideal reinforcement. However, to fully realize the potential of CNT as reinforcement for any given matrix, it is necessary to ensure that individual CNT are dispersed in the matrix and there is good interfacial bonding between CNT and the matrix. This continues to be the biggest challenge to accomplish till date. A lot of attention has been given to the development of CNT reinforced ceramic, polymer and metal matrix composites. Extensive research has been conducted on CNT reinforced polymer matrix composites due to their ease of processing at relatively low temperatures and the fact that the elastic modulus of CNT is 1000 times more than that of polymers. Addition of CNT to ceramics has shown to increase their fracture toughness and wear resistance. Several review papers have been published on polymer-CNT composites [11-13], whereas a few publications are available on ceramic matrix composites [14, 15]. CNT reinforced Al matrix composites have also received a lot of attention since the last 2 decades. To the authors’ knowledge, the only commercial application of CNT reinforced aluminium matrix composites is as spikes for sport shoes produced by California Nanotechnologies and used by Adidas for the world's lightest track shoe [16]. The 4

achievements of the last one decade have been very promising and many more applications may come up in future. A significant advantage of Al-CNT composites is the possibility to achieve required properties with addition of low amount of CNT. There are some recent indications on use of CNT as reinforcement for preparing high strength filler wires for welding [17]. Another possible application is in fuel cells for hydrogen storage [18]. Thostenson et al. have included a brief discussion in a review paper about Al-CNT composite for electrical applications [19]. Curtin and Sheldon have mentioned about metal matrix composites for electromagnetic applications [20]. The information provided in these reviews compares only the electrical and magnetic applications of ceramic and polymer matrix composites. Bakshi et al. have published the first major review on CNT reinforced metal matrix composites [21]. Tjong has published a review paper on recent developments and properties of metal matrix composites reinforced with carbon nanotubes and graphene [22]. These reviews were focussed on all metal matrices. There are two short review papers on AlCNT composites. One is by Shadakshari et al. in 2012 which is a terse and is based on 15 papers only [23]. It does not analyse the reported data critically and does not provide significant conclusions. The second paper is by Fan et al. [24] which is focused mainly on the interfacial phenomena in Al-CNT composites and talks about various modifications undergone by CNT during MMC fabrication. However, it does not describe in detail the processing of Al-CNT composites and its effect on the tensile properties of the composites. Huang et al. and Suryanarayana have also provided short reviews on Al-CNT composites, but the information provided is very small [25, 26]. Agarwal et al. have written a book on Carbon Nanotubes Reinforced Metal Matrix Composites which summarizes the progress in this area based on research reported up to 2010 [27]. Bakshi et al. have analysed the factors affecting strengthening in bulk Al-CNT composites based on the data available up to 2010 [28]. Figure 1 shows the histogram of the number of research articles published on Al-CNT composites since the first paper was published in 1998. The papers using pure Al and Al alloys as matrix have been shown separately. From Fig. 1, it is observed that the interest in developing AlCNT composites has been increasing since 2004. It is also observed that most researchers have used pure Al as the matrix (270 papers) as compared to Al alloys (140 publications). Considering that there are about 410 research papers on Al-CNT composites with 320 published between 2011 and 2019, a comprehensive review of tensile properties of Al-CNT composites is found to be necessary. It is necessary to understand the factors which affect the strengthening due to addition of CNT and whether the increase in strength and stiffness be 5

predicted using micro-mechanical models. The effect of processing techniques on the strength and failure strain of the composites needs to be understood. The present review addresses these points by analysing the tensile properties of CNT reinforced Al and Al alloy composites prepared by various techniques.

Fig. 1: Year-wise plot of publications on Al-CNT composites [Source: www.scopus.com].

2. Tensile properties of Al-CNT composites Tables S1 and S2 provided in the supplementary material summarise the data available on AlCNT composites using pure Al and Al alloy as matrix, respectively. Only those papers have been considered which have reported tensile properties of the composites. The tensile data has been analyzed to understand the effect of CNT content and processing route on the strengthening due to CNT addition. The data on pure Al and Al alloy matrix composites has been separated and this provides several new insights on their tensile behaviour.

2.1. Young’s Modulus of Al-CNT composites Figure 2 shows the Young’s modulus reported for CNT reinforced Al/Al-alloy composites obtained by tensile testing of bulk samples. It is observed that the elastic modulus of the composite increases from 70 GPa to about 110 GPa for CNT addition from 1 to 5 vol. %. There are some studies which have reported a decrease in the elastic modulus by addition of 6

CNT [29, 30]. These are cases where the dispersion is poor in the powder and the final microstructure had large CNT clusters. In case of plasma sprayed Al-CNT composites, it was observed that the clusters were not infiltrated with molten metal [31] and they acted similar to porosity. It is observed that the increase in Young’s modulus is more for pure Al matrix as compared to Al alloy matrix. It is also seen that rate of increase of stiffness reduces with increase in CNT content. This is due to the formation of CNT clusters and a decrease in the load transfer to individual CNT for higher CNT loading [32].

Fig. 2: Variation of elastic modulus of Al-CNT composites with CNT content.

A very simple treatment for long-fibre composites gives the following equations: E || = E M (1 − V F ) + E F V F

(1)

1 − VF VF 1 = + E⊥ EM EF

(2)

Where E|| is the elastic modulus calculated along the direction of the fibres (Voigt model) and E⊥ is the elastic modulus calculated along a direction perpendicular to the fibre (Reuss 7

model). EM and EF are the elastic modulus of matrix and fibre respectively and VF is the volume fraction of fibre. For discontinuous randomly oriented short fibre composites, a combined Voigt-Reuss model is proposed as given below:

EC =

3 5 E || + E ⊥ 8 8

(3)

For CNT, EF can vary from 500 GPa to 1000 GPa. Thus, these predictions can be calculated and represented as a band as shown in Fig. 2. It is observed that, most of the values reported for pure Al matrix fall in the E|| band. In most of the cases these samples are made by ball milling followed by hot extrusion and the CNT get aligned to some degree along the extrusion direction. Pure Al, being softer, allows for a higher alignment of CNT in the extrusion direction. Some investigations have reported Young’s modulus values higher than these predictions. There could be two reasons for this. One is that the elastic modulus of CNT is greater than 1000 GPa and another reason is that there may be formation of aluminium oxide during processing leading to further increase in stiffness. Such small amount may not be detected by XRD as reported by most of the researchers. It is seen that the E⊥ is very small compared to experimental values. Most Al alloy-CNT composites fall in the band of combined Voigt-Reuss model. This suggests that the alignment of CNT in Al alloys is difficult.

2.2. Strength of Al-CNT composites Figure 3a shows the increase in the Yield Strength (YS) of the composites as a function of CNT content. It is seen that there is a lot of scatter in the YS values reported in literature. This shows that the processing route has a strong influence. Another factor that contributes to this is the quality of CNT used. In general, thinner CNT are likely to be defect free and have very high strength. This was also observed experimentally by Peng et al. [7]. In the literature, CNT diameters ranging from 20 to 140 nm have been used and this is expected to contribute to this scatter as well. It is observed that the increase in YS is more for Al alloy-CNT composites compared to pure Al-CNT composites. This could be attributed to the fact that most of the Al alloys used are precipitation hardened and CNT addition improved the precipitation hardening characteristics as discussed later. Due to their small size and the fact that dislocations cannot cut through CNT, Orowan looping mechanism has been proposed by several authors as the feasible mechanism of strengthening. The increase in strength obtained due to this mechanism is given by the following equation [33]: 8

∆ σ = 0 .264 G

V F1 / 2 ln (r / b ) 1/ 2 (1 − 1.085V F ) r / b

(4)

Where r is the volume equivalent radius for CNT and G the shear modulus of Aluminum (~26 GPa), b the Burgers vector (~0.286 nm for Al) and VF the volume fraction of CNT. Taking the r = 20 and 140 nm, the calculated increase in yield strength due to this mechanism has been shown as a band in Fig. 3a. It is observed that the dispersion strengthening model satisfies most of the data for pure Al-CNT composite.

Fig. 3: Plots showing variation of tensile properties of Al-CNT composites as a function of CNT content. (a) Increase in the YS, (b) Increase in TS and (c) Strengthening efficiency.

Another model proposed by Arsenault and Shi for strengthening is based on increase in the dislocation density produced during processing due to the thermal coefficient mismatch [34]. The increase in yield strength due to this mechanism is given as:

σ = 1.25Gb ρ 1 / 2

(5) 9

Where ρ = 10V F ε / br (1 − V F ) is the dislocation density due to the mismatch in thermal expansion coefficient, b is the Burgers vector and r is the radius of the particle (CNT), ε is the thermal mismatch strain given by ε = ∆(CTE )∆T . For, a ∆T of 600 °C and ∆(CTE) of 25 x 10-6 m/m. K (= CTE of AA1050), ε turns out to be 0.015. Using this value, ∆σ has been calculated for radius of CNT = 20 and 140 nm and has been plotted in Fig. 3a.

It is seen that the thermal mismatch model predicts very high strengthening. It is seen that, some of the data on pure Al-CNT composite and most of the data for Al Alloy-CNT composites fall in this band. Arsenault and Shi have shown presence of dislocations emanating from particles in composites supporting this mechanism. Some studies have shown increased dislocation densities in CNT composites [35] leading to strengthening, but it may be attributed to the deformation rather than CTE mismatch. Reduction in grain size also contributes to the increase in strength. Although, it is obvious that CNT may pin grain boundaries and prevent grain growth, very few studies have actually focused on the strengthening due to difference in grain size [36-40]. CNT presence also affects the precipitation hardening beneficially. The sum of increase in YS due to these two mechanisms predicts the observed values closely. However, for higher CNT contents, the measured values are lower than predicted values due to CNT clustering which is not accounted for in the models. Figure 3b shows the increase in Tensile Strength (TS) of the composites with CNT content. It is evident from Fig. 3b that the % increase in the tensile strength is high for pure Al-CNT composites compared to Al alloy-CNT composites. The large improvement in the TS of pure Al-CNT composites is due to two major reasons. Firstly, pure Al-CNT have lower porosity compared to Al alloys-CNT composites and secondly, lower TS of pure Al produced by similar methods compared to Al-alloys which are also strengthened by precipitates. Several models have been developed for predicting the strength of short fibres reinforced composites [21, 27]. Many authors have used the Kelly-Tyson formula [41] based on interfacial load transfer as given below.

σ C = σ FVF

l + σ M (1 − V F ) For l


σ C = σ FVF 1 − 

lc   + σ M (1 − VF ) For l >lc 2l  10

(6)

(7)

Where, σC is the TS of the composite, l c =

σFD is called the critical length at which the 2τ M

maximum stress transferred to the fiber is equal to the fracture strength of the fiber, VF is the volume fraction of the fiber, σF is the TS of the fiber (~ 50 GPa), τM (=σM/2) is the shear strength of the interface, and σM is the TS of the matrix. For τM value of 50 MPa (pure Al) and CNT diameter equal to 20 nm (most commonly used CNT dia.), the critical length turns out to be 10 µm, while for τM value of 200 MPa (for Al alloy), it is 2.5 µm. Figure 3b shows the bands predicted by equation 6 and 7 for σM values of 100 and 400 MPa. It is seen that only a few data fall in the band given by Eq. 7 (for l > lc). Further it is observed that a lot of experimental data fall in the band corresponding to Eq. 6 (for l < lc). In most of the studies, the original CNT length is below 10 µm which is further reduced during ball milling or composite fabrication. So it is expected that the lengths of the CNT is far below the critical length. In our previous work, we have measured CNT length of the order of 100 nm in Al-Si matrix after ball milling for 20 h [42]. Thus, most of the data can be explained based on the reduction of CNT length during processing. Further, it is noted that ball milling of CNT with harder Al-alloy particles is expected to break down the CNT to a larger extent as compared to pure Al powders. This explains why most of the data for Al-alloy CNT composites are lower. If length of CNT is lower than critical length, then the failure mode will be pull-out mode. On the other hand, if it is higher than critical length, the failure mode turns into breaking mode [43]. Hence, in most of the studies of Al-CNT composites, the failure of CNT is pull-out from the matrix. Halpin and Tsai have proposed empirical equations to predict the property of discontinuously reinforced composites [44]. The strength of randomly oriented composite is given as:

σ C 1 + ξη V F = σM 1 − ηV F Where ξ = 2

(8)

l α (σ F / σ M ) − 1 and η is given by the expression η = D α (σ F / σ M ) + 2l / D

with α=1/6 for

short fiber composites. Figure 3b also shows the band corresponding to values calculated from Eq. 8 for σM corresponding to 100 and 400 MPa and l/D = 50. This aspect ratio corresponds to a length of 1 µm for a CNT of 20 nm in diameter. It is seen that most of the data with good strengthening fall in this band. If l/D is reduced, then the band is expected to shift to lower strength values. At high CNT content, increase in strength is lower than that 11

predicted by Halpin-Tsai equations. This is due to the fact that these models do not account for the CNT clustering effect at high CNT content and assume the dispersion to be uniform. A new model called the generalized shear lag model has been proposed by Ryu et al. [45] which take into account the fiber orientation distribution. This predicts the strength of the composite as:

σ C VF = S eff + 1 2 σM

(9)

Where, Seff is the effective aspect ratio of the fiber which depends on the orientation. In Fig. 3b, Eq. 9 is also plotted for a Seff value of 50. It is seen that this line is independent of the initial strength of the matrix and is like an average line for the reported data. Again it is seen that for higher CNT content samples, the experimental values are below that predicted by the generalized shear lag model. It can be seen that the rate of increase in strength due to CNT addition decreases with increase in CNT content. This property has been quantified by a term called Strengthening Efficiency as defined below [46]: R=

σ Al − CNT − σ Al V F σ Al

(10)

Here, σ denotes the Tensile Strength and VF is the volume fraction of CNT.

Figure 3c shows the strengthening efficiency of CNT in Al-CNT composites as a function of CNT content. It is observed that the strengthening efficiency is high for pure Al-CNT composites compared to Al alloy-CNT composites. One of the reasons for this is the low strength of pure Al (denominator in eq. 10) prepared by the same processing techniques as compared to Al alloys. It is seen that the strengthening efficiency in fact drops down at higher CNT contents. This is due to the formation of CNT clusters which reduces the effective CNT content participating in the strengthening process. CNT clusters not only reduce the strengthening efficiency but they decrease the strength by acting as porosities and crack nucleation sites. Improvement in dispersion improves the strengthening efficiency, but it is seen that the highest values are obtained for CNT content less than 1 vol. %.

2.3. Numerical modelling of tensile properties of Al-CNT composites Modelling of the tensile properties of Al-CNT composites can provide understanding into the deformation behaviour and mechanisms of strengthening and load transfer in the composites. 12

Numerical modelling of Al-CNT composites has gained attention only in the last few years. Compared to continuous fibre reinforced composites, there are several unique challenges in numerical modelling of carbon nanotube composites. The in-homogeneous distribution of the CNT makes it difficult to assume a representative volume element for the modelling. The computed properties are expected to be sensitive to several factors such as curvature and orientation of the CNTs, presence of reaction products at interface, changes in the length of the CNTs, etc. In order to compare the computed properties with measured one, a true threedimensional microstructure of the composite is necessary which is difficult to obtain due to the fine size of the CNT which cannot be imaged using the available tomography techniques. Nevertheless, several studies have explored correlation of microstructure and tensile behavior of Al-CNT composites. Su et al. have modelled Al-CNT composites by considering CNTs as flexible co-axial cylinders and used an algorithm to disperse the CNT randomly in the matrix with different orientations [47]. They have modelled CNT distribution in lamellar configuration as well as throughout the volume and the calculated true stress-strain matched closely with the measured values for composites prepared by them. Alfonso et al. have modelled the effect of formation of different thickness of Al4C3 on the CNT on the Young’s modulus using Finite Element Analysis (FEA) [48]. They used single CNT cylinder in a cylindrical matrix with Al4C3 at the interface as the representative volume element (RVE). They showed that the presence of Al4C3 increased the Young’s modulus of the composite marginally. Perez et al. have further carried out 3D simulations of randomly oriented straight CNTs in aluminum matrix using discrete element method [49]. They also observed that the Young’s modulus of different randomly oriented Al-CNT structures was between the longitudinal and transverse modulus values for a unit cell containing single CNT. The Young’s modulus increased linearly with increase in Al4C3 thickness and CNT content. A geometry similar to Alfonso et al. [48] was used (without Al4C3) and the deformation behaviour was analyzed using molecular dynamics (MD) simulations by Choi et al. [50]. They have used single walled CNT in their analysis. Their results predicted very high fracture strength for the composites (in GPa) while the elastic modulus was found to be close to experimental values. Results from such simulations can be used in multiscale models. Similar results were obtained by Junfeng et al. [51] who further showed that larger diameter CNT and armchair type of CNT provided better tensile properties. Ansari et al. have studied the effect of inter-phase properties on the initial yield surface for Al-CNT composites [52]. They used a square geometry and showed that the size of the bi-axial yield surfaces depended strongly on 13

the inter-phase properties. They have further shown that the elastic modulus increases with increase in the thickness of Al4C3 and the FEM results match well with rule of mixtures predictions [53]. Iacobellis et al. have done multi-scale modeling of tensile properties of AlCNT composites using a combination of atomic scale finite element method (AFEM) and FEA simulations [54]. They have used single-walled as well as multi-walled CNTs and obtained values of tensile strength of the order of GPa which was similar for different types of CNT. It is noted that the RVE used is such that the CNT makes up for a considerable volume of it. They have also modeled crack propagation through the composite and observed that it becomes difficult when crack encounters the CNT. Park et al. have carried out MD simulation on single-walled CNT reinforced Al composites [55] and studied the effect of the CNT orientation on the tensile properties. They showed that the strength reduced significantly (from 7.3 to 4.3 GPa) while the elastic modulus reduced from 91.84 to 86.62 GPa as the angle between the CNT and the tensile axis increased from 0 to 40°. Increase in CNT diameter resulted in a marginal increase in the tensile strength but a large increase in Young’s modulus. Thus, numerical modelling can provide a lot of information to correlate microstructure to the properties. Bridging simulations at multiple-length scales needs to be studied further. It can also provide insights into the effect of CNT agglomeration on the tensile properties of Al-CNT composites.

2.4. Failure strain and toughness of Al-CNT composites It is important to analyse the toughness of Al-CNT composites. Figure 4a shows the % change in failure strain as compared to the matrix prepared by same technique and similar experimental conditions. It is observed that the failure strain reduces significantly in the composites indicating a significant drop in the ductility. Most of the composites having more than 3 vol. % CNT have shown 60% or more decrease in failure strain. Failure strain reduction is significant in case of Al alloy matrices for CNT content above 3 vol. %, which is due to the difficulty in dispersing CNT in Al alloys. Figure 4b shows the difference in the (TS×εf) value (which can be considered as an indicator of toughness) for the composite and the matrix as a function of CNT content. It is seen that for both pure Al and Al alloy matrices, the toughness reduces with increasing CNT content. It is seen that there is a significant decrease in toughness in case of Al-alloy CNT composites. In case of pure Al-CNT composites, it is seen that for lower CNT content (< 2 vol. %), toughness is increased. This is 14

due to the large increase in TS value. It is seen that for more than 3 vol. % CNT addition, toughness is reduced for both pure Al and Al alloys. It was shown in previous analysis [28] that there was a toughness decrease in case of poor CNT dispersion (poor strength) as well as in samples having good dispersion (high strength). Another quantity of interest is the ratio TS/YS. The closer the value of this ratio to 1, the more brittle the material is. Also it shows that the material has a very small capacity for work hardening and the formability may also be poor. Figure 4c shows the value of the TS/YS ratio as a function of CNT content. It is seen that in general this ratio decreases with increase in CNT content. This means that by addition of CNT, the YS is increased by a larger extent compared to the TS. This is also an indication that the brittleness of the material is increasing. When compared to Al alloy-CNT composites, pure Al-CNT composites seem to have higher TS/YS ratio. It is observed that the improvement in the strength and stiffness by addition of carbon nanotubes comes with a decrease in ductility and toughness.

Fig. 4: (a) Change in the failure strain, (b) Change in the toughness, and (c) Variation of the ratio of TS to YS of Al-CNT composites with respect to CNT content. 15

2.5 Insights from in situ tensile and nano-mechanical tests of Al-CNT composites Boesl et al. have prepared Al-CNT (1 vol. %) composites using wet-mixing method [56]. In this method, CNTs were mixed with Al powder using ultrasonication consolidated by SPS at 500 °C with heating rate of 50-60 °C/min under 80 MPa load for a holding time of 1 h. The TS and failure strain increased to 95.5 MPa from 68 MPa and 5.5% from 3% due to addition of CNT. It is noted that the values are lower since ball milling was not employed which results in grain refinement and no secondary processing was employed. The values were comparable with calculated values using Kelly and Tyson shear lag model (90-100 MPa). The in situ tensile tests clearly exhibited that fibre strengthening was the major dominating mechanism in Al-CNT composites. Chen et al. [43, 57-59] and Zhou et al. [60] have also performed in-situ tensile test for Al-CNT composites and analyzed the interface phenomena, interface shear strength and fracture surface during in-situ tensile testing. They have not reported the tensile properties of Al-CNT composites. CNT pull-out is the dominant failure mechanism observed during in-situ tensile test. They have also observed that partially reacted CNTs with interfacial carbides are ideal reinforcement to obtain high load transfer efficiency in Al-CNT composites. The load transfer strengthening was effective in Al-CNT composites. The peeling behaviour of CNT was observed and the defective structures (inter-wall bridges) cross-linked the adjacent walls of CNT. This phenomenon also leads to the effective load transfer between CNT walls. They have also observed that nano-particle modified Al-CNT interfaces enhance the load transfer efficiency, while neat interfaces lead to CNT pull-out. Nanoindentation is a quick tool for assessing mechanical properties and sample preparation is also easy. If tensile properties can be calculated using this technique, it can save a lot of time. However, the method is known to be good for homogeneous and dense materials and may result in the determination of localized properties which may not be representative of the bulk material. Further, only the matrix properties are known and effect of porosity and CNT clusters is not accounted for. There are a lot of reports on hardness and elastic modulus of AlCNT composites by nanoidentation. It is always noticed that due to the localized nature of the testing, the values obtained are almost always higher than macro-scale testing. Bakshi et al. have prepared 5 mm thick coatings of Al-12%Si eutectic alloy reinforced with 5 and 10 wt.% CNT on 25.4 mm diameter steel pipe using Plasma spraying [31]. Analysis of the nanoindentation load displacement curves resulted in YS values of a few tens of GPa while the tensile tests showed a TS of ~120 MPa. This was attributed to the presence of CNT clusters which act like porosities due to poor wetting and lack of infiltration by the molten 16

metal. Kwon et al. [61] have measured the YS of Al- 1 vol.% CNT composite by tensile test to be 263 MPa (78 MPa for Al). They have also measured YS of the composite by plotting the indentation stress vs. strain from nanoindentation data by analyzing the initial data for low depths of indentation. The strength and elastic modulus obtained by nanoindentation were found to be similar to the tensile test results. This analysis may not hold true for different samples having microstructural complexity. The effect of the CNT clusters and oxide particles at grain boundaries can only be ascertained from bulk tensile tests.

3. Effect of processing techniques on properties of Pure Al-CNT composites Various processing techniques have been employed for fabrication of Al-CNT composites. Different processing techniques will result in different degree of CNT dispersion, CNT alignment and matrix-CNT interfacial contact affecting the load transfer to the CNT which determines the tensile properties. Powder metallurgy techniques have been most suitable and successful for preparing high strength Al-CNT composites [28]. Based on the literature, the processing techniques for pure Al-CNT composites can be divided broadly into 5 types. The first one is sintering by conventional (CS), hot pressing (HP) or spark plasma sintering (SPS) without any post processing such as extrusion, rolling etc. The second type refers to processes involving conventional sintering/hot pressing followed by hot extrusion/hot rolling. The third type includes spark plasma sintering followed by hot extrusion/hot rolling. The fourth type of processing is by deformation processing of powders without any primary sintering step. The fifth type includes all other processes such as casting, thermal spraying, etc. These processes are described below along with tensile properties of composites. A simple nomenclature using abbreviations has been used to describe the fabrication processes used in sequence. The effect of processing on the properties of Al alloy-CNT composites is discussed separately.

3.1. Sintering without post processing There are only few reports on use of sintering without any post processing for synthesis of pure Al-CNT composites. Figure 5 shows the tensile strength, percentage increase in TS and failure strain of Al-CNT composites with respect to CNT content. Nie et al. [62] have prepared Al composites reinforced with up to 3 vol.% Mo coated CNT by SPS at 580 °C for 5 min under 40 MPa (SPS route). They have used magnetic stirring (for 8 h) to prepare powders and observed poor dispersion and TS of Al-3 vol.% CNT composites was 80 MPa (89 MPa for Al) with a failure strain of 10 % (35 % for Al). Wu et al. [63] have synthesized 17

Al-CNT (0.75-7.5 vol.%) composites by SPS at 580 ºC for 10 min under 40 MPa pressure (SPS route). They have used ultrasonication for 30 min and observed poor dispersion of CNT. They have found that the TS of the Al-3 vol.% CNT composites was 119 MPa (120 MPa for Al) with the failure strain of 9 % (37 % for Al sample). Figure 5 shows that Mocoated CNT show slightly higher TS than normal CNT. However, the strength values and % increase in strength is found to be very low due to poor dispersion of CNT. For CNT content of 3 or more vol. %, the strength was found to be lower than Al sample. For low CNT content, failure strain is found to be high and it reduces with increase in CNT content due to the agglomeration of CNT in composites. Thus, it is seen that dispersion has a strong effect on the ductility and strength of the composites. The increase in SPS time from 5 min [62] to 10 min [63] increased the strength of Al-CNT composites due to improved densification. Singh et al. [64] have used ball milled Al powders (180 RPM) and ultrasonicated them with functionalized CNT (4 h) and ball milled further (100 RPM for 15 min). The Al-CNT (0.75 and 1.5 vol.%) composites were prepared using SPS (550 °C at 80 MPa for 20 min) (BM+SPS route). They have achieved a TS of 217 MPa for Al-0.75 vol.% CNT composites (161 MPa for Al) with a failure strain of 4.7% (6.6% for Al). It was reported that the use of low milling speed and milling time lead to presence of CNT clusters and lower tensile properties (126 MPa TS and 3.3% elongation) for the Al-1.5 vol.% CNT composite which was poor than the Al sample. Hence, appropriate milling parameters are necessary to improve the tensile properties of composites. Recently, Park et al. have synthesized vertically CNT sheet and transferred them on to Al foil [65]. Then, Al was deposited on CNT by sputtering and the foils were stacked and SPS was carried out at 580 °C for 30 min [SPS route]. The SPS samples were heat-treated at 500 °C for 60 min under Ar. They reported a low TS of ~85 MPa for Al-0.15 vol.% CNT composites (~71 MPa for Al) with a failure strain of 12.3% (13% for Al). The obtained lower tensile properties of composites may be due to the small dimensions of tensile samples (gauge length of 2 mm and width of 1.5 mm). Zhao and co-workers have prepared Al-CNT composite powders by in-situ CVD growth of CNT on Al with Ni as catalyst [66-72]. Using this dispersion technique, He et al. have fabricated up to 9.8 vol.% CNT-Al composites [66, 68] by cold compaction under 600 MPa and sintering at 640 °C for 3 h in vacuum. In order to improve the density, the sintered samples were re-pressed under 2 GPa and annealed at 850 °C [68] (In Situ CVD+CS+Rep route). They have found that CNT were uniformly dispersed and retained their structural integrity. However, for higher CNT content, clusters of CNT were observed along with Al4C3 18

formation. They have also prepared samples using ball milled powders with same technique (BM+CS+Rep route). It is observed from the Fig. 5 that very high strength is obtained for (In Situ CVD+CS+Rep) route compared to (BM+CS+Rep) techniques. This is due to better dispersion and less damage to the CNT during processing. The TS of Al-7.5 vol.% CNT composites was enhanced by 184 % (398 MPa) while for ball milled powders of Al-7.5 vol.% CNT showed only 52 % (213 MPa) improvement. Although the strength and % increase in strength was found to be very high, the ductility was very low (<5 %) as seen in Fig. 5c.

Fig. 5: Tensile properties of conventionally processed Al-CNT for varying CNT content. (BM = Ball milling, CS = Conventional Sintering, Rep = Repressed after sintering, In Situ CVD = Al powder with CNT grown on it in situ, SPS = Spark Plasma Sintering).

3.2. Conventional sintering/hot pressing followed by hot extrusion/hot rolling It is observed that conventional processing methods without post-processing give poor tensile properties for Al-CNT composites. Hence, several post-processing operations have been carried out to improve the densification and tensile properties of the composites. The post19

processing methods used are hot extrusion and hot rolling for pure Al-CNT composites. Figure 6 shows the tensile properties of composites as a function of CNT content. The legends for each plot have been arranged in increased order of properties obtained for the process. The first paper published on Al-CNT (5 and 10 vol. %) composites used magnetic stirring (at 300 rpm) and hot pressing (at 600 ºC and 100 MPa) followed by hot extrusion at 500 ºC with an extrusion ratio (ER) of 25 (HP+HE route) to fabricate the composites [73]. Large CNT clusters were observed due to poor powder dispersion technique and consequently resulted in a lower TS of 80 MPa (89 MPa for Al sample) for Al-10 vol.% CNT composite with a failure strain of 16 % (41 % for Al). Turan et al. [74] have used sonication and stirring for mixing and prepared Al-0.4 vol.% CNT composites by compaction (at 300 MPa), sintering (600 °C for 150 min in Ar) and hot extrusion at 400 °C with ER 16 (CS+HE route). They have also obtained low TS of 148 MPa for composites (116 MPa for Al) with a failure strain of ~14.8% (16.5% for Al). The Al-3 vol.% CNT composites [29] prepared by mechanical mixed powders (2 h in blender at 200 rpm), compaction (at 200 MPa) and sintering at 520 °C for 90 min followed by cold extrusion (CS+CE route) increased the TS to 184 MPa (98 MPa for Al) with the failure strain of < 4 %. The ball milling of Al and CNT powders lead to the enhancement in tensile properties of composites due to the grain refinement and better dispersion of CNT in composites. Liu et al. have reported the effect of milling time (2 to 12 h at 300 rpm) on the dispersion and tensile properties of Al-0.75 vol.% CNT composites [75] prepared by hot pressing (560 °C) and hot forging (450 °C) (BM+HP+HF route). They have found that tensile properties are increased with milling time and above 6 h resulted in significant damage to CNT. The TS of Al-CNT composite with 6 h ball-milled powders was 206 MPa (170 MPa for Al) with a failure strain of 7 % (14 % for Al). Deposition of metallic coating on CNT lead to improvement in the tensile properties of Al-CNT composites which could be due to better load transfer to the CNT. Maqbool et al. have prepared Al-Cu coated CNT composites by ball milling (for 1h), compaction (at 600 MPa), sintering (at 550 °C for 3 h), hot rolling (80% reduction) with 0.25 mm per step at 480 °C [76] followed by annealing at 500 °C for 6 h (BM+CS+HR+AN route). They have achieved a TS of 290 MPa (140 MPa for Al) for Al-1.5 vol.% Cu coated CNT with the failure strain of 5%, whereas the TS was 227 MPa for Al-1.5 vol. % uncoated CNT composite with the failure strain of 7%.

20

Fig.6: Tensile properties of Al-CNT composites with the function of CNT content processed by sintering/hot pressing followed by hot extrusion/hot rolling. (HP = Hot Pressing, HE = Hot Extrusion, BM = Ball Milling, CS = Conventional Sintering, CE = Cold Extrusion, HR = Hot Rolling, Flake Al = Al flakes produced by milling, VHP = Vacuum Hot Pressing, HF = Hot Forging, AN = Annealing, In Situ CVD = Al powder with CNT grown on it in situ).

It is observed from the Fig. 6 that hot extrusion after primary processing of Al-CNT composites has given high tensile properties compared to hot rolling. Bustamante et al. have synthesized Al-CNT (up to 3 vol.%) composites [77] using 5 h ball milled powders by sintering at 600 ºC and then extrusion at 500 ºC with an ER of 16 (BM+CS+HE route). They have reported a TS of 252 MPa (160 MPa for Al) with failure strain of 15.6 % (19.6 % for Al) for 3 vol.% CNT addition. Liao et al. [78] have milled Al-CNT powders (200 rpm for 220 min) and then consolidated by vacuum sintering (at 630 ºC for 3.5 h) and hot extrusion (at 520 ºC with ER of 9) (BM+CS+HE). They have found a TS of 300 MPa for Al-3 vol.% CNT composite (136 MPa for Al) with the failure strain of 18 % (28 % for Al). Thus, vacuum 21

sintering of ball milled powders followed by hot extrusion provides good strength and failure strain. They have further studied the effect of hot rolling (85% reduction at 500 °C) on properties of extruded rods (CS+HE+HR route) [79]. However, they have used roll mixing to disperse the CNT and found to have lower properties (TS of 200 MPa) compared to extruded rods prepared from ball milled powders. Thus, it can be concluded that CNT dispersion method is very important for achieving good properties of Al-CNT composites. Liao et al. have further rolled Al-0.75 vol.% CNT composites for 6 times [80] using previous process parameters [79] and found TS increase to 295 MPa (175 MPa for Al) but the failure strain was reduced to 5.2 %. Thus, higher deformation during processing improves the strength of Al-CNT composites. Liu et al. [81] have prepared Al-2.56 vol.% CNT composites by ball milling (at 150 rpm for 2-12 h), compaction (at 190 MPa for 10 min), sintering at 560 °C for 2 h in Ar followed by hot extrusion at 560 °C with ER of 36 (BM+CS+HE route). They have observed that tensile properties of composites were improved with increase in milling time. The TS of 12 h milled composite was 253 MPa (98 MPa for Al), with an elongation of about 16 % (25% for Al). It was observed that though the extrusion ratio was higher as compared to previous work [78], higher extrusion temperature (560 °C vs. 520 °C) resulted in lower tensile properties of Al-CNT composites. It is observed from Fig. 6 that high energy ball milling (HEBM) of Al-CNT powders resulted in highest tensile properties. Salama et al. have used an innovative approach of using Al-CNT powders as reinforcement in Al with aim to improve the ductility of the composites [82]. The Al and CNT powders were mixed by HEBM for 2 h at 400 rpm under Ar. They prepared composites from the milled powders (single matrix) and a mixture of Al and milled Al-CNT powders (dual matrix) and studied the properties of both for the same CNT content (1.5 and 3.7 vol.%). The powders were cold compacted at 500 MPa for 1 h and then sintered at 500 °C for 1 h. The sintered samples are hot extruded at 500 °C with ER 4 (BM+CS+HE route). They showed that increasing the milling time up to 1 h increased the TS and failure strain for the dual matrix composites. A 1:1 mixture was found to result in good properties. The strength was lower for the dual matrix composite (348.3 MPa vs. 380.2 MPa for single matrix) while the elongation was slightly improved (7.2% vs. 6.2% for single matrix) for the 3.7 vol.% CNT composite. Salama et al. also investigated the tensile behaviour of functionally graded Al-CNT composites with similar experimental conditions containing a total of 1.5 and 2.2 vol.% of CNT [83]. They obtained a TS of 215 MPa (102 MPa for Al) and failure strain 28% [48% for Al) for the 2.2 vol.% CNT-Al FG material with a. Thus, dual matrix and functionally graded 22

materials may have improved failure strains. Xu et al. [84] have fabricated Al-CNT (1.5, 3, 4.5 vol.%) composites using ball milling (for 24 h at 150 rpm), compaction at 187 MPa, sintering at 560 °C for 4 h and hot extrusion at 500 °C with an ER of 36 (BM+CS+HE route). They have found a maximum TS of 312 MPa for Al-3 vol.% CNT composites (145 MPa for Al) with a failure strain ~15.8 (17% for Al). This study shows that HEBM with high ER enhances the tensile properties of Al-CNT composites. Xu et al. have used various milling techniques to disperse 2.2 vol.% CNT in pure Al powders in a planetary mill under Ar. They have used (a) low-speed ball milling (LSBM) at 135 rpm for 9 h, (b) high-speed ball milling (HSBM) at 270 rpm for 9 h, and (c) shift-speed ball milling (SSBM) consisting of 8 h milling at 135 rpm followed by 1 h milling at 270 rpm [85-86]. The powders were consolidated by cold pressing (at 500 MPa), sintering at 550 °C for 2 h followed by hot extrusion at 400 °C with ER of 25 (BM+CS+HE route). The TS of LSBM, SSBM and HSBM composites were 367 MPa, 376 MPa and 408 MPa, respectively with 41 %, 33 % and 31 % enhancements over Al matrices. However, failure strain was significantly decreased for LSBM (from 17 % to 6.3 %) and HSBM samples (10.2 % to 4 %), while the ductility loss in SSBM sample was much smaller (from 14.1 % to 12.4 %). They have used SSBM powders and prepared Al-CNT (1 and 2 vol. %) composites with similar sintering parameters and hot extrusion at 320 and 350 °C with ER of 4 [87]. They found that the grain size reduced with decreasing extrusion temperature and increasing CNT content. A maximum TS of 355 MPa with 14.8% failure strain was obtained for the Al-2 vol.% CNT composite extruded at 320 °C. Yuan et al. [88] have prepared Al-CNT (2.2 and 4.5 vol.%) composites using SSBM (at 135 rpm for 8h followed by 270 rpm for 30 min), cold compaction (at 500 MPa), sintering (at 540 °C for 2h in vacuum), hot forging (at 450 C with 30% reduction and further hot rolling (at 480 °C with 85% rolling reduction [BM+CS+HF+HR route]. They have obtained TS of 453 MPa for Al4.5 vol.% CNT composite (295 MPa for Al) with failure strain of 5% (11.6% for Al). Thus, higher degree of deformation leads to higher strength but lower failure strains. Li et al. have prepared Al-CNT (up to 1 vol.%) composites using in situ CVD grown CNT/Al powders [72] by cold pressing (at 450 MPa) followed by sintering (at 610 °C for 2 h in vacuum) followed by hot extrusion at 450 °C with ER of 10 (In situ CVD+CS+HE route). They have observed moderate TS (185 MPa) for Al-0.75 vol.% CNT composites (138 MPa for Al) due to poor dispersion. Yang et al. have synthesized in-situ CNT-Al powders (up to 6.75 vol.%) and then ball milled them at 500 rpm for 90 min followed by compaction at 600 MPa [89]. The 23

compacts were sintered at 630 °C for 1 h in Ar followed by extrusion at 500 °C with ER of 16 (In situ CVD+BM+CS+HE route). They have observed good strength and strengthening efficiency (marked as ball milled in Fig. 6). The failure strain was found to be good up to 4 vol.% CNT addition. The TS of Al-6.75 vol.% CNT composites was found to be 420 MPa (173 MPa for Al), but the failure strain was reduced drastically to 5.3 % (24 % for Al). Thus, the (In situ CVD+BM+CS+HE) route gives better properties than (BM+CS+HE) route followed by (In situ CVD+CS+HE) route. This shows the importance of ball milling and its synergistic relation with in situ CVD growth of CNT on Al powders for achieving improved dispersion.

Jiang et al. have developed new process employing disc shaped powders and termed it Flake powder metallurgy (flake PM) [46, 90-91]. The 2D flaky Al powders having a thickness of up to 0.2 µm were prepared by ball milling of 10 µm diameter near-spherical Al powders at 423 rpm for 1-2 h in flowing Ar. It was observed that this method can uniformly distribute CNT on a sub-micron level and has a huge potential for composite development. They have synthesized compacts from Flake Al-CNT (0.5 and 2 vol.%) powders by compacting (at 500 MPa) and sintering (at 550 °C for 2 h in Ar) followed by hot extrusion at 440 ºC with ER of 20 (Flake Al+CS+HE route) [46, 91]. The Flake Al-2 vol.% CNT composite exhibited TS of 430 MPa and failure strain of 6% [91]. Fan et al. have synthesized Flake Al-3 vol.% CNT composites by ball milling (at 426 rpm for 2 h in Ar), compaction and vacuum hot pressing (at 530 °C and 500 MPa for 2 h) followed by hot extrusion at 490 °C with ER of 20 (Flake Al+BM+VHP+HE route) [92] and found that ball milled flake Al-CNT composites resulted in highest TS of 406 MPa (245 MPa for Al) compared to milled spherical Al-CNT (365 MPa) (BM+VHP+HE route) and stirred flake Al-CNT (298 MPa) (Flake Al+VHP+HE route) composites. The failure strain was also high for ball milled Flake Al-CNT (8.8 %) compared to spherical Al-CNT (4 %) and stirred Flake Al-CNT composites (1.9 %). Annealing of samples prepared by (Flake Al+VHP+HE) route was found to increase the tensile strength. Li et al. have produced flake Al-CNT (0.5, 1.5 and 3 vol.%) composites by cold compaction (at 500 MPa), VHP (at 500 °C under 10-2 Pa for 1 h) followed by hot extrusion at 440 °C with an ER of 20 (Flake Al+VHP+HE route) [93] and then subjected them to annealing under H2/Ar flow at 500 °C for 2 h. The TS values of Al-3 vol.% CNT composites were reported to increase from 298 to 370 MPa (250 MPa for Al) while the failure strain increased from 1.9 % to 4.6 % (17.3 % for Al) after annealing. 24

Liu et al. [94] have studied the tensile properties of Flake Al-3 vol.% CNT composites by ball milling (for 4 h at 300 rpm), compaction (at 250 MPa), sintering (at 620 °C for 4 h in Ar) followed by hot extrusion at 600 °C with ER of 32 (Flake Al+BM+CS+HE) route. The TS of the composites was measured to be 167 MPa (139 MPa for Al) with a failure strain of 9 % (38 % for Al). It is observed from the Fig. 6a that the strength of these composites are very low while the failure strain was similar compared to (Flake Al+BM+VHP+HE) route reported by Fan et al. [92] though the ER (=32) was high. This is due to the high pressure (500 MPa) used during VHP as well as lower extrusion temperature of 490 °C. These conditions are expected to provide better density and finer microstructure. Though the strength values are higher, it is observed that the % increase in TS of composites using flake Al powders is lower than (BM+CS+HE) route as seen in Fig. 6b. Figure 6c shows that the failure strain of composites was also higher for (BM+CS+HE) comparative to flake Al-CNT samples. It is observed from Fig. 6b that the % increase in TS was highest for (BM+VHP+HE) processed Al-CNT composites along with higher TS. Christopher et al. have produced Al-CNT composites by hot pressing (at 350 °C for 60 min using 570 MPa for 10 s) and hot extrusion (at 500 °C with an ER of 14) of 20 h ball milled (at 360 rpm) powders (BM+VHP+HE route) [95]. They have found that the TS increased to 428 MPa for 4.5 vol.% CNT (118 MPa for Al) due to good dispersion and CNT-matrix bonding during hot extrusion. However, the failure strain was reduced to 3.7 % (25 % for Al). The % increase in TS of AlCNT composite for (BM+VHP+HE) route are much higher than the Flake Al+ (CS/VHP) +HE route. However, failure strain values for (BM+VHP+HE) route is low compared to Flake Al-CNT composites.

3.3. Spark plasma sintering followed by hot extrusion/hot rolling A detailed discussion on spark plasma sintering (SPS) process and its applications can be found elsewhere [96]. It was observed from Fig. 5 that use of only SPS process did not result in good strengthening and the strengthening efficiency was negative for CNT content of 3 vol.% or more. Hence, several researchers have carried out post processing for improvement of properties. Figure 7 shows the variation of TS, % increase in TS, and failure strain as a function of CNT content. The legend has been arranged from top to bottom in increasing order of the properties. Kawasaki group have performed systematic studies on SPS followed by hot extrusion. Kurita et al. have used ultrasonication of acid treated CNT and Al powders for 3 h for preparing the powders (1 and 5 vol.%) [97]. The powders were subjected to SPS 25

(600 °C for 20 min at pressure of 50 MPa) followed by hot extrusion at 550 ºC with ER of 20 (SPS+HE route). The TS of 1 vol.% CNT composite was found to be 165 MPa (110 MPa for Al) with a failure strain of 24 % while that of 5 vol.% CNT composite was 160 MPa with a failure strain of 12 %. In another study, they have reported the properties of composites with various CNT content up to 5 vol.% using the same technique [98]. They have observed that beyond 0.6 vol.% CNT, the strength was found to be the same while the failure strain decreased with increasing CNT content. However, the failure strains were among the highest compared to other routes. Zhou et al. [99] have further heat-treated Al-CNT (0.6, 1 and 1.5 vol.%) composites prepared by (SPS+HE) route at 600 °C for 6 min [98]. In their study, the TS of hot extruded Al-1.5 vol.% CNT composite was found to be 178 MPa (117 MPa for Al) with a failure strain of 15 % (25 % for Al) and heat treatment did not result in significant improvement of TS (183 MPa) and failure strain (15 %). Liao et al. have used can-rolling to mix Al-CNT (0.75, 1.5 and 3 vol.%) powders [37] and prepared composites by SPS at 500 ºC for 20 min followed by extrusion at 500 ºC with ER of 9 (SPS+HE route). They have observed poor dispersion of CNT and the TS was found to be 172 MPa (155 MPa for Al) for Al-0.75 vol.% CNT composite with a failure strain of 34 % (36% for Al). They have further used various dispersion techniques [100], namely polymer assisted binding (PAB) and high energy milling (HEBM) at 200 rpm for 4 h to disperse the CNT (0.75 vol.%) in composites (BM+SPS+HE route). The TS of composite was increased to 165 MPa (132 MPa for Al) in case of PBA mixing and to 206 MPa (166 MPa for Al) in case of HEBM. Zhang et al. [101] have also used milled Al-CNT powders (200 rpm for 4 h) and prepared the Al-CNT (0.25, 0.5, 1, 1.5 vol.%) composites by SPS (630 °C for 30 min with 30 MPa) and hot extrusion (400 °C with ER of 18) (BM+SPS+HE route). The TS of Al0.25 CNT increased to 164 MPa (127 MPa for Al) with failure strain of 19.5% (29.8% for Al). Ogawa et al. [102] have prepared Al-0.5 vol.% CNT composite using milling (90 rpm for 3h), SPS (560 °C for 2 h at 50 MPa) and hot extrusion (500 °C with ER of 9) (BM+SPS+HE route) and obtained TS of 155 MPa (135 MPa for Al) with a failure strain of 6.2% (6.5% for Al). These studies indicate that ultrasonication, acid functionalization, using low ball milling times and milling speeds does not lead to significant dispersion of CNT and results in lower strength of the Al-CNT composites as seen in Fig. 7a. It is observed from Fig. 7b that the % increase in TS was also lower for (SPS+HE) route. This indicates that good powder dispersion method is necessary to obtain better tensile properties of Al-CNT composites for high CNT content. 26

Fig. 7: Tensile properties of Al-CNT composites as a function of CNT content processed by spark plasma sintering followed by hot extrusion/hot rolling. (NSD = Nano Scale Dispersion of Al-CNT powders, BM = Ball milled, Flake Al = Al flakes produced by milling, SP = Spark Plasma Sintering, HE = Hot Extrusion, HR = Hot Rolling).

Kwon et al. [103-104] have used Al-5 vol.% CNT powders produced by nanoscale dispersion (NSD) process and subjected to SPS at 600 ºC for 20 min under a pressure of 50 MPa followed by extrusion at 400 °C with ER of 20 (NSD+SPS+HE route). They have found a TS of 194 MPa (85 MPa for Al) and failure strain of 10 %. The low TS of composite was attributed to CNT clusters as the NSD process disperses the CNT only on the powder surface. Chen et al. have done extensive work using Flake PM method which employs Al flakes produced by ball milling [38, 57, 105-106]. They have used a solution ball milling (SBM) process to mix the Al-CNT slurry at 200 rpm for 1 h followed by decantation of excess CNT suspension to obtain dispersed Flaky Al-CNT (0.51 and 0.88 vol.%) powders [105]. The Flake Al-CNT powders were subjected to SPS at 550-600 ºC for 30 min at 30 MPa pressure 27

followed by extrusion at 500 ºC with ER of 37 (Flake Al+SPS+HE route). The TS was moderately increased to 180 MPa (157 MPa for Al) and 192 MPa for 0.51 vol.% and 0.88 vol. % CNT composites, respectively. They have investigated the in-situ tensile behaviour of Al-0.88 vol.% CNT composites prepared by same process parameters with 4 h milled powders to prepare the flakes and 1 h mixing of CNT by roll mill and ER of 12 [57]. They have found the TS of the composite to be 123 MPa (101 MPa for Al), which was lower than the previous reported values with ER of 37. This indicates that higher extrusion ratio gives better tensile properties. They have further studied the effect of heat treatment on hot extruded Al-0.88 vol. % CNT composites (ER = 12) [106] at 450, 550 and 650 ºC for 60 min in a vacuum of 20-50 Pa. The TS of heat treated composites at 450 ºC (184 MPa) was better than the hot extruded composites (123 MPa). However, samples heat treated at 550 and 650 ºC showed lower strength than extruded samples due to generation of cracks within the material because of residual stresses. They have further studied the effect of SPS temperature (at 523-623 ºC for 60 min under 30 MPa pressure) on tensile properties of flake Al-CNT composites (Flake Al+SPS+HE route) [38] and observed that both the strength and failure strain were improved with increase in SPS temperature due to better bonding between the grains as well as between CNT and Al. The TS and failure strain of the Al-1.5 vol.% CNT composite sintered at 523 ºC were measured to be 186 MPa and 11.2 %, respectively, while they were 212 MPa and 20.4 % for sintering temperature of 623 °C. They have also prepared flake Al-1.5 vol.% CNT powder by wet mixing process which were subjected to SPS at different temperatures (427-627 °C) for 60 min at 30 MPa pressure followed by hot extrusion at 427 °C with ER of 37 (Flake Al+SPS+HE route) and found improved tensile properties of composites with sintering temperature [43]. The TS of composites sintered at 627 °C was 212 MPa (168 MPa for Al). Thus, wet mixing and solution ball milling methods result in Flaky Al morphology and gives moderate strength and failure strains. It is also observed from Fig. 7b that the strengthening efficiency of Al-CNT composites processed by (Flake Al+SPS+HE) route found to be moderate, which is due to the fact that the TS of Al sample was also high due to flaky nature (deformed structure). However, failures strain of flake Al-CNT composites was high as seen in Fig.7c. Use of ball milling results in significant increase in TS and strengthening efficiency compared to flake Al-CNT composites as seen in Fig.7a and 7b. However, the failure strain was found to be lower as seen in Fig.7c. Guo et al. [107] have prepared Al-CNT composites using SPS of ball milled powders at 590 and 630 °C for 30 min followed by hot rolling with 28

75% reduction (BM+SPS+HR route). The increase in the sintering temperature from 590 to 630 °C gave higher TS of 220 MPa (180 MPa for Al) for the Al-0.75 vol.% CNT composite with an enhanced failure strain of 21 % (13 % for Al). The TS of Al-1 vol.% CNT composite was found to be 235 MPa with the failure strain of 17%. Guo et al. have oxidized CNT using different reagents and dispersed (1 vol. %) in 2 µm Flake Al powders [108] using ball milling for 5 h at 300 rpm. The composites were prepared using SPS at 600 °C for 20 min at 30 MPa pressure under Ar followed by hot rolling (30% reduction in 6 passes at 450 °C) (Flake Al+BM+SPS+HR route). They have obtained a low value of TS (191 MPa) with failure strain of 6 % for Al-1 vol.% pristine CNT composite which is due to the lower deformation during hot rolling. However, oxidation of the CNT using H2SO4-H2O2 mixture gave a high value of TS (340 MPa) with good failure strain of 18%. Zhang et al. have obtained Flake Al powders by HEBM at 1400 rpm for 1 h [40]. The Flake Al-CNT (0.25-1.5 vol. %) composites were prepared using ball milling (at 200 rpm for 2 h under Ar), SPS (at 630 °C for 60 min, under 30 MPa pressure) followed by hot extrusion at 400 °C in Ar with ER of 18 (Flake Al+BM+SPS+HE route). They have obtained TS value of 228 MPa for Flake Al-0.5 vol.% CNT composites (160 MPa for Flake Al) with failure strain of 13 % (20 % for Flake Al). Guo et al. have prepared Flake Al-1 vol.% CNT composites by ball milling (at 300 rpm for 5 h), SPS (at 630 °C for 30 min under 30 MPa pressure) followed by hot extrusion at 500 °C in Ar and with ER of 5 (Flake Al+BM+SPS+HE route) [109]. The TS of Al-CNT composite was increased to 305 MPa (95 MPa for Al) with a failure strain of 12 % (19 % for Al). It is observed from Figs. 7a and 7b that both the TS and strengthening efficiency of (Flake Al+BM+SPS+HE) route composites were low than (Flake Al+BM+SPS+HR) composites. It is noted that Guo et al. [109] have used lower ER, low content of CNT and higher extrusion temperature, but they have obtained higher TS values compared to Zhang et al. [40]. This could be due to higher milling time, speed and better dispersion of CNT. These studies again indicate that better dispersion method and optimization of parameters are necessary to obtain high tensile properties of Al-CNT composites. It is observed from Figs. 7a and 7c that Al-CNT composites processed through (BM+SPS+HE) route exhibit the highest tensile strength with good ductility. Chen et al. have prepared Al-1.5 vol.% CNT composite powders using HEBM (for 2-48 h at 200 rpm), SPS (at various temperatures from 527-627 °C for 30 min under 30 MPa pressure) followed by extrusion at 427 °C with ER of 37 (BM+SPS+HE route) [39] and they have found that tensile properties of composites increased with increasing milling times. The TS of composites was 29

observed to be 368 MPa (168 MPa for Al) for 48 h milled powders with failure strain of 16 % (25 % for Al). The decrease in milling time leads to lower tensile properties of Al-CNT composites. Zhang et al. [110] have dispersed the 0.5 CNT (0.5, 1, 1.5 vol. %) in Al using low speed planetary mill for 4 h at 200 rpm under Ar using same process parameters [40] (BM+SPS+HE route) and obtained highest TS of 225 MPa (127 MPa for Al) with a failure strain of 11.5 % (30 % for Al) for Al-0.5 vol.% CNT composite.

Fig. 8: (a-b) Optical images of Al–2.0 wt.% CNT composite (a) SPS [37], (b) SPS+HE [37] (arrow in (b) indicates the extrusion direction), (c-d) SEM EBSD images of Al-1 wt. % CNT; (c) Flake Al+SPS+HE [38] and (d) Flake Al+BM+SPS+HE route [40].

Figure 8(a-d) show micrographs of Al-CNT composites prepared by various processes. It is observed from these images that the grain size (GS) of the matrix is reduced in the following sequence: SPS (GS ~ 9 µm) [37] > SPS+HE (GS ~ 6.5 µm) [37] > Flake Al+SPS+HE (GS ~ 30

1.75 µm) [38] > BM+SPS+HE (GS ~ 1.7 µm) [39] > Flake Al+BM+SPS+HE (GS ~ 1.45 µm) routes [40]. The densification of Al-CNT composites is also high for SPS+HE (> 99 %) process compared to only SPS (~ 95 %). Hence, grain refinement is also one of the major strengthening mechanisms to improve the strength of Al-CNT composites processed by ball milling followed by SPS. The densification of SPS processed samples is increased with increase in the SPS temperature from 527-627 °C [43]. Sintering at 527 °C resulted in insufficient grain bonding of Al matrix, while bonding was good at 627 °C without grain growth. The grain size dependence of SPS samples on the SPS temperature is shown in Fig. 9 (a) which indicates small increase in grain size from 1.67 and 1.75 µm for increase in SPS temperature from 527 °C to 627 °C [43]. Figure 9(b) shows that the contribution of effective load transfer is increased with increase in SPS temperature and contribution of grain refinement is small. It is also seen that formation of Al4C3 at temperature of 627 °C reduces the strengthening due to low load transfer.

Fig. 9: (a) Average grain size and (b) strengthening factors in Al-1 wt. % CNT composites with SPS temperature 427-627 °C [43]; Insets in (a) show colored grains of typical materials.

It is known that ball milling also reduces the length of CNT due to fracture. Figure 10 shows a comparison between the experimental values and predicted curves of the strengthening effect with respect to the aspect ratio of CNT for Al-CNT composites prepared by ball milling and SPS followed by hot extrusion (BM+SPS+HE) route [39].

31

Fig.10: Dependence of strength contribution by CNT on the aspect ratio of CNT for Al-CNT composites prepared by (BM+SPS+HE) route [39].

It was observed that Orowan strengthening and load transfer mechanisms have opposite trends when the aspect ratio of CNT varies and the strengthening behaviour exhibits three regimes. If the aspect ratio is small (≤ 10, Regime I), strength of Al-CNT composites follows the high strength predicted by Orowan mechanism while the load transfer effect is very small. In the Regime III (large lengths or aspect ratios of CNT), load transfer becomes the dominant strengthening mechanism. In the intermediate Regime II, the experimental strength values are located between the strengths predicted via Orowan strengthening and the load transfer model. The strengths are lower than the predictions by Orowan strengthening, but much higher than the values predicted by the load transfer model. It is noted that the best combination of strength and failure strain had been reported for (BM+SPS+HE) route.

32

3.4. Powder deformation processing Powder deformation processing refers to techniques that prepare compacts directly using powders by subjecting them to hot deformation. The intermediate sintering step is not used in this process. The mixed Al-CNT powders are processed through different powder deformation methods such as hot extrusion or hot rolling. Figure 11 shows tensile properties of Al-CNT composites as a function of CNT content prepared by this technique. The processes in legend have been arranged in increasing order of properties from top to bottom. It is noticed from Figs. 11a and 11b that both the tensile strength and % increase in TS of composites were lower for samples prepared by hot rolling of blended powders subsequently subjected to conventional sintering (HR+CS route). However, the failure strain was observed to be good. Esawi and El Borady were the first to use hot rolling of canned powders to prepare plates of Al-CNT composites which were further sintered at 300 ºC for 3 h in vacuum and then at 550 ºC for 45 min in air [111]. They have prepared up to 3 vol.% CNT-Al composites using powders mixed using Turbula shaker and milling without any milling media which resulted in poor dispersion and agglomeration at higher CNT content (HR+CS route). The TS of 0.75 vol.% CNT composite was found to be 142 MPa (130 MPa for Al) with failure strain of 18 % (25 % for Al). For better dispersion of CNT in composites, Noguchi et al. have developed a new CNT dispersing technique called nano-scale dispersion (NSD) [112]. Although the dispersion was uniform on the surface of the powder, it is noted that the CNT were located at the particle boundaries after processing. They have utilized NSD processed powders and synthesized Al-1.5 vol.% CNT composite by hot extrusion at 400 ºC with ER of 10 and 20 (NSD+HE route) [113]. The TS of composites increased with increase in ER and were found to be 170 MPa and 230 MPa for ER of 10 and 20 (150 MPa for Al) with corresponding failure strains of 8 % and 6.5 % (11.5% for Al), respectively. Hence, the increase in extrusion ratio increases the tensile strength of Al-CNT composites. Prior to powder deformation processing of Al-CNT composites, ball milling has been shown to result in good tensile strength and strengthening efficiency. Kwon et al. [61] have prepared Al-1 vol.% CNT composite by hot extrusion of milled powders (at 360 rpm for 160 min under Ar) at 550 °C with ER of 14 (BM+HE route). The TS of the composites was found to be 298 MPa (116 MPa for Al) with a failure strain of 9 % (25 % for Al). Ogawa et al. have milled Al and CNT (0.5, 1, 2, and 4 vol.%) powders at 200 rpm for 3 h [114] and filled them in Al containers and compressed them in vacuum (10-5 torr) followed by extrusion at 550 °C with ER of 9 with holding time 30 min [BM+HE route]. They have obtained highest TS of 33

458 MPa (378 MPa for Al) with 37.2 % failure strain (15.5 % for Al) for the Al-0.5 vol.% CNT, while the TS of Al-4 vol.% CNT composite was 402 MPa with the failure strain of 4.2 %. The compression of powders in vacuum resulted in high density of samples while ball milling resulted in good dispersion leading to high TS values. Further they [102] have used high ER of 16 with holding time 90 min and prepared the composites and found low TS of 405 MPa (400 MPa for Al) with failure strain of 28% (35% for Al) for Al-0.5 vol.% CNT composites. It is seen from these studies that the heating time also plays a major role on tensile properties of the composites.

Fig. 11: Tensile properties of Al-CNT composites as a function of CNT amount processed by powder hot deformation process. (NSD = Nano Scale Dispersion of Al-CNT powders, BM = Ball milled, Powder HE = Powder Hot Extrusion, Powder HR = Powder Hot Rolling, CS = Conventional Sintering, AN = Annealing).

Esawi et al. have prepared Al-3 vol.% CNT powders using ball milling (200 rpm for 0-48 h) and concluded that mechanical milling is a promising technique to overcome the clustering of CNT in composites [115]. They have prepared Al-3 vol.% CNT composites using ball 34

milling (3 h and 6 h at 200 rpm in Ar), compaction (at 475 MPa) followed by hot extrusion (at 500 ºC using ER of 4) [116] and annealing (at 400 ºC and 500 ºC for 10 h) (BM+HE+AN route). They have found that the 3 h ball milled composite showed TS of 345 MPa (285 MPa for Al) with a decrease in failure stain to 5.7 % (8.6 % for Al). However, TS of the 6 h milled composites was similar (348 MPa) to Al (348.5 MPa) after annealing at 500 ºC. Comparatively, the composite had better TS of 365.5 MPa (377.4 MPa for Al) when annealed at 400 ºC. They have also prepared Al-CNT (0.75, 1.5, 3, and 7.5 vol. %) using 30 min milled powders [117] and found improved TS for the Al-3 vol.% CNT composite (255 MPa vs. 170 MPa for Al). This shows that shorter milling times and lower extrusion ratios result in lower strength of Al-CNT composites. It is also found from these results that the increase in milling time provided higher strength of composites (255 MPa from 30 min and 366 MPa for 6 h milled samples). Esawi et al. have also studied the effect of CNT diameter (40 and 140 nm) and CNT content (0-5 wt. %) on dispersion by ball milling for 30 min at 400 rpm [118] with same process parameters (BM+HE+AN) route [116] and found that the CNT with large diameter were dispersed easily in Al. They have observed that for the same amount of CNT, the 40 nm CNT provided more strengthening than the 140 nm CNT up to 2.25 vol.% CNT. However, this was reversed at 3 and 7.5 vol.% CNT in composites. Overall, Al-3 vol.% CNT (140 nm) provided significant strengthening (96%, 335 MPa from 170 MPa). It is known that fine CNT are difficult to disperse, but have higher TS. Their results indicate that there is trade-off between the quality of dispersion and inherent strength of CNT. Esawi’s group have also studied the effect of damaged CNT (intentionally prepared using harsh ball milling conditions) on strength of Al-3 vol.% composites [119] and have obtained high TS of 333 MPa, 330 MPa, 288 MPa (169 MPa for Al) with corresponding failure strains of 5.3 %, 7.2 %, and 4.4 % (10 % for Al), respectively for mildly damaged, as received, and severely damaged CNT-Al composites prepared by (BM+HE+AN) route. Figure 11 shows that hot rolling of ball milled powders (BM+HR route) provide highest TS and % increase in TS with moderate failure strain. Bae’s group [120-124] have reported extensively on the microstructure and tensile properties of Al-CNT composites fabricated through ball milling and hot rolling (BM+HR route). Choi et al. have used hot rolling to prepare plate type specimens of Al-CNT (1.5, 3, 4.5 and 6 vol. %) composites [121]. They have placed the milled powders (at 500 rpm for 6 h in Argon) in a Cu can followed by hot rolling at 480 ºC for 27 times with 12 % reduction on each pass and have obtained very high TS (620 MPa) for 4.5 vol.% CNT composite (270 MPa for milled Al) with the failure strain 35

of 2.5 % (6.5 % for Al sample). They have also reported the effect of grain size on tensile properties of these composites in which grain size was altered by changing milling condition and hot rolling temperature [124]. The effect of milling time (3, 6 and 12 h) and speed (425, 520 and 600 rpm) on tensile properties of Al-4.5 vol.% CNT composite was further studied [120]. The TS and failure strain values of composites fabricated with powders milled at 425 rpm for 12 h, 520 rpm for 6 h and 600 rpm for 3 h were found to be 580 MPa and 4 %, 600 MPa and 2.5 %, and 530 MPa and 5 % respectively. The grain size of the matrix found to be 72, 151, and 157 nm for samples fabricated under milling conditions of 425 rpm for 12 h, 520 rpm for 6 h, and 600 rpm for 3 h, respectively indicating the effect of grain refinement. It is also noticed that optimized milling parameters are necessary to achieve better dispersion of CNT and tensile properties. Choi et al. have further shown that the Al-4.5 vol.% CNT composite has a high creep resistance at 250 ºC compared to Al sample in the high stress (200 MPa) region [122]. They have also studied the warm temperature properties of the same composite and found it to be better compared to Al, which has been explained by change in grain boundary character towards equilibrium low angle boundaries [123]. Yoo et al. have prepared Al-CNT (1 and 3 vol.%) composites by ball milling (at 400 rpm for 6 h) and hot rolling at 450 ºC after soaking for 1 h, to a thickness of 2 mm in 8 passes (BM+HR route) [125] and have found that the Al-3 vol.% CNT composite had the high TS of 610 MPa (360 MPa for Al) with the failure strain of 6.5 % (16 % for Al). They have further reduced rolled samples to 0.2 mm thickness by high-ratio differential speed rolling at 200 ºC [126] and obtained TS of 420 MPa (350 MPa for Al) for 3 vol.% CNT-Al composites with a failure strain of 10 % (16 % for Al). The improved strength of composites by (BM+HR) route is attributed to high density, high strain hardening, grain refinement, alignment of carbon nanotubes and better load transfer to CNTs.

3.5 Other Processing routes It is found from literature that few researchers have used other processes which are different from previously mentioned techniques. Liu et al. have placed CNT in six holes of variable diameters (0, 2, 4, 6, 8 and 10 mm) having a depth of 3.5 mm and then have synthesized AlCNT (0, 1.6, 2.5, 4.4, 5.3 and 6 vol.%) surface composite by Friction Stir Processing (FSP route) for five passes using a 27 mm dia. tool [127]. They have obtained increased tensile properties of composites with increasing CNT content. The Al-6 vol. % of CNT composite showed a double fold improvement in the TS (from 93 MPa to 190 MPa) but an almost 4 36

times reduction in the ductility (37 % to 10 %). The effect of energy input on tensile properties has been investigated by Zhang et al. [128]. They have fabricated Al-CNT (1.6 and 3.2 vol.%) composites by FSP for 3 passes with various rotational speeds (600-950 rpm) and feed rates (30-150 mm/min) with 1.5 mm and 2 mm grooves made on Al plates. The dispersion of CNT was enhanced for high energy input and tensile properties of composites were improved. A maximum TS of 111 MPa was obtained for Al-1.6 vol. CNT composites at 750 rpm and 30 mm/min with 1.5 mm groove width, while the TS of Al-3.2 vol.% CNT composite (2 mm groove width) was 139 MPa (90 MPa for Al) with failure strain 31.2% (36.8% for Al) at 900 rpm and 30 mm/min. It was found that the TS of Al-1.6 vol.% CNT composites was 128 MPa with 5 passes of FSP at 950 rpm and 30 mm/min [127] compared to 111 MPa with 3 passes of FSP at 750 rpm and 30 mm/min [128]. This shows that number of FSP passes plays a major influence to improve the tensile properties of Al-CNT composites as it affects the dispersion. Park et al. have studied the tensile properties of Al-CNT (0.15-0.6 vol.%) composites prepared by ball milling (at 300 rpm for 4 h), hot pressing (at 580 °C for 20 min under 500 MPa in Ar), melt blending (at 680 °C under a vacuum of 10-3 Torr with stirring at 500 rpm for 20 min) followed by hot extrusion at 550 °C with an ER of 10 (BM+HP+Melt+HE route) [129]. They have observed that use of paraffin oil prevents the oxidation of Al during ball milling and dispersion of CNT in Al was also good. They have found TS of 114 MPa (from 92 MPa for Al) for Al-0.3 vol.% CNT composite with a failure strain of ~9 % (21 % for Al). The strengthening of composites in this study was lower due to the poor dispersion of CNT. Mansoor et al. have prepared Al-CNT (0.1 and 0.2 vol.%) composites by induction melting at 760 °C for 10 min followed by casting [130]. They have obtained a TS of 125 MPa for Al-0.2 vol.% CNT composites (82 MPa for Al), but the failure strain was very low (3.1% compared to 2.15% for Al). Liu et al. [131] have used in situ AlCNT (0, 0.5, 1.5, and 2.5 vol.%) powders and subjected to shift speed ball milling (at 150 rpm for 3 h and then 280 rpm for 1 h in Ar atmosphere) followed by cold compaction and induction melting and hot extrusion at 550 °C with ER of 16 (In Situ CVD+BM+Melt+HE route) and have found that TS of the Al-1.5 vol.% CNT composite was 185 MPa (90 MPa for Al) with failure strain of 33 %. However, TS was not improved for Al-2.5 vol.% CNT (182 MPa) and failure strain was reduced to 18 % (39 % for Al). Lahiri et al. have prepared AlCNT (0, 2, 7.5 and 9.5 vol.%) composite by roll bonding process [35]. They sprayed the CNT suspension (ultrasonicated in acetone) on to Al foils and 4 layers of Al foils with 3 intermediate layers of sprayed CNT were stacked together and then cold rolled. The presence 37

of aligned CNT was also observed. They have observed an improved CNT dispersion in case of 2 vol.% CNT and clusters were observed at higher CNT contents (7 and 9.5 %). However, the tensile strength was increased with increase in CNT content. The TS was observed to be 98 MPa for Al-9.5 vol.% CNT composite foils (28 MPa for Al). Larianovsky et al. [132] have prepared Al-CNT (0.37 and 0.75 vol.%) composites by inserting the CNT in the shot sleeve before high pressure die casting (HPDC) of molten metal at 760 °C. Further, Cyclic extrusion was carried out at 375-400 °C with ER of 12 under 375-400 MPa pressure for 1-10 cycles. The tensile strength of Al-CNT composites increased with increase in the number of extrusion cycles. The maximum TS achieved was 132 MPa for Al-0.7 vol.% CNT composites (94 MPa for Al) with a failure strain 18.5% (27.7% for Al) for 10 cycles. It can be seen that methods other than powder metallurgy result in poor tensile properties but moderate/good ductility. It is observed from casting and selective laser melting routes of Al-CNT composites that complex structures can be made, however, clustering, poor wettability, CNT damage and aggressive metal-CNT reaction are major issues for obtaining improved tensile properties.

3.6. Comparison of various processes for Al-CNT composites Figures 3-4 shown earlier indicate a large amount of scatter in the tensile properties of AlCNT composites for a given CNT vol.%. Jiang et al. have plotted the Strengthening Efficiency (R) vs. Tensile Strength and Tensile Strength vs. Failure Strain of the composite as shown in Fig. 12(a) and 12(b), respectively, and argued that R is a function of the damage to CNT [46, 91]. They have mentioned that with flake-powder metallurgy route, where the CNT are not damaged due to ball milling, both the strength and strengthening efficiency are the highest (Fig. 12a). They have mentioned that the strengthening efficiencies of flake PM AlCNT composites showed 20 times of LEBM and 10 times of HEBM Al-CNT composites. The strength and failure strain of flake PM was higher than NSD Al-CNT composites. It was also argued that both the strength and failure strain were good in case of flake PM Al-CNT composites, while conventional methods had a strength-ductility trade-off. They reasoned that ball milling reduces the length of the CNT as well as introduces defects and results in lower strengthening efficiency. It is noted that these diagrams are incorrect since they have used tensile data for different CNT vol.% in these plots. It is known that for higher CNT vol.% addition, dispersion is a problem and strengthening efficiencies are low. Hence, a comparison of different processes must be done for the same vol.% CNT addition. Moreover, strengthening efficiency is not a criteria for judging the efficiency of a process, rather the 38

strength and failure strain obtained are of importance for applications. In the present analysis, the tensile data of 1.5 vol.% and 3 vol.% CNT reinforced Al composites prepared by various processing techniques have been considered to compare different processes with respect to the strength, strengthening efficiency and failure strain that can be achieved. This is because, most studies using various processes have used either 1.5 vol.% or 3 vol.% CNT as reinforcement in Al-CNT composites.

Fig. 12: (a) Strengthening Efficiency vs. Engineering Stress [91] and (b) Tensile strength vs. Tensile strain of Al-CNT composites [46].

Figure 13 shows similar graphs of Strengthening efficiency and Failure Strain vs. Tensile strength for Al-1.5 vol.% CNT and Al-3 vol.% CNT composites, respectively. It is noted that the conclusions from these graphs are different compared to Jiang et al. [46, 91]. The processes shown in the legend have been arranged in increased order of TS (Figs. 13a and b) and failure strain (Figs. 13c and 13d) from top to bottom. It is observed from Fig. 13 that SPS process without any post-processing gives low tensile strength and good failure strain [6263]. It is observed that strengthening efficiency is higher at 1.5 vol.% CNT addition and becomes less than zero for 3 vol.% addition due to poor dispersion. FSP process resulted in increased strength and failure strain compared to SPS [127] due to severe deformation and dynamic recrystallization. Post-processing of composites resulted in improved tensile properties. However, post-processing without ball milling gave moderate TS and strengthening efficiency (even negative for 3 vol.%) and lower failure strain (< 10 %) as seen in Figs. 13a and 13c (HR+CS route). Al-1.5 vol.% CNT composites prepared by mechanical mixed powders followed by (CS+CE route) resulted in improved TS and strengthening efficiency but lowest failure strain (< 4 %) [29]. The Al-CNT composites prepared by (SPS+HE route) have resulted in moderate TS and good failure strain [37]. Increase in SPS 39

temperature and hot extrusion ratio resulted moderate improvement in TS with moderate failure strain [99]. Thus, CNT dispersion process is very important to obtain high tensile properties especially at higher CNT content. Composites prepared from ball milling of in situ CVD synthesized 1.5 vol.% CNT-Al powder (In Situ CVD+BM+Melt+HE route) [131] gave moderate strength and good ductility (> 30 %). The Al-CNT composites prepared by (CS+HE+HR route) had further improved TS and failure strain [78-79]. Use of Flake Al powders have provided increased tensile properties of composites, but the strengthening efficiency of composites processed by this route was less with moderate failure strain as seen from Figs. 13a and 13b, respectively. Chen et al. have studied the effect of heat treatment on (Flake Al+SPS+HE route) prepared samples [106] and reported that TS of composites heat-treated at 450 ºC (184 MPa) was higher than the hot extruded composites (123 MPa). They have also consolidated Al-1.5 vol.% CNT powders using SPS at different temperatures followed by hot extrusion [38, 43] and reported that both the TS and failure strain were improved with increase in SPS temperature due to better bonding between the grains as well as between CNT and Al. The use of Flake Al and ball milling for Al-CNT powders further enhanced tensile properties of composites. Zhang et al. have obtained Flake Al powders by HEBM [40] and prepared Flake Al-CNT composites by (Flake Al+BM+SPS+HE route). The TS was found to be higher than (Flake Al+SPS+HE) route. The TS of Flake Al-3 vol.% CNT composites prepared by (Flake Al+BM+CS+HE) route [94] was measured to be lower than (Flake Al+SPS+HE route) as seen in Figs. 13a and 13c indicating that SPS is better than CS. It is observed that the strengthening efficiencies were less in the case of Flake Al-CNT as seen from Fig. 13a, which could be due to lower milling time used and high strength of Flake Al sample. However, failure strain values of composites were moderate. Nano-scale dispersion of Al-CNT (NSD+HE route) [113] exhibited higher TS and strengthening efficiency compared to (Flake Al+SPS+HE) and (Flake Al+BM+SPS+HE) as seen from Fig. 13a. Figures 13a and 13b show that the % increase in TS prepared by (BM+HE+AN route) exhibited lower and negative strengthening efficiencies for Al-1.5 vol.% CNT and Al-3 vol.% CNT composites with lower failure strain values. However, TS of composites prepared by this route were higher. The TS of Al-1.5 vol.% CNT composites prepared by (BM+CS+HE) [81] have been also reported to be improved with increased milling time. It is also seen from Fig. 13a that the strengthening efficiency prepared by this route was also high and increased with increase in milling time. It is observed from Fig. 13a that the samples 40

prepared by (BM+CS+HE route) have increased TS and failure strains [78]. Thus, vacuum sintering of ball milled powders results in good properties. It is observed that functionally graded 1.5 vol.% CNT-Al composites prepared by (BM+CS+HE) improved the failure strain of composites to 17%, but TS was only 195 MPa [83]. Xu et al. [84] have prepared Al-CNT composites by (BM+CS+HE route) and found a maximum TS of 312 MPa for Al-3 vol.% CNT composites with a failure strain of 15.8% as shown in Figs. 13a&c. This study shows that HEBM with higher milling time and high ER enhance the tensile properties of Al-CNT composites. It is observed from Figs. 13a&c that use of coated CNT resulted in enhanced TS of Al-1.5 vol.% CNT composites prepared by (BM+CS+HR+AN route) [76]. However, the failure strains of composites prepared by this method are low and reduced with use of coated CNT.

Fig. 13: Variation of (a-b) Strengthening Efficiency and (c-d) Failure Strain with Tensile strength of Al – 1.5 vol. % CNT and Al-3 Vol. % CNT composites, respectively produced by various routes. (In Situ CVD = Al powder with CNT grown on it in situ, BM = Ball Milling, Melt = Melting of Al-CNT composite, SPS = Spark Plasma Sintering, HE = Hot Extrusion, 41

CS = Conventional Sintering, VHP = Vacuum Hot Pressing, HR = Hot Rolling, FSP = Friction Stir Processing, HT = Heat Treated, AN = Annealed, NSD = Nano Scale Dispersion of Al-CNT powders, Flake Al = Al flakes produced by milling, CE = Cold Extrusion).

Vacuum hot pressing (VHP) of Al-CNT composites results in high tensile properties compared to conventional sintering as seen in Figs. 13a&b. Flake Al-3 vol.% CNT composite prepared by (Flake Al+VHP+HE route) [69] showed lower TS (298 MPa) with very low failure strain (1.9 %). A decrease in extrusion temperature [93] resulted in higher TS values of 326 MPa and 370 MPa, respectively with good failure strains of 12.6 % and 4.6 % for 1.5 vol. % and 3 vol. % CNT-Al composites. The Al-3 vol.% CNT composites prepared by (BM+VHP+HE route) [92] had a TS of 365 MPa (Fig. 13b) with moderate strengthening efficiency, but the failure strain was low (4 %). It is also seen from Fig. 13b that Flake Al-3 vol.% CNT composites with the same ball milling and processing conditions (Flake Al+BM+VHP+HE route) had a higher TS (406 MPa) with moderate strengthening efficiency and failure strain (8.8 %) [92]. It is noticed from the Fig. 13a that tensile properties of Al-CNT composites prepared by (BM+SPS+HE) route with ER of 37 are highly enhanced compared to (BM+CS+HE) route. The TS of Al-1.5 vol.% CNT composites prepared by (BM+SPS+HE) route [39] was found to increase with increasing milling times from 4 h (250 MPa with 14 % failure strain) to 48 h (368 MPa with 16 % failure strain). The strengthening efficiency values prepared by this method are highest among all processes of Al-CNT composites with moderate failure strains as seen in Figs. 13a&c, respectively. It is seen that lower ER (18) of Al-1.5 vol.% CNT composites prepared by same processing method resulted in low TS values of 155 MPa [110] and 138 MPa [101] with failure strains of 24% and 15%, respectively. These studies again show that the higher ER of composites results in improved TS of the composite. It is seen from Fig. 13 that TS of Al-CNT composites were highest for the (BM+HR) route but with poor failure strains [121, 125-126]. The strengthening efficiencies were low compared to (BM+SPS+HE), (BM+HE+AN), and (BM+CS+HE) processed Al-CNT composites. Choi et al. [121] have reported TS values of 395 MPa and 492 MPa, respectively with failure strains of 6.5 % and 4.5 % for Al-1.5 vol.% CNT and Al-3 vol.% CNT composites. Yoo et al. [125] have reported the highest TS of 610 MPa with a failure strain of 6.5 % for Al-3 vol.% CNT composites. The strength enhancement is attributed to the highest rolling reduction which causes grain refinement and higher work hardening rate. Figure 14 42

shows TEM images of Al-CNT composites prepared by (a) BM+VHP+HE [92] and (b) BM+HR [125]. The grain sizes of Al matrix prepared by BM+VHP+HE (480 nm) and BM+HR (500 nm) are similar as seen in Fig. 14. The milling parameters used for both methods are also identical (6 h @426 rpm for BM+VHP+HE and 6h@400 rpm for BM+HR). Moreover, the densification is also identical for both processes. However, the TS of Al-3 vol.% CNT composites prepared by (BM+HR) route is higher than by (BM+VHP+HE). This is due to the large work hardening (Orowan strengthening) of samples during hot rolling compared to vacuum hot pressing.

Fig. 14: TEM micrographs of Al-CNT (3 vol. %) composites prepared by (a) BM+VHP+HE [92] and (b) BM+HR [125] processes.

4. Reinforcement of Al alloys with CNT It is evident that most researchers have worked on pure Al as matrix in composites. About 30 % of the papers published in Al-CNT composites have Al alloy as a matrix. This may be due to the ease of processing, no heat treatment requirements, inexpensive powders and easy availability. It is known that pure Al is very soft, ductile and has good formability and is not suitable for most of the structural applications due to its low strength. However, it was shown in previous section that pure Al-CNT composites can have strength more than 400 MPa with a failure strain of more than 15 %. Typically, Al alloys are strengthened by solid solution strengthening or precipitation hardening. The major alloying elements used in Al are Cu, Mg, Si, Mn, Zn, and Li [133]. Various researchers have demonstrated that the tensile properties of Al can be increased by alloying and CNT reinforcement. Table S2 in the supplementary 43

provides the details on dispersion methods, fabrication processes and tensile properties of the different Al alloy composites reinforced with CNT. Most of the work has been carried out on 2xxx and 6xxx series alloys as matrix. Hence, they are summarised separately in the following sections.

4.1. Al-Cu alloys It is well known that the 2XXX series Al-alloys, consisting of copper as the main alloying element, can be strengthened by precipitation hardening due to the formation of the CuAl2 phase. Composites have been fabricated by a combination of PM and secondary processing techniques such as hot extrusion and hot rolling. Figure 15 shows tensile properties of Al-Cu alloy-CNT composites as a function of CNT content reported in the literature. The process details in the legend have been arranged in increasing order of TS from top to bottom. Meng et al. have prepared 2.5 wt.% In Situ CVD synthesized CNT-Al-4 wt.% Cu composite [134] by ball milling (at 400 rpm for 1 h in Ar), compaction (at 600 MPa), sintering (at 600 °C for 1 h in Ar) followed by hot extrusion (at 550 °C with ER of 16) (In Situ CVD+BM+CS+HE route). The TS of composites was found to be 375 MPa (280 MPa for Al alloy) with 34 % increment in TS. The hot extruded samples were further subjected to T6 ageing and TS was increased to 451 MPa. Synergistic effect of aging treatment and CNT was observed which resulted in improved TS. However, failure strain was drastically reduced to 3.6 % (15.8 % for Al-Cu alloy). Use of Flake Al powders and HIP process resulted in improved tensile properties compared to (InSituCVD+BM+CS+HE) processed composites. Li et al. [135] have prepared Flake Al-4 wt. % Cu powders by ball milling (at 423 rpm for 4 h) and mixed with 2.2 vol.% CNT using stirrer and then composites are prepared using hot pressing (at 600 °C and 50 MPa) followed by hot extrusion (at 475 °C with ER of 10) (FlakeAl+HIP+HE route). The TS of the composite was observed to be 482 MPa (376 MPa for Al-Cu alloy) with a failure strain of 6 % (10 % for Al-Cu alloy). The improvement in TS was attributed to homogeneous dispersion of the CNT and nano Al4C3. It is found from Fig. 15 that ball milling of Al alloy-CNT composites enhanced the TS and improvement in TS was also highest for ball milled composites. Nam et al. have used Cufunctionalized CNT powders prepared by molecular level mixing method [136] for synthesis of Al-4 wt. % Cu-CNT composites [137-138] by ball milling (for 3 h at 200 rpm) followed by SPS (at 500 ºC for 5 min under 50 MPa pressure) (BM+SPS route). They have observed improved tensile properties with increasing CNT content. The acid functionalized CNT (4 44

vol. %) reinforced Al-4 wt.% Cu composite had better TS of 494 MPa (237 MPa for Al alloy) with a failure strain of 8.6 % (29 % for Al alloy) [137]. The SPS samples were further subjected to T6 (BM+SPS+T6 route) [138] and TS was found to drastically increase to 600 MPa (309 MPa for T6 Al alloy) with a failure strain of 8 % (29 % for T6 Al alloy). Liu et al. have used a bi-axis rotary mixer at 60 pm for 8 h to mix Al and CNT powders and composites were prepared by compaction and VHP at 560 °C for 1 h followed by hot forging at 450 °C (VHP+FG) [139-140]. It is seen from Fig. 15 that the TS of composites processed by (VHP+FG) route exhibit lower TS of 392 MPa and 298 MPa for 1.5 and 4.5 vol.% CNT reinforcement (411 MPa for Al sample), respectively with lower failure strains of 8 % and 1 % (12 % for Al). The negative % increase in TS was attributed to the presence of CNT cluster and coarser grain size (5-10 µm) after the forging process. It is observed from Fig. 15(a) that the FSP of Al alloy-CNT after (VHP+FG) process resulted in improved tensile properties compared to (BM+SPS route). Ma et al. group have conducted extensive investigations on FSP of Al alloy-CNT composites [139-143]. Liu et al. have studied the effect of FSP on hot pressed (560 °C for 1h) and hot forged (450 °C) AA2009-CNT (1.5 and 4.5 vol.%) composite disk [139-140]. The FSP was carried out for 1-5 passes and then solutionized at 495 °C for 2 h, quenched and naturally aged for 96 h (VHP+FG+FSP+T4 route). They have observed that the intense tool stirring during FSP processing significantly reduced the CNT clusters and dispersion of CNT was improved with increased number of FSP passes. However, CNT were damaged progressively with increase in Al4C3 formation. Higher degree of grain refinement with increased number of passes and with increase in CNT content was reported. The obtained TS value was 477 MPa (417 MPa for Al Alloy) for 1.5 vol.% CNT-Al composite after 4 passes. Liu et al. have further determined high temperature properties of FSP composites [142] and found that at higher testing temperatures, the difference between the strength of alloy and the composite was reduced and the TS of 4.5 vol.% CNT-Al at 300 ºC (150 MPa) was less than the alloy (180 MPa). This was attributed to the temperature being above the equi-cohesive temperature where smaller grain size was detrimental. However, they found that room temperature TS of Al-1.5 vol.% CNT composite was high (490 MPa vs. 410 MPa for Al alloy) due to grain refinement and fibre strengthening as evident from the CNT pull out. Liu et al. [141] have explored the effect of hot rolling (at 480 ºC with 80 % reduction) on tensile properties of FSP AA2009-CNT composites (VHP+FG+FSP+HR+T4 route) and obtained superior TS of 650 MPa for 4.5 vol.% CNT composite (400 MPa for Al alloy) with high strengthening efficiency as shown in Fig. 15b. 45

However, failure strain was drastically reduced (5 %) compared to Al alloy (15 %) as seen in Fig. 15c. The increased TS was due to increased dislocation density, better load transfer and alignment of CNT during hot rolling. They have measured effect of CNT alignment on anisotropy of properties of composites [143] and observed that TS was higher (640 MPa) along rolling direction compared to normal direction (470 MPa). Deng et al. [144-146] have prepared AA2024-CNT composites using ball milling (10 min), cold isostatic pressing (300 MPa for 5 min) followed by hot extrusion at 460 ºC with ER of 25 (BM+CIP+HE route). They have achieved good density (> 98.5 %) up to 1.5 vol.% CNT and found increased TS (from 384 MPa to 522 MPa). They reported decrease in strength for 2 vol.% CNT addition due to lower density of 95 %. He et al. have used cryogenic milling (in liquid N2) to disperse CNT (0.45, 1, 1.5, 2.2 vol.%) in AA2009 alloy [147] followed by HIP at 465 °C under 120 MPa for 3 h and hot extrusion at 460 °C with ER of 18. Further, hot extruded samples were subjected to T4 heat-treatment (BM+HIP+HE+T4 route). The CNT dispersion was good up to 1.5 vol.% and further addition of CNT lead to agglomeration of CNT. The TS of 1.5 vol.% CNT-Al alloy composite was 560 MPa (448 MPa for Al alloy) with failure strain of 10 % (14 % for Al alloy). These studies again indicate that good CNT dispersion is necessary for higher CNT content to obtain improved tensile properties. It is noted that higher tensile properties are obtained for HIP processed composites compared to CIP processed Al-CNT composites. This is due to the improved densification of samples after HIP process. Hence, densification also plays an important role to obtain improved properties of composites. Anas et al. have prepared Al-4.4 wt.% Cu-0.5 wt. %- CNT (0.75-3.75 vol.%) composites by (BM+FG+HE) route. They have prepared powders using an Attritor mill at 700 rpm for 4 h under Ar [148] and filled them in Al cans and then forged at 450 °C with 350 MPa followed by hot extrusion at 550°C with ER of 16. Further, hot extruded samples were solutionized at 490 °C for 2 h, quenched and aged at 150 °C for 4 h (BM+FG+HE+T6 route). They observed that grain size of the Al-4.4 wt.% Cu-0.5 wt.%Mg-3.75 vol.% CNT composite was decreased to 193 nm (336 nm for Al-4.4Cu) and TS of composites increased with increasing CNT content. The maximum TS of 682 MPa (428 MPa for Al alloy) was obtained for Al-4.4Cu3.75 vol.% CNT composite after T6-ageing with a failure strain of 3.3 % (11.6 % for Al alloy). However, the % increase in TS of composites was lower as seen in Fig. 15b. Anas et al. [149] have also prepared Al-4.4 wt.% Cu based composite powders reinforced with 2.2 vol.% CNT or Ni coated CNT using same milling and artificial ageing parameters used in [148], but, primary and secondary processes used were hot pressing at 500 °C at 350 MPa 46

and hot extrusion at 500 C with ER 16 (BM+HP+HE). It is reported that the TS of Ni coated CNT composites is increased to 638 MPa from 600 MPa for pure CNT based composites (464 MPa for Al alloy). However, the failure of the Ni coated CNT composite was drastically decreased to 1.5% from 7% for CNT composites (9.5% for Al alloy). This study show that the metal deposition on CNT enhances the TS of composites, but the failure strain is concern for Ni coated CNT based composites.

Fig.15: Tensile properties of CNT reinforced AA 2XXX matrix composites as a function of CNT content prepared by various processing routes. (In Situ CVD = Al powder with CNT grown on it in situ, BM = Ball Milling, HIP = Hot Isostatic Processing, CIP = Cold Isostatic Processing, FG = Forging, T4 = Natural ageing, T6 = Artificial ageing, SPS = Spark Plasma Sintering, HE = Hot Extrusion, CS = Conventional Sintering, VHP = Vacuum Hot Pressing, HR = Hot Rolling, FSP = Friction Stir Processing, Flake Al = Al flakes produced by milling).

The combination of ball milling process and hot rolling have provided highest tensile strength for Al-Cu as seen in Fig. 15a. Choi et al. [150] have prepare AA2024-CNT (up to 3 vol.%) composites using 18 h pre-milled AA2024 powders. The powders were milled with CNT (at 47

500 rpm for 6 h) and containerized in a Cu tube and compacted under pressure and subjected to hot rolling (at 450 ºC with 12 % reduction). The rolled samples were then solutionized at 530 ºC for 2 h followed by quenching and artificial aging at 180 °C (BM+HR+T6 route) and have obtained the highest TS (820 MPa) for 3 vol.% CNT-AA2024 composite (570 MPa for Al alloy) with failure strain of 3 % (8.5 % for Al alloy).

Fig. 16: Contribution of various mechanisms to Yield strength of the Al alloy-CNT composites in peak-aged T6 condition [148].

The contribution of strengthening mechanisms in improving the strength of the Al alloy-CNT composites has been explained by Anas et al. [148] as shown in Fig. 16. The YS of the Al alloy-CNT composites in aged condition can be written as:
() =
+
 +
 +
 +


(11)

Where,
 is the yield strength,
is the matrix strength of Al,
 =   / is the Hall/ /

Petch strength due to grain refinement,
 =  ε 48

is the strengthening by solid

solution,
 is the precipitate strengthening by fine precipitates, and
 is the strengthening due to the CNT in composites. In the above equations,  is the Hall-Petch coefficient which is 0.13 MPa√m, d is the grain size, M is the mean orientation factor or Taylor factor (0.36 for Al), G is the shear modulus (25.4 GPa for Al), b is the Burgers vector (0.286 nm), ε is the lattice strain due to solid solution formation, and c is the atomic fraction of solute atoms. It is observed from the Fig. 16 that the strengthening by matrix and solid solution effect remains unchanged with increase in CNT content in the composites. The grain size contribution is more in increasing the strength of composites with increase in CNT content. It is also seen from the Fig. 16 that the precipitation strengthening contribution is lower and CNT strengthening contribution increases with increase in CNT content. These concepts are true for all Al alloy-CNT composites, in general.

Fig. 17: TEM micrographs of precipitates for (a) Al-Cu alloy; Inset of fig (a) is high magnification of fig (a), (b) 2 vol. % CNT/Al-Cu, and (c) 4 vol. % CNT/Al-Cu composites prepared by SPS+T6 route [138].

It has been observed that the ageing kinetics are enhanced with increase in CNT content in Al alloy-CNT composites as seen in Fig. 17 [138]. In the case of the Al-Cu alloy (Fig. 17a), θ’’ precipitates (disk shape) were observed after 3 h aging and its volume fraction and size are increased with increasing aging time until 24 h. After 72 h aging, θ precipitates were observed and θ’’precipitates disappeared. However, in case of Al-Cu-2 vol.% CNT composites (Fig. 17b), volume fraction of θ’’ precipitates rapidly increased after 12 h aging and θ precipitates were observed after 24 h aging. This increased volume fraction and decreased inter-particle spacing of θ’’ precipitates increased the strength of Al-Cu-CNT composites according to Orowan looping mechanism in the early stages of aging. In case of 49

Al-Cu-4 vol.% CNT composites (Fig. 17c), a more accelerated precipitation behavior was observed and after 12 h aging, θ precipitates were observed. This behavior mainly results in a different hardening behavior for Al-Cu-CNT composites compared to Al-Cu alloy matrix. Choi et al. have also found that the peak age hardening time of AA2024-CNT composites was reduced to 4 h (20 h for AA2024) due to finer grain size and enhanced nano-precipitate formation due to the presence of CNT [150]. The large deformation induced by milling had a strong effect on aging behaviour. Meng at al. has found similar decrease in peak aging time for Al-Cu/CNT composites [134] and also showed increase in TS and reduction in ductility of CNT reinforced composites compared to Al-Cu alloy.

4.2. Al-Mg-Si alloys The 6xxx series Al alloys have Mg and Si as the principle alloying element and are heat treatable and they are easy to extrude and forge. These are most widely used in structural applications. Figure 18 shows the variation of the tensile properties with CNT content for AA6xxx alloys. Again the processes in the legend have been arranged in increasing order of properties from top to bottom. Du et al. have prepared AA6061-0.75 vol.% CNT composites by FSP for 3 passes [151] and have observed a reduction in the TS (from 193 MPa to 178 MPa) and failure strain (from 18 % to 10 %). Thus, FSP alone could not disperse the CNT in the matrix effectively even at lower CNT contents. Kondoh et al. have studied the effect of artificial aging on the properties of AA6063- CNT (up to 1.22 vol.%) composites [152-153]. The initial powders of Al-Mg-Si were mixed with CNT using a wet route and then SPS was carried out at 550 °C for 30 min under 30 MPa followed by hot extrusion at 350 °C (SPS+HE route). They have found that the enhancement in tensile properties was not significant [152] and TS was increased to 175 MPa for 0.56 vol.% CNT composites (165 MPa for Al alloy), while failure strain was 19 % (23 % for Al alloy). They have also found that T6 aging treatment (SPS+HE+T6 route) improved the TS of the alloy, but TS of the composite was lower than the alloy (170 MPa for 1.22 vol.% CNT vs. 210 MPa for Al alloy) with a failure strain of 16 % (18 % for aged Al alloy) [153]. This is due to poor dispersion technique used. In this study, CNT significantly reduced the age hardening tendency and the composite properties after aging was poor than alloys. This is due to segregation of Mg to CNT clusters resulting in depletion from matrix which reduced the formation of Mg2Si precipitate and hence the strength. 50

Liu et al. have studied the effect of FSP (for 3 passes) on the improvement of properties of AA6061-1.5 vol. % CNT composites prepared by (VHP+FG+FSP+T6) route [154]. In their study, Al and CNT powders are mixed using rotary mixer (at 60 rpm for 8 h), compacted, hot pressed (at 560 °C for 1 h) and then forged at 450 °C. The forged samples were processed by FSP followed by solutionizing treated at 530 °C for 1 h followed by water quenching and artificial aging at 160 °C for 12 h (T6 treatment). The TS of composite was increased from 323 MPa (for VHP+FG route) to 394 MPa (337 MPa for Al alloy) with a failure strain of 15 % after FSP and T6 treatment. Chen et al. [155] have prepared AA6061-2.2 vol.% CNT composites by pre-dispersion at 300 rpm for 30 min, shift-speed ball milling (at 135 rpm for 6 h and then at 270 rpm for 1 h), cold pressing (at 500 MPa), vacuum sintering (at 510 °C for 2 h) followed by hot extrusion at 420 °C with ER of 25 (BM+CS+HE route) and have found TS of 428 MPa (290 MPa for Al alloy) with a failure strain of 12.6 % (14.7 % for Al alloy). They have also studied the effect of pre-dispersion milling speed (1600, 2000, 2400 and 2800 rpm) on tensile properties of composites by using same SSBM and process parameters [156] and the maximum obtained TS of composite was 453 MPa (308 MPa for Al alloy) with a failure strain of 11.3% (17.6% for Al alloy) at 2000 rpm milling speed. Further, they have performed solutionizing at 530 °C for 1h, water quenched, and artificially aged (T6) at 175 °C for 6h [157] and obtained TS of 469 MPa with a failure strain of 8.9%. Further, the excess Mg addition in AA6061 enhanced the TS of T6 aged composite to 507 MPa (488 MPa for extruded composite) with a failure strain of 6.3% (5.1% for extruded composite). Haung et al. [158] have milled AA6061 and 2.2 vol.% CNT powders under Ar by SSBM at 135 rpm for 8 h, then at 270 rpm for 1 h. Then milled powders were compacted and then sintered at 550 °C for 2 h in Ar and extruded at 480 °C with ER of 25 and further rolled at 400 °C to 4 mm thick (BM+CS+HE+HR route). The obtained TS and failure strain of composites were 465 MPa and 10.4%, respectively. Thus, SSBM can be a good technique for obtaining high strength with reasonable ductility.

51

Fig.18: Tensile properties CNT reinforced Al 6XXX alloys matrix composites as a function of CNT content prepared by various processing methods (BM = Ball Milling, HIP = Hot Isostatic Processing, FG = Hot Forging, T6 = Artificial ageing, SPS = Spark Plasma Sintering, HE = Hot Extrusion, CS = Conventional Sintering, FSP = Friction Stir Processing)

Recently, Guo et al. prepared flake Al-10Mg-4.5Si powders by milling at 300 rpm for 10 h [159] and further milled them with functionalized CNT (0.5, 0.75 and 1 vol.%) for 5h at 300 rpm. Further, the milled powders are cold compacted under 400 MPa, sintered at 600 °C for 5 h and hot rolled at 450 °C for 6 passes with total reduction of 30% (BM+CS+HR route). They have obtained a maximum TS of 265 MPa for Al-1 vol.% CNT composites (175 MPa for Al alloy) with failure strain of 10% (16% for Al alloy). It is seen that in this study, the obtained tensile properties were lower compared to conventional AA6061-CNT composites [155]. This could be due to higher amount of Mg in the Al alloy. Najimi et al. [160] have used various ball milling techniques to disperse 2.2 vol.% CNT in AA 6061 such as (a) Planetary mill at 200 rpm for 24 h, (b) Horizontal attritor at 1000 rpm for 2 h, and (c) 52

Horizontal attritor in two steps; at 200 rpm for 2 h, then followed by milling at 1000 rpm for 1 h. Then, SPS was carried out at 550 °C under 35 MPa pressure followed by hot extrusion at 350 °C with ER of 12 (BM+SPS+HE route). They have found uniform CNT dispersion without CNT damage and highest TS after two stage milling. Figures 18a and 18b reveal that the TS of composites obtained by this process was found to be 580 MPa (273 MPa for Al alloy), highest increase in TS (112%), but failure strains were very low (< 4 %) as seen in Fig. 18c. Thus, milling parameters, type of process and process parameters plays a major in enhancing the tensile properties of composites. It is seen that the ball milling results in improved tensile properties. The composites prepared by (BM+SPS+HE) route had higher strength than (BM+CS+HE), (VHP+FG) and (VHP+FG+FSP+T6) processed composites. This is due to higher densification of composites and grain refinement of matrix.

4.3. Other alloys Agarwal and his group have carried out extensive studies on thermally sprayed Al-Si alloy coatings and bulk structures reinforced with CNT. HVOF [161-165], plasma spaying [30-31, 161, 163, 166-167] and cold spraying [30, 168] have been used for spraying Al alloy-CNT composites. Laha et al. have prepared free standing hypereutectic Al- 23 wt.% Si alloy coatings reinforced with 10 wt.% CNT by plasma spraying [161, 163, 166] and high velocity oxygen-fuel (HVOF) spraying [161-163]. The CNT were mixed with Al-Si powder by roll mixing. Laha et al. have prepared near net shape cylinders having diameter of 62 mm, length of 100 mm and wall thickness of 0.3 mm using plasma spraying [166]. Laha et al. [163] have sintered the PSF and HVOF composites and observed a small increase in the primary silicon size and decrease in size of porosity. Laha et al. showed that the TS of the 12 wt.% CNT composite was only 120 MPa (68 MPa for Al-Si coating) which could be due to the improper mixing achieved from roll mixing [169]. Bakshi et al. have prepared 5 mm thick coatings of Al-12%Si eutectic alloy reinforced with 5 and 10 wt.% CNT on 25.4 mm diameter steel pipe [31]. They have used spray drying method to prepare Al (1-3 µm size) -12 wt.% Si powders and showed good dispersion of CNT [30-31]. However, the strength of composites was found to be lower due to presence of CNT clusters acting like porosities. This was attributed to the poor wetting and lack of infiltration of CNT clusters with molten metal [30]. Elshalakany et al. have prepared A356-CNT (up to 2.5 wt.%) composite pellets by ball milling (8 h), compaction (at 70 MPa) and then pellets were introduced into the molten metal at semisolid temperature of 620 ºC and stirred for 1 min. The melt was poured into steel 53

mould heated to 250 ºC and squeeze cast (BM+Melt route) [170]. The TS of A356-2.25 vol.% CNT composite was found to be 243 MPa (162 MPa for A356 alloy) with failure strain of 6.9 % (1.7 % for A356 alloy). Hanizam et al. have prepared A356-0.75 vol.% CNT composites using stir casting and thixo-forming followed by artificial ageing [171-172]. In their studies, A356 was heated up to 700 °C for melting and the temperature was reduced to 650 °C to reduce the degradation of CNT. Then CNT were injected and mechanically stirred at 500 rpm. The mixture was poured into a pre-heated (150 °C) mould to get thixotropic billets. The billet was re-heated up to 580 °C which yielded 50% liquid and then rammed with forging load 5 tons and speed 1 m/s into pre-heated (100 °C) tool steel mould on top of coil and then the thixoformed samples were solutionized at 540 °C for 1h, water quench and artificial aged at 180 °C for 2 h [Melt+Thixoform+T6]. The TS and failure strain were found to be 180 MPa and 3%, respectively for as cast composite (135 MPa and 2.2% for as cast Al alloy), while they were 256 MPa and 6%, and 277 MPa and 7.2% for thixoformed and aged A356-CNT composites. The increase in TS from as cast to thixoformed composites was due to reduction in the porosity and precipitation of Mg2Si. Selective laser melting (SLM) has also been used for Al-CNT composites. Du et al. used magnetic stirring to mix the Al-10Si-Mg alloy and 0.75 vol.% CNT powders and composites (50 µm thick) were prepared using SLM [173]. In their study, the TS and failure strain obtained were lower for composites (287 MPa and 2%) compared to Al alloy (315 MPa and 10%). This could be due to CNT clusters and porosity due to poor dispersion technique used. However, Gu et al. [174] have utilized milling (at 200 rpm for 4h) to mix the Al-10Si-Mg and 0.75 vol.% CNT powders and studied the effect of scan speed (1.8, 2, 2.2 and 2.4 m/s) of SLM process (BM+Melt route). They have found that 2 m/s scan speed and 350 W power were the optimized parameters for better densification (99%) and they have obtained maximum TS of 425 MPa for composite (350 MPa for Al alloy) with a failure strain of 8.8% (5.5% for Al alloy). Wang et al. have prepared Al-10Si-1Mg-1.5 vol.% CNT composite by SLM process [175]. They have performed the milling of powders at 100 rpm for 10 h under Ar and carried out selective laser melting using Nd: YAG laser under Ar. They have obtained improved TS of 412 MPa (356 MPa for Al alloy) with a failure strain of 4.3 % (5.5 % for Al alloy). These studies on SLM process also show that appropriate dispersion method and optimization of SLM parameters are necessary to obtain good tensile properties of composites. Choi et al. have used 18 h pre-milled Al-5 wt.% Si powders and mixed 3 vol.% CNT by ball milling (at 500 rpm for 6 h), followed by hot rolling (BM+HR route) and have 54

showed improvement in TS (530 MPa) compared to Al-Si alloy (465 MPa) with more than 5 % ductility [176]. The pure Al-3 vol.% CNT composite had a TS of 483 MPa [121] indicating the strengthening effect of Si addition. Khan et al. have prepared AA5083-CNT (0.5 and 1 vol.%) composites by single pass FSP [177-178] and observed that tensile properties were increased with CNT content but the enhancement was very small. The TS of 0.5 and 1 vol.% CNT composites were 279 MPa and 294 MPa respectively (270 MPa for Al alloy) with failure strain of 4.8 % for 1 vol.% CNT-Al composite (12 % for Al alloy). This shows that single pass FSP is not good enough to disperse the CNT in composites. Stein et al. [179] have studied the effect of dispersion conditions on tensile behaviour of AA5083-2.2 vol.% CNT composites processed by ball milling (at 300 rpm for 30 min, 600 rpm for 3-7 h), HIP (at 350 °C under 100 MPa in Ar) and hot extrusion at 350 °C with ER of 16 (BM+HIP+HE route). It was observed that low energy ball milling resulted in reduced TS of composites (352 MPa) compared to Al alloy (376 MPa) with a failure strain of 11.3 % (14.5 % for Al alloy). However, TS and failure strain of composite was observed to be 427 MPa and 4.8 % in case of high energy milling conditions. Hence, it is seen that milling condition affects the properties significantly and HEBM is necessary to improve the CNT dispersion and tensile properties. They have again shown that the higher density steel balls are more effective for dispersing CNT [180]. The TS of composites was improved with CNT content and was found to be 674 MPa for Al-2.2 vol. % CNT composites (420 MPa for Al alloy), but the failure strain was very low (0.5 %) compared to Al alloy (12 %). Further, addition of CNT lead to decrease in TS to 492 MPa for Al-3 vol.% CNT composites with a failure strain of 0.4 %. Yu et al. have prepared AA50832.2 vol.% CNT composites using SSBM (at 135 rpm for 8h + 270 rpm for 1 h), cold pressing, sintering (at 510 °C for 2 h), hot extrusion at 450 C with ER of 8 and annealing at 500 °C under Ar for 0.5, 1, 2 and 3h [181]. They have obtained TS of 440 MPa with a failure strain of 5.4% for extruded composites. They have found that increasing the annealing time of extruded composites increasesd the TS up to 1 h and further annealing reduced the TS value. The maximum TS value was 455 MPa with failure strain of 5.9% for 1 h annealed composite. Jagan et al. have reported tensile properties of AA7075-CNT (0.5, 1, 1.5 and 2 wt.%) composites [182] prepared using powders mixed in a turbula mixer (for 1 h) followed by cold compaction (at 400 MPa), sintering (at 600 °C for 2 h) and hot extrusion at 450 °C with ER of 16 (CS+HE route). It was observed that tensile properties of composites were improved with increased CNT content up to 1.5 vol.% and further addition of CNT lead to CNT 55

clusters in the composites. The maximum TS was found to be 289 MPa (219 MPa for Al alloy) for 1.5 vol.% CNT addition with failure strain of 9.8 % (13.2 % for Al alloy). However, % increase in TS (32%) was lower. Thus, better dispersion technique and aging treatment is necessary for obtaining high strength of Al-CNT composites. Zhang et al. [183] have utilized stirring (for 1 h) and milling (at 200 rpm for 4 h) to mix the AA7075 and CNT powders and prepared AA7075-2.2 vol.% CNT composites using hot pressing at 540 °C for 1 h under 40 MPa and hot extrusion at 470 °C with ER of 16. Further hot extruded composites were solution treated (at 480 °C for 3 h), water quenched, and artificially aged (T6) at 120 °C for 10 h (BM+HP+HE+T6 route). They have found that the TS of composite is increased to 474 MPa (439 MPa for Al alloy) with a failure strain of 10.5% (12.7% for Al alloy). The T6 heat-treatment is enhanced the TS of composite to 558 MPa (519 MPa for Al alloy-T6) with a failure stain of 7.7% (11% for Al alloy-T6). Wei et al. have fabricated AA7075-2 vol.% CNT laminated and disordered composites by flake PM and hot extrusion [184]. The powders obtained from flake PM route [67] were cold compacted and then vacuum hot pressed at 600 °C under 10 MPa followed by hot extrusion at 440 °C with ER of 25. The extruded rods were subjected to T4 heat treatment to obtain laminated composites (FlakeAl+VHP+HE+T4 route). To obtain disordered composites, they have used 8 h ball milled powders and prepared the composite by same method. It was reported that compared to disordered composites (780 MPa and 3 %), the tensile properties were greater for laminated composites (820 MPa and 5 %). It is observed from above studies that (CS+HE) route of AA7075-CNT composites [182] resulted in lower TS while the composites processed by (Flake Al+VHP+HE+T4) had highest TS value [184]. The single pass FSP of AA5083-CNT composites [177-178] resulted in clusters of CNT and hence low TS values of composites were obtained. The AA5083-CNT composites prepared by (BM+HIP+HE) route resulted in higher TS values [179-180]. These studies indicate the uniform dispersion of CNT is necessary to enhance the tensile properties of Al alloy-CNT composites. The tensile properties of Al alloy-CNT composites are increased drastically with use of ball milling for CNT dispersion. It is also seen that Al-Si alloy-CNT composites prepare by additive manufacturing process had enhanced tensile properties [175] compared to conventional melting route [170]. However, Al-Si alloy-CNT composites processed by (BM+HR route) [176] had higher TS compared to melting route of composites. Thus, dispersion method, process type and process parameters have significant effect on tensile properties of composites. 56

5. Role of Interfacial phenomena It is well known that the stiffness and strength of composites are governed by load transfer efficiency from the matrix to reinforcement, while toughness is improved by phenomena like crack deflection at the interface and crack bridging, and the ductility is affected by stress relaxation phenomena near the interface [185-186]. In general, limited interfacial reactions are beneficial for the composite. For MMC prepared by casting route, the interfacial characteristics are governed by the wettability between the reinforcement and molten matrix. Good wettability between the reinforcement and the matrix is essential to avoid micro-scale cavity formation and to obtain good adherence at the interface, which will avoid delamination [187-188]. Good wetting is also important for good dispersion of CNT as well due to their large specific surface area. Several studies have shown that wetting characteristics can be altered by the formation of the reaction product at the interface [189-190]. The interfacial characteristics of CNT with molten Al can be altered primarily in two ways. Firstly, the composition of the matrix alloy can be changed [192-193]. Here, the segregation of an alloying element to the interface or its reaction with the reinforcement is used to modify the wetting behaviour. Ebbesen has reported that liquids having surface tension between 100 and 200 mN.m-1 show good wetting with CNT [194]. The surface tension of pure Al is about 1090 mN.m-1 which is quite high, due to which it does not wet the CNT [195]. Addition of Si to aluminium reduces the surface tension to about 880 mN.m-1 which helps in improving wetting [196]. Additionally, the reaction of Al and Si with CNT leading to formation of Al4C3 or SiC on the surface of CNT which reduces the contact angle significantly promoting wetting (known as reactive wetting). Bakshi et al. [30] have studied the effect of the Si content of Al-Si alloys on the interfacial reaction with CNT. Using the FactSageTM thermo-chemical software and database, formation of Al4C3 was shown to be thermodynamically feasible for Al-11.6 wt. % Si alloy while SiC formation was feasible for Al–23 wt. % Si alloy during reaction with CNT. The predictions are in line with results of Laha et al. [191]. Formation of both Al4C3 and SiC is good for the wettability. However, only the formation of SiC is desirable for good tensile properties. Al4C3 is undesirable since it is hygroscopic and reacts with moisture and can be highly detrimental for the ductility and long term stability of the composite. It has been shown that compacts have crumbled to powders with time due to reaction of Al4C3 and moisture in the air [197]. However, it is to be noted that in space applications, presence of Al4C3 may not be a concern 57

due to lack of atmosphere. Such problems can also be minimised by cladding with pure Al on composites. The second method of altering the interfacial characteristics involves modifying surface features of reinforcement. The surface of CNT can be modified either by metal coating or creation of defects on the surface of CNT. The surface energy of CNT (γ) has been reported to be 45.3 mJ.m-2, which is similar to carbon fibre [198]. Such a low surface energy is not good for wetting. Coating the surface of CNT can increase the wettability by decreasing the surface tension and also help in avoiding the formation of deleterious carbides. Niraj et al. have observed that formation of Al4C3 can be avoided by electroless Silver coating of CNT [199]. Interface in solid state processes is mainly determined by the extent of reaction between CNT and the matrix during processing. This can be controlled to some extent by controlling the process parameters. Choi et al. [36] have prepared Al-CNT composite from ball milled powders using hot extrusion at 470 ºC [BM+HE route] and no reaction was observed between Al and CNT. However, Deng et al. [200] have prepared AA2024-CNT composites by ball milling, SPS at 600 ºC followed by hot extrusion at 460 °C [BM+SPS+HE route] and show that all the CNT were converted to carbide. Deng et al. have found using DSC that the reaction between AA2024 and CNT starts as soon as the melting happens [201]. This explains the fact that most of the studies involving solid state processing such as extrusion have reported either very low or no formation of aluminium carbide. This is supported by results of Park et al., who have observed strong Al4C3 peaks in melted compact compared to sintered sample [129]. Their results indicate that addition of 0.3 wt. % CNT reduces the ductility drastically making the composite brittle which could be related to the formation of Al4C3. The temperature of processing has a strong effect on the formation of carbide, which is obvious due to the higher reactivity. With increased number of intermediate processing steps such as hot pressing or spark plasma sintering or use of higher temperatures, carbide formation cannot be avoided. It was seen earlier that low hot extrusion temperatures result in very high properties [78, 92-93] which could be due to limited Al4C3 formation. It has been shown by Ci et al. that the reaction between CNT and Al happens at the open ends of the CNT or on the damaged CNT surface [202]. Hence, the use of good quality CNT of high purity can reduce the reaction products with the matrix. The surface of CNT is modified during processing as well. During mechanical milling, CNT are broken down and more defects are created which can assist in the formation of aluminium carbide. This is supported by DSC studies on milled Al-CNT powders reported by Niraj et al. [203] and Poirier et al. 58

[204] which report solid state Al4C3 formation (indicated by exothermic peak) before melting. Choi et al. [121] have studied the effect of milling speed on interfacial characteristics of Al-4.5 vol.% CNT composite prepared by powder hot rolling at 480 ºC (BM+HR route). They have observed that the CNT were intact for 520 rpm milling speed while at 600 rpm milling speed, CNT damage was increased and Al4C3 formation was observed. They reported that tensile strength of the composites prepared from 520 rpm powders was slightly higher than the 600 rpm milled powder. The growth of reaction between Al and CNT cluster has been observed using in situ heating experiments inside TEM by Housaer et al. [205] and it was reported that aluminium carbide growth increased with reaction time. Zhou et al. [99] have reported in recently that the quantity and average length of Al4C3 increased with the increase in heat-treatment time as shown in Fig. 19a. It was also observed that the CNT cluster size decreases considerably as the Al4C3 grows. These observations prove that the reaction between Al and the CNT occurs below the melting temperature of Al (at 600 °C). The TS of the Al-CNT composite increased with increasing Al4C3 quantity up to 11 wt. % to 148 MPa (136 MPa without Al4C3 formation) with a failure strain of 24.7 % (25.5 % without Al4C3). This is due to enhanced interfacial shear strength and load transfer ability. Numerical modelling studies have also established beneficial effect of inter-phase, but studies have not been carried out to determine the desirable amount of Al4C3 formation. However, correlation of Al4C3 quantity and the strength of composites depend on several factors such processing parameters, quality of CNT used, measurement techniques to quantify the Al4C3 in composites. Numerical modelling studies have also established this. The interfacial shear strength of composites can be calculated using the following equation [43]:

τ=

"#$% &'

+

#(

(12)



Where,
) = Load transfer strength =
* -
;
* is strength of composite,
is the strength of matrix, S is aspect ratio = (l/D); l is length and D is diameter of CNT, VF is volume fraction of the CNT. The load transfer efficiency (δ)+ ) of CNT in composites can be calculated by using the following formula [43]: "#

τ  #(

$% δ)+ = "#(,+ .

%$(13)

(,- #(

59

/ Where,
) is maximum load transfer effect, when τ = τ / , which is equal to half of the

maximum tensile yield strength (
, / ) of the matrix. It is noticed from the Eq. 13 that the load transfer efficiency increases with increase in τ value. Figure 19b describes the dependence of interface strength at various SPS temperatures from 427-627 °C [43]. It is seen that composites have mechanical bonding at lower sintering temperatures (427-527 °C) and interaction between Al and CNT is low. The mechanical bonding contributed to small interface strength (~14 MPa) and δLTE (17-21%). At the temperature of 527 °C, the formation of Al4C3 started and the mechanical bonding is transformed to chemical bonding. At 577 °C, Al4C3 was formed in composites, resulting in improved interface strength of 22 MPa and it is increased to 54 MPa with the increased quantity of Al4C3 (at 577 °C) with estimated δLTE of 86%. At high temperature of 900 K, some of CNTs were completely transformed to Al4C3. Although the interfacial strength is high (60 MPa), the strengthening effect of composites was decreased compared with 577 °C sample. This phenomenon was related to structural difference of reinforcements. Hence, it can be concluded that the strengthening effect is increased with increased quantity of Al4C3 up to a certain limit.

Fig.19: (a) The quantity and average length of formed Al4C3 in the 0.6 vol. % CNT/Al composite as a function of HT time at 600 °C. The quantity of Al4C3 was defined as the ratio of the average length of formed Al4C3 and average length of the incorporated CNTs. The average length of each was measured from 45 HRTEM photographs [99] and (b) Dependence of interfacial strength on interfacial characteristics at various SPS temperatures. Red discontinuous line indicates the tendency [43].

60

6. Challenges in fabrication of Al-CNT composites There are several challenges associated with processing of CNT reinforced composites that have limited the development of useful products for structural applications. The major challenges in developing Al matrix composites reinforced with carbon nanotubes include: 1. Production of good quality, defect free, functionalized CNT for fabrication of composites 2. Fabrication of Al-CNT composites with > 6 vol. % CNT having uniform dispersion 3. Homogeneous dispersion while retaining the structural integrity of CNT 4. Reduced damage of CNT during processing at higher temperatures and pressures 5. Improving wettability of CNT with molten Al and its alloys for casting based techniques

It is proven that the CNT with minimum defects result in enhancement of properties as compared to CNT with high quantity of defects [119]. It is known that the quality of CNT produced by arc discharge method [206] is better compared to that of the one prepared by CVD techniques [207]. The perfect cylindrical structure of CNT is thermodynamically stable even at higher temperatures [202]. However, CNT synthesis by arc discharge method is expensive. For the preparation of better composites, the chemical stability of CNT is also a major concern in order to avoid interfacial reactions with the matrix at higher temperatures. Coated/functionalized CNT can have better interface characteristics and wettability than nonfunctionalized CNT and have been shown to result in better properties. However, cost is again a factor in use of coated CNT. There is no method to produce Al-CNT composites with good dispersion at high volume fractions (> 6 vol. %). Till date the most successful processing route for the preparation of Al-CNT composites is powder metallurgy route. Cast Al-CNT composites are difficult to produce as the CNTs are rejected from the melt and agglomerated due to their non-wettability with the molten alloy. Methods such as thermal spray techniques [31, 162] are promising for near net shape component fabrication, but aggressive reaction of the CNT with molten matrix is a concern due to higher process temperature. Attempts were made to produce Al-CNT composites by metal infiltration, but have not succeeded in getting good dispersion and tensile properties. It is possible to get good dispersion of CNT by FSP as evidenced by few studies [139-141]. However, maintaining a uniform composition of CNT throughout the composite is very difficult and CNT have been shown to break down during the process. The tensile strength of composites is affected by a variety of parameters such as the degree of dispersion, interfacial bonding and amount of porosity. It has been reported that silicon 61

carbide formation is beneficial as compared to aluminium carbide [191]. It is also mentioned that small amount of interfacial reaction between Al and CNT may be helpful in enhancing the load transfer but large amount will degrade the properties of composites [103]. However, the boundaries are not clear and more systematic in-depth studies are needed. Secondary processes that involve high temperature and stresses result in damage of the tubular structure of CNT which enhances the reaction of CNT with the matrix. The tendency of Al to oxidize has also been suggested as the cause for poor tensile properties of Al-CNT composites [208]. Recent studies using in situ tensile testing are providing better knowledge and these must be explored further [56, 129]. Proper selection of processing route and parameters for large scale production of composites is necessary.

7. Scope for future work The scope for future research on CNT reinforced Al matrix composites are as follows: • Methods to improve wettability and dispersion of CNT in molten aluminium needs to be studied to make use of large scale manufacturing possibilities by the melt route. • Few studies with coated CNT have shown promising results. Low cost metallization of CNT can be useful for the preparation of Al-CNT composites by casting, which is very difficult with uncoated CNT because of the difference in density between Al and CNT. • Bulk CNT structures such as Bucky papers are available and have not been used so far for Al-CNT composites. • Studies on corrosion and stress corrosion cracking needs to be carried out to understand durability of these composites. Also there is a need for evaluation of fatigue properties. • Recent research has shown that CNTs can be added as reinforcement for Al foams [209211]. The compressive strength and energy absorbed were higher for the CNT containing foams. There is a large scope for preparation of ultra-strong foams using CNT. • Comparison of the strengthening of composites due to CNT synthesized by different methods (arc discharge, chemical vapour deposition) need to be studied. • Multi-scale models to analyze the combined effect of interfacial reaction, CNT dispersion, CNT morphology on tensile properties of Al-CNT composites need to be developed. • High temperature deformation and effect of strain rate on properties needs to be studied.

62

8. Conclusions •

The increase in Young’s modulus is more for pure Al compared to Al alloy matrix. The rate of increase of stiffness reduces with increase in CNT content due to formation of CNT clusters and a decrease in the load transfer to individual CNT. Rule of mixtures provides good estimation of elastic modulus for pure Al matrix composites, while the combined Voigt-Reuss model holds well for Al alloy composites.

•

The increase in YS is more for Al alloy-CNT compared to pure Al-CNT composites. This could be attributed to the fact that most of the Al alloys used are precipitation hardened and CNT addition improved the precipitation hardening characteristics.

•

The strengthening efficiency is high for pure Al-CNT composites compared to Al alloyCNT composites due to low strength of pure Al prepared by the same processing techniques as compared to Al alloys. The strengthening efficiency drops down at higher CNT contents due to the formation of CNT clusters, which reduces the effective CNT content participating in the strengthening process. CNT clusters also decrease the strength by acting as porosities and crack nucleation sites. Improvement in dispersion improves the strengthening efficiency, but the highest values are obtained for CNT content less than 2 vol. %.

•

Failure strain reduction is significant in case of Al alloy matrices for CNT content above 3 vol. %, which is due to the difficulty in dispersing CNT in Al alloys.

•

Toughness is significantly decreased in case of Al-alloy CNT composites. In case of pure Al-CNT composites, it is seen that for lower CNT content (< 2 vol. %), toughness is increased. It is seen that for more than 3 vol. % CNT addition, toughness is reduced for both pure Al and Al alloys.

•

The TS/YS ratio decreases with increase in CNT content indicating increase in the brittleness. Pure Al-CNT composites seem to have higher TS/YS ratio compared to Al alloy-CNT composites.

•

Processing for Al-CNT powders through blending, stirring, ultrasonication, roll mixing and low energy ball milling do not lead to good tensile properties. High energy ball milling of Al and CNT powders leads to the enhancement in tensile properties of composites due to the grain refinement and better dispersion of CNT. Increase in the ball milling speed and time results in higher strength but lower failure strain. Use of in situ CVD powder with ball milling provides good tensile properties. Metallic coating of CNTs leads to enhancement in tensile properties. 63

•

Primary processes such as conventional sintering, SPS or friction stir processing alone do not result in good properties. Secondary processing methods such as hot extrusion and hot rolling enhance the tensile properties by increasing density, breaking down CNT clusters and distributing them, improving CNT matrix contact with stronger interfaces and strain hardening of composites.

•

Vacuum sintering of ball milled powders followed by hot extrusion provides good strength and failure strain. Vacuum sintering or hot isostatic pressing of ball milled powders followed by secondary processing methods resulted in good tensile properties over conventional sintering due to good densification and better CNT dispersion. Combination of Flake Al powders and ball milling provides highest TS with better failure strain compared to ball milled powders in case of samples prepared by vacuum hot pressing followed by hot extrusion. However, the strengthening efficiency values of Flake PM processed composites are lower than ball milled based processed composites due to the higher strength of Flake Al matrix.

•

Superior tensile properties are obtained in Al-CNT composites processed through spark plasma sintering as compared to conventional sintering due to short sintering time which inhibits grain growth. The tensile properties of Al-CNT composites increase with increased sintering temperature due to better densification of composites. Spark plasma sintering of ball milled powders followed by hot extrusion provides very good strength with good failure strain.

•

Hot extrusion results in better tensile properties compared to hot rolling. Lower extrusion temperature and higher extrusion ratio provides higher strength.

•

Hot rolling of ball milled powders resulted in highest TS, but the failure strain is very low. The strength is better compared to hot extrusion of ball milled powders, primarily due to very high degree of deformation.

•

Various studies indicate that a small amount of Al4C3 is good, while large amount is deleterious for the tensile properties.

•

Studies reveal that the ageing kinetics of Al alloy-CNT composites is enhanced with increase in CNT content.

Acknowledgments Jagannatham and Srinivasa Rao Bakshi acknowledges funding from IIT Madras under Institute Research and Development Award (Grant no. MET/16-17/839/RFIR/SRRB). 64

Prathap Chandran and Srinivasa Rao Bakshi would like to acknowledge funding from ISROIITM cell of IIT Madras under the grant no. ICSR/ISRO-IITM/MET/11-12/134/SRRB.

Data Availability Statement Data will be made available upon request.

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Tensile Properties of Carbon Nanotubes Reinforced Aluminum Matrix Composites: A Review M. Jagannathama, Prathap Chandrana, S. Sankarana, Prathap Haridossa, Niraj Nayanb and Srinivasa R. Bakshia* a

Department of Metallurgical and Materials Engineering

Indian Institute of Technology Madras, Chennai, India 600036 b

Materials and Mechanical Entity

Vikram Sarabhai Space Centre, Thiruvananthapuram, India 695022 *Corresponding Author Email: [email protected]

Declaration of Interest

It is hereby stated that there is no actual or potential conflict of interest including any financial, personal or other relationships with other people or organizations within three years of beginning the submitted work that could inappropriately influence, or be perceived to influence this work. All authors have contributed, read and approve the final manuscript.