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Elevated temperature compressive properties and energy absorption response of in-situ grown CNT-reinforced Al composite foams
Author’s Accepted Manuscript Elevated temperature compressive properties and energy absorption response of in-situ grown CNTreinforced Al composite foams Kunming Yang, Xudong Yang, Enzuo Liu, Chunsheng Shi, Liying Ma, Chunnian He, Qunying Li, Jiajun Li, Naiqin Zhao www.elsevier.com/locate/msea
PII: DOI: Reference:
S0921-5093(17)30294-0 http://dx.doi.org/10.1016/j.msea.2017.03.004 MSA34787
To appear in: Materials Science & Engineering A Received date: 4 January 2017 Revised date: 1 March 2017 Accepted date: 2 March 2017 Cite this article as: Kunming Yang, Xudong Yang, Enzuo Liu, Chunsheng Shi, Liying Ma, Chunnian He, Qunying Li, Jiajun Li and Naiqin Zhao, Elevated temperature compressive properties and energy absorption response of in-situ grown CNT-reinforced Al composite foams, Materials Science & Engineering A, http://dx.doi.org/10.1016/j.msea.2017.03.004 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. 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.
Elevated temperature compressive properties and energy absorption response of in-situ grown CNT-reinforced Al composite foams Kunming Yang 1 , Xudong Yang 2, * , Enzuo Liu 1, 3 , Chunsheng Shi 1 , Liying Ma 1 , Chunnian He 1, 3 , Qunying Li 1 , Jiajun Li 1 , Naiqin Zhao 1, 3 * 1
Tianjin Key Laboratory of Composite and Functional Materials, School of Materials
Science and Engineering, Tianjin University, Tianjin 300072, China 2
Sino-European Institute of Aviation Engineering, Civil Aviation University of China,
Tianjin 300300, China 3
Collaborative Innovation Center of Chemical Science and Engineering, Tianjin
University, Tianjin 300072, China [email protected] [email protected] *
Corresponding author to Yang XD and Zhao NQ, Tel: +86 27891371
Abstract Carbon nanotube (CNT) reinforced Al composite foams were successfully fabricated by the combination of an in-situ chemical vapor deposition (CVD), short-time ball-milling and space-holder method. The CNTs are homogeneously dispersed and embedded in the Al foam matrix after 90 min ball-milling while maintaining the structural integrity. Both compressive properties and energy absorption capacity of the composite foams increase with the increment of CNT content but decrease with the temperature rising between 25 and 250 oC. The compressive yield
strength and the plateau stress of 3.0 wt.%-CNT/Al composite foams maintain 16.8 and 20.2 MPa at 150 oC, respectively, which are much higher than the corresponding yield stress (5.7 MPa) and plateau stress (8.6 MPa) of the pure Al foam. Especially, the energy absorption capacity of the 3.0 wt.%-CNT/Al composite foams reaches 19.8 MJ/m3 at 150 oC, which is ~2.5 times higher than that of pure Al foam. Fracture analysis shows that the failure mode of the Al foam changes from ductile type to brittle type combined with ductile type, as a result of the CNT addition in the matrix. Keywords: Powder metallurgy; Metal-matrix composites; Carbon nanotubes; Porous materials; High-temperature properties Introduction Aluminum foam has been widely used as functional and structural engineering materials due to its large specific surface area, high specific strength, excellent energy absorption capacity, flame resistance and acoustic insulation [1-3]. In particular, compared with polymer foam materials, Al foam could be used under relative high temperature and stress condition such as the transpiration cooled rocket nozzles, the cooling system of steam turbines and the heat shielding for aircraft exhaust [4]. The huge potential of Al foam is the main motivation to further explore its high temperature properties. Unfortunately, few literatures can be available in this aspect. The compressive tests were performed at 25-620 oC by Aly. et al. [4] and they found that the mechanical properties of Al foam were dependent on the foam density, as well as the testing temperature. Hakamada et al. [5] investigated the compressive properties of Al
foam at 300-500 oC, and concluded that the stress exponent of the Al foam and activation energy for the deformation at elevated temperatures were in agreement with those in Al alloy. Sahu et al. [6] studied the compressive deformation behaviors of ZA27 foam at 100-250 oC, and the result showed that the plateau stress and energy absorption capacity of the ZA27 foam increased with an increase of relative density but decreased with elevated temperatures. It can be seen that the properties of Al foam are not only determined by its own characteristics (e.g. density), but also seriously affected by the ambient temperature. Thus, it is quite necessary to systematically investigate the compressive properties of the Al foam at elevated temperatures. Owing to low density, excellent mechanical, physical, chemical and electronic properties [7-9], carbon nanotubes (CNTs) have been regarded as the most attractive reinforcement to fabricate light weight and high strength metal-matrix composites. However, because of the high tendency of CNTs to form clusters, poor wettability between CNTs and metal, it is challenging to fabricate the CNT-reinforced metal matrix composites with prominent properties. Previous efforts have focused on the fabrication and properties of CNT-reinforced Al matrix dense composites [10-12]. High energy ball milling (HEM) technique [13, 14] and the pre-treatment of CNTs [15] route are often used to solve the above-mentioned problems. Whereas, the properties of CNT-reinforced Al composite foams have rarely been reported up to date [16, 17], and the field of which is still in the infancy. The major hurdle is how to foam molding while maintaining the well dispersion of CNT reinforcement. Duarte et al. [16] reported an
approach of combining colloidal-processing and powder metallurgy to fabricate CNT/Al composite foams, but did not show the mechanical properties of the foams. Zhang et al. [17] developed a closed-cell Al foam reinforced with different contents of CNT by using a modified melt foaming method. Whereas, the strength of the composite foams decreased as the MWCNTs content exceeded only 0.5 vol.%, owing to the clustering of the CNTs. Our group has firstly reported that in-situ chemical vapor deposition (CVD) method is a highly effective approach to make the CNTs uniformly dispersed in Al powders [18-20]. Recently, the CNT/Al composite foams with 2.0 wt.% CNT content were successfully fabricated by a combination of CVD, a short time ball milling and the space holder method, with carbamide particles as the space-holder materials [21]. The CNT/Al composite foams exhibited an obvious improved compressive property at room temperature, which makes it possible to further evaluate mechanical properties and the energy absorption capacity of CNT/Al composite foams under elevated temperatures. In the present study, the compressive properties of in-situ grown CNT-reinforced Al composite foams with different CNT contents, were investigated at ambient temperature between 25 and 250 oC. The strengthening mechanism of the composite foams under high temperature environment has also been discussed. 1. Experimental works 1.1 Raw materials and fabrication of CNT/Al composite foams Pure Al powders with an average particle size of 70 μm (99% purity, Tianjin
Weichen Co., Ltd ) and spherical carbamide particles with an average diameter of 1.5 mm (99% purity, Tianjin Fengchuan Chemical Reagent Technology Co., Ltd ) were applied as the matrix and space-holder materials, respectively (as shown in Fig. 1a-b). The impregnation method [19] was employed to produce Co/Al catalyst containing 0.5 wt.% Co. By introducing a mixture flow of C2H2/Ar (20/240 mL/min) into the tubular furnace, CNTs were synthesized at 600 oC for 0-60 min using Co/Al catalyst. Finally, the system was cooled down to room temperature under Ar atmosphere and the CNT/Al composite powders were obtained. The as-obtained CNT/Al composite powders with different CNT contents were transferred into the 250 mL stainless steel jars containing 6 mm stainless steel balls. The mixing jars were filled with Ar and then were agitated in the planetary ball mill (QM-3SP4, Nanjing Nanda Instrument Plant, China) at 400 rpm for 90 min. Here, the ball to powder weight ratio was 10:1. The ball-milled CNT/Al composite powders and spherical carbamide particles were cold-compacted in a cylinder die under the pressure of 500 MPa. Then, the as-pressed green compact was immersed in a 200 mL beaker containing 80 oC hot water for 6 h. Finally, the CNT/Al composite foams with a porosity (P) of 60% were achieved through a sintering process at 650 oC for 3 h. In this investigation, the P is calculated by: 𝑃 = (1 −
𝜌∗ 𝜌𝑠
) ∗ 100%
where ρ* is the density of Al foam and ρs is the density of the pore wall, respectively. 1.2 Characterization
(1)
Field emission scanning electron microscope (SEM, Hitachi S4800) and high-resolution transmission electron microscope (TEM, Philips Tecnai G2 F20, 200kV) were employed to characterize the sizes, distributions and structural integrity of CNTs in the matrix. Raman spectroscopy of the CNT/Al composite powders was performed by using the 532 nm line of Ar+ laser as the excitation source to validate the quality of CNTs. Compressive specimens were prepared with a diameter of 20 mm and a height of 20 mm, which were held at each measure temperature for 30 min and then tested at temperatures between 25 and 250 oC under a crosshead speed of 1mm/min on an Instron 5982 tester. To ensure the reproducibility and reliability of the results, three samples were tested under the same condition. 2. Results and discussion 2.1 Microstructure Fig. 1c shows that the pores of CNT/Al composite foams with an average size of 1.5 mm are uniformly distributed in the composite foams, which well replicate the shape of original spherical carbamide particles. The average pore wall thickness of the composite foams is about 200 μm, and the pore wall also shows a smooth surface (seen in Fig. 1d). Actually, compared with other space-holder materials such as NaCl [22], the spherical-shaped pores can help reduce the surface roughness with less edges and corners, which benefits to lessen the local stress concentrations and inhomogeneous deformation during compression, and thus, improves the strength of CNT/Al composite
foams. Besides, the pore size and porosity as well as the morphology of the composite foams could be easily tailored by the spacer method, which has been confirmed by our previous works [21, 23].
Fig. 1. (a) SEM image of pure Al powders. Photographs of (b) original carbamide particles and (c) 3.0 wt.%-CNT/Al composite foams. (d) is the magnified image of (c). Fig. 2 shows the SEM images of CNT distributions in CNT/Al composite powders obtained by CVD. With the reaction time increasing, the surface of Al particles is gradually homogeneously covered by CNTs of several microns. When the reaction time increases to 60 min, the CNT content reaches 3.0 wt.%, and the Al matrix is fully covered by CNTs (seen in Fig. 2c). Thus, the CNT content could be well controlled by adjusting the reaction time. Even though the in-situ CVD method alleviates the
agglomeration of CNTs to a certain extent, some CNTs still present an entangled morphology (Fig. 2d-f) due to the large aspect ratio of CNTs and the large van der Waals force among the CNTs. Moreover, the high percentage of CNTs distributed on the Al surface will greatly hinder the effective combination of Al particles during the forming process of composite foams, finally becoming the failure initiation sources and weakening the strength of CNT/Al composite foams. Herein, the short-time ball-milling technique is introduced to further improve the CNT dispersion and the interfacial bonding between CNTs and Al matrix.
Fig. 2. SEM images of (a,d) 2.0 wt.%, (b,e) 2.5 wt.% and (c, f) 3.0 wt.% CNT reinforced Al composite powders synthesized by 15, 30 and 60 min. (d), (e) and (f) are the magnification of (a), (b) and (c), respectively. SEM images in Fig. 3 display that the average size of 3.0 wt.%-CNT/Al composite powders is about 120 μm after ball milling of 90 min, a little larger than that of the original Al powders (~70 μm, see Fig. 1a) because of the repetitive process of fracturing
and welding [24]. Meanwhile, the separately dispersed CNTs are deeply embeded inside the Al matrix instead of covering on the surface of Al particles (Fig. 3b). In practice, the final distribution of CNTs in the composite foams depends on its initial distribution in the original CNT/Al composite powders, because the dispersion of CNTs in the Al matrix can not be affected and improved during the next process, including the cold compaction and sintering [25]. Therefore, the as-ball-milled CNT/Al composite powders can be very effective to ensure the good dispersion of CNTs in the finally formed composite foams.
Fig. 3. (a) SEM image of 3.0 wt.%-CNT/Al composite powders after 90 min ball milling. (b) is the magnified image of (a). Fig. 4 shows the typical TEM images of CNTs. The synthesized CNTs with multi-walled hollow tubular structures and outer diameters of 10-20 nm present relatively clean and smooth wall surfaces (Fig. 4a). The apparent graphitic sheets of CNTs with 0.34 nm of the interlayer spacing are observed in Fig. 4b, which is consistent with the ideal graphitic interlayer space (0.34 nm). This suggests that the purity and quality of CNTs is sufficient to act as an excellent reinforcement in CNT/Al composite
foams without an additional treatment. After ball milling of 90 min, the CNTs are implanted into the Al matrix with mild structure damage (Fig. 4c-d), forming a strong interfacial bonding between CNTs and Al matrix. Fig. 4e-f shows the TEM images of composite foam wall after compressive test at room temperature. Although the CNTs are still embedded in Al matrix, it merits the attention that some defects, such as the broken carbon layers (pointed by the red arrow), occur around the interface between CNTs and Al matrix, indicating that the structure integrity of CNTs is injured during the compressive tests. It also confirms that CNTs are subjected to the heavy loads by the means of interfacial load transfer.
Fig. 4. TEM images of (a,b) raw CNTs, 3.0 wt.%-CNT/Al composite powders, (c, d) after 90 min ball milling, and (e, f) after being compacted at 25 oC. (b), (d) and (f) are the magnification of (a), (c) and (e), respectively. To further assess the quality variations of CNTs during the experiment, Raman spectrum measurements are carried out. The relative density between the two peaks (G-line and D-line) is known to provide information about the quality of internal CNTs [18]. The relative intensity ratio of D to G peak (ID/IG) (Fig. 5) increases from 0.74 (raw CNTs) to 0.89 (after ball milling), indicating that there is negligible damage to the CNTs
graphite structure during the 90 min of ball milling. Nevertheless, the ID/IG value of CNTs after being compacted reaches 1.02, demonstrating that the structure of CNTs is injured, which is in agreement with the observations in Fig. 4e-f. But overall, the feasibility of keeping balance between uniform dispersion and structural integrity of CNTs, makes the in-situ CVD method followed by a short time ball milling technique as a promising route to realize the potential of CNTs as the reinforcement in Al composite foams.
Fig. 5. Raman spectrum of the CNT/Al composite foams. 2.2 Compressive property Fig. 6 shows the compressive stress-strain curves of pure Al foam and CNT/Al composite foams at different ambient temperatures ranging from 25 to 250 oC. The specific parameters of compressive properties are also summarized in Fig. 7. The stress-strain curves at both room temperature and elevated temperatures all exhibit three distinct regions, just as described in previous literatures [4-6]: a linear elastic region in the initial stage of deformation, then a plateau region with nearly constant stress over a
wide range of strain, and finally a densification region where the stress increases rapidly with the strain increasing. Generally, two types of metallic foams can be distinguished from the compressive stress-strain curves [26]. One is ductile metallic foams whose curve is characterized by smooth and steadily rising stress. The failure is mainly controlled by the buckling of the pore walls. On the contrary, another is brittle foams which exbihit severe fluctuations during the plateau region. The failure of these foams is mainly dominated by fracture and collapse of the pore walls. According to Fig. 6a, the stress-strain curves of pure Al foam show an extremely smooth tendency, because of the strong plastic deformation ability of pure Al matrix. However, Fig. 6b-d shows that the stress-strain curves of CNT/Al composite foams display some slight fluctuations during the plateau region. Nevertheness, by comparison with other ceramic phases reinforced Al composite foams [5, 27], the fluctuations in our present work are comparatively small. This is mainly ascribed to the high intrinsic toughness of CNTs.
Fig. 6. Compressive stress-strain curves of (a) pure Al foam, (b) 2.0 wt.%, (c) 2.5 wt.% and (d) 3.0 wt.% CNT reinforced Al composite foams at 25-250 oC. Fig. 7 shows the significant parameters of the metallic foams including the compressive yield stress, plateau stress and densification strain of the as-fabricated Al foams at different temperatures. In general, the yield stress is regarded as the first peak stress of metallic foams, and the average stress in the plateau region is the plateau stress. As Ramamury [28] suggested, the densification strain is the intersection of tangents to stress-strain curve for the plateau region and densification region. It is obvious that both the yield stress and plateau stress decrease along with the temperature elevation, showing an significant temperature softening effect. But the densification strains of both pure Al foam and CNT/Al composite foams are almost invariant to the temperture
( shown in Fig. 7c). This may be determined by the nature of metallic foams themselves. Among temperatures of 25-250 oC, the properties of 3.0 wt.%-CNT/Al composite foams are always the most pronounced. At 25 oC, the yield stress and plateau stress of 3.0 wt.%-CNT/Al composite foams are 22.8 and 33.1 MPa, which are ~1.6 and ~0.8 times higher than that of the pure Al foam, respectively. Though the stresses of CNT/Al composite foams decrease with the increase of temperature, the improvement is still evident compared to the pure Al foam. Especially at 150 oC, the yield stress and plateau stress of 3.0 wt.%-CNT/Al composite foams are 16.8 and 20.2 MPa, respectively, which are much higher than the corresponding yield stress (5.7 MPa) and plateau stress (8.6 MPa) of the pure Al foam.
Fig. 7. Graphs of (a) yield stress, (b) plateau stress and (c) densification strain varying with the temperature and the content of CNT.
2.3 Energy absorption capacity of the as-prepared Al foams One of the most prominent features of metallic foams is that they can absorb energy because of the bending, yielding and collapse of pore walls as well as the friction between pore walls when they contact with each other during the compression [1]. The energy absorption capacity (W) of metallic foams is usually characterized by: 𝜀
𝑊 = ∫0 𝜎𝑑𝜀
(2)
where W is defined as the energy absorbed per unit volume, ε is the compressive strain and σ is the corresponding compressive stress. Here, the maximum value of ε is set as the densification strain. The energy absorption capacity (W) evolutions of Al foam with different CNT contents at 25, 150 and 250 oC are shown in Fig. 8a-c, respectively. It can be seen that increasing the testing temperature can lead to a decreased W level, for both pure Al foam and composite foams. However, the value of W increases with the increae of CNT content. Meanwhile, the W values of CNT/Al composite foams are dramatically higher that of the pure Al foam at temperature 25-250 oC, which is due to higher yield stress and plateau stress of the composite foams. When it is compressed to the densification strain, according to Eq. (1), the values of W for the 3.0 wt.%-CNT/Al composite foams at 150 and 250 oC are calculated to be 19.8 and 11.6 MJ/m3, respectively. Whereas, for the pure Al foam, the values of W at 150 and 250 oC decrease to be only 5.6 and 3.9 MJ/m3.
Fig. 8. (a), (b) and (c) are the diagrams of energy absorption capacity (W) at 25, 150 and 250 oC, respectively. For the engineering applications, the Al foam as an energy-absorbing material has a promising prospect. Considering the different preparation processes of the Al composite foams, a dimensionless quantity named enhanced coefficient (η) is introduced to quantitatively evaluate the reinforcement effects on the energy absorption capacity of the composite foams. The expression of η is given as follows: 𝜂=
𝑊1−𝑊2 𝑊2
∗ 100%
(3)
where η is the enhanced coefficient, W1 and W2 are the energy absorption capacities of the composite foams and matrix, respectively. For comparison, the values of η for Al composite foams in other literatures are calculated using Eq. (2) and the results are
listed in Table 1. The intuitive statistical data suggests that the contribution of CNTs to the enhancement of energy absorption capacity in this work is higher than most of the referenced reinforcements [29-33]. Especially when the temperature increases 150 oC, the value of η even reaches 253.6%. This demonstrates that the improved powder metallurgy approach we used can bring a significant improvement in energy absorption of the Al composite foams, attributing to the uniform dispersion of reinforcement in the matrix. Table 1 Enhanced coefficient of Al composite foams fabricated by different methods Composite
foams
(Reinforcement/Mat
Preparation
Reinforcement
processing
content
Porosity (%)
Pore (mm)
size
Testing
Enhanced
temperature
coefficient (η)
o
rix)
( C)
CNT/Al [this work]
In-situ
CVD
+
3.0 wt.%
~60
~1.5
25
117.3%
150
253.6%
250
197.4%
space holder
CNT/Al [17]
Melt foaming
0.5 vol.%
~85.0
~2.4
25
73.3%
SiCp/Al [29]
Powder metallurgy
1.0 vol.%
~60.1
~0.5
25
69.4%
Melt foaming
2.5 vol.%
~73.5
2.0~4.0
25
142.9%
In-situ synthesis +
10.0 wt.%
~60.0
~0.5
25
198.2%
+ foaming Ceramic microsphere/Al [30] MgAl2O4/Al [31]
space holder Al2O3/Al [32]
Melt foaming
15.0 vol.%
~77.0
~3.0
25
100.0%
SiC/Al-Si [33]
Stir casting
18.0 wt.%
12.2~26.3
~2.9
25
181.3%
12.5~26.7
~2.9
200
160.0%
10.7~26.9
~2.9
400
25.4%
2.4 Fractograph and strengthening mechanism Fig. 9a-c is the fractographs of pure Al foam at 25 oC, showing that there are many dimples on the fracture surface, which present a typical ductile failure mode. Moreover,
when the temperature is elevated to 250 oC, lots of tearing ridges instead of the dimples occurr on the fracture surface (Fig. 9d-f), which implies that the Al matrix is markedly softened under the high temperature environment. The result is consistent with the experimental result observed by Aly et al. [4], which also verified the softening effect of Al matrix at high temperatures. On the other hand, Fig. 10a and b show that the fracture of 3.0 wt.%-CNT/Al composite foams is not completely caved and partly keeps the original morphology of the pore at 25 oC. Compared with the fractograph of pure Al foam, the dimple number of composite foams is obviously reduced and the depth becomes shallow. Besides, some notches also are observed. It should be noted that, when the testing temperature increases to 250 oC, the composite foams are almost pressed into a pie shape, as indicated in Fig. 10e. Some cleavage facets as well as scratches with varying depths are exposed to the fracture surface, showing an intergranular fracture characteristic. In addition, CNTs are not pulled out in the fracture surface by observation (Fig. 10c-d and Fig. 10g-h), manifesting the strong interfacial bonding between CNTs and Al matrix. Combined with the above observation, it can be inferred that the failure characteristics of the Al foam change from a ductile type to the combination of brittle and ductile type, resulting from the CNT addition. It is generally accepted that the CNTs are thermodynamically stable and do not react with Al matrix even at high temperatures, as long as they keep the perfect cylindrical structure which consists of several rolled up graphitic basal planes [34]. Similarly, no interfacial reactant (e.g. Al4C3) is formed in the
observation (Fig. 4), which also has been confirmed by our previous study [35]. Thereby, the brittle behavior of the CNT/Al composite foams may mainly be caused by the incompatible deformation between CNTs and the Al matrix under the condition of compression.
Fig. 9. Fractographs of pure Al foam at (a-c) 25 and (d-f) 250 oC.
Fig. 10. Fractographs of 3.0 wt.%-CNT/Al composite foams at (a-d) 25 oC and (e-h) 250 oC. The strengthening mechansim of CNT/Al composite foams in present work can be
summarized to several factors. Firstly, like the case of CNT-reinforced Al matrix composites [36, 37], the fiber strengthening is the dominating mechanism. Due to the well interfacial bonding between CNTs and the Al matrix, an applied force can be effectively transmitted from the matrix to the CNTs by the means of shear stresses. Liu et al. [38] studied the tensile properties of CNT/2009Al composites at temperatures of 20-300 oC and found that the load transfer mechanism play an important role at temperatures elevated up to 300
o
C, thus the yield strength of the 1.5
vol.%-CNT/2009Al composites at 150-300 oC, was improved compared with the 2009Al matrix. Secondly, a three-dimensional network of uniformly distributed nano-reinforcement (CNTs) are formed in the matrix of composite foams, causing a dispersion strengthening effect [10]. Furthermore, when the temperature is elevated from 25 to 250 oC, the soften of Al foam matrix can be effectively impeded by CNTs, because of the extremely low thermal expansion coefficient (CET ≈ 0) [12, 20, 38] and the excellent high temperature mechanical properties of CNTs [38]. Certainly, the above mentioned strengthening mechanisms do not exist in isolation and the intercoupling of these strengthening effects finally improve the elevated temperature compressive properties and the energy absorption capacity of CNT/Al composite foams. 3. Conclusions In this investigation, 60% porosity of CNT/Al composite foams with an average pore size of 1.5 mm were successfully fabricated. The in-situ grown CNTs with 10-20 nm diameters and several microns length are homogeneously dispersed and embedded
in the Al foam matrix after ball-milling of 90 min. The composite foams with uniformly distributed pores replicate the size and shape of carbamide particles. Both the compressive properties and energy absorption capacity decrease with the temperature rising, but increase with the increment of CNT content. The failure mode of the Al foam changes from ductile type to brittle type combined with ductile type, as a result of the CNT addition in the matrix. The interfacial load transfer and the excellent high temperature mechanical properties of CNTs are supposed to be the strengthening mechanisms for CNT/Al composite foams. In addition, the high temperature performance of the CNT/Al composite foams is expected to be further improved with the optimization of CNT content and composite microstructure. Such research is still in progress. Acknowledgements This work is supported by the Key Program of National Natural Science Foundation of China (No. 51531004), the National Natural Science Foundation of China for Young Scholars (No. 51301198). References [1] L.J. Gibson, M.F. Ashby, Cellular Solids: structure and properties. Cambridge University Press: Cambridge, UK, 1997. [2] M.F. Ashby, T. Evans, N.A. Fleck, L.J. Gibson, J.W. Hutchinson, H.N.G. Wadley, Metal Foams: A Design Guide., Butterworth Heinemann: Elsevier, 2000. [3] J. Banhart, Manufacture, characterization and application of cellular metals and
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PII: DOI: Reference:
S0921-5093(17)30294-0 http://dx.doi.org/10.1016/j.msea.2017.03.004 MSA34787
To appear in: Materials Science & Engineering A Received date: 4 January 2017 Revised date: 1 March 2017 Accepted date: 2 March 2017 Cite this article as: Kunming Yang, Xudong Yang, Enzuo Liu, Chunsheng Shi, Liying Ma, Chunnian He, Qunying Li, Jiajun Li and Naiqin Zhao, Elevated temperature compressive properties and energy absorption response of in-situ grown CNT-reinforced Al composite foams, Materials Science & Engineering A, http://dx.doi.org/10.1016/j.msea.2017.03.004 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. 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.
Elevated temperature compressive properties and energy absorption response of in-situ grown CNT-reinforced Al composite foams Kunming Yang 1 , Xudong Yang 2, * , Enzuo Liu 1, 3 , Chunsheng Shi 1 , Liying Ma 1 , Chunnian He 1, 3 , Qunying Li 1 , Jiajun Li 1 , Naiqin Zhao 1, 3 * 1
Tianjin Key Laboratory of Composite and Functional Materials, School of Materials
Science and Engineering, Tianjin University, Tianjin 300072, China 2
Sino-European Institute of Aviation Engineering, Civil Aviation University of China,
Tianjin 300300, China 3
Collaborative Innovation Center of Chemical Science and Engineering, Tianjin
University, Tianjin 300072, China [email protected] [email protected] *
Corresponding author to Yang XD and Zhao NQ, Tel: +86 27891371
Abstract Carbon nanotube (CNT) reinforced Al composite foams were successfully fabricated by the combination of an in-situ chemical vapor deposition (CVD), short-time ball-milling and space-holder method. The CNTs are homogeneously dispersed and embedded in the Al foam matrix after 90 min ball-milling while maintaining the structural integrity. Both compressive properties and energy absorption capacity of the composite foams increase with the increment of CNT content but decrease with the temperature rising between 25 and 250 oC. The compressive yield
strength and the plateau stress of 3.0 wt.%-CNT/Al composite foams maintain 16.8 and 20.2 MPa at 150 oC, respectively, which are much higher than the corresponding yield stress (5.7 MPa) and plateau stress (8.6 MPa) of the pure Al foam. Especially, the energy absorption capacity of the 3.0 wt.%-CNT/Al composite foams reaches 19.8 MJ/m3 at 150 oC, which is ~2.5 times higher than that of pure Al foam. Fracture analysis shows that the failure mode of the Al foam changes from ductile type to brittle type combined with ductile type, as a result of the CNT addition in the matrix. Keywords: Powder metallurgy; Metal-matrix composites; Carbon nanotubes; Porous materials; High-temperature properties Introduction Aluminum foam has been widely used as functional and structural engineering materials due to its large specific surface area, high specific strength, excellent energy absorption capacity, flame resistance and acoustic insulation [1-3]. In particular, compared with polymer foam materials, Al foam could be used under relative high temperature and stress condition such as the transpiration cooled rocket nozzles, the cooling system of steam turbines and the heat shielding for aircraft exhaust [4]. The huge potential of Al foam is the main motivation to further explore its high temperature properties. Unfortunately, few literatures can be available in this aspect. The compressive tests were performed at 25-620 oC by Aly. et al. [4] and they found that the mechanical properties of Al foam were dependent on the foam density, as well as the testing temperature. Hakamada et al. [5] investigated the compressive properties of Al
foam at 300-500 oC, and concluded that the stress exponent of the Al foam and activation energy for the deformation at elevated temperatures were in agreement with those in Al alloy. Sahu et al. [6] studied the compressive deformation behaviors of ZA27 foam at 100-250 oC, and the result showed that the plateau stress and energy absorption capacity of the ZA27 foam increased with an increase of relative density but decreased with elevated temperatures. It can be seen that the properties of Al foam are not only determined by its own characteristics (e.g. density), but also seriously affected by the ambient temperature. Thus, it is quite necessary to systematically investigate the compressive properties of the Al foam at elevated temperatures. Owing to low density, excellent mechanical, physical, chemical and electronic properties [7-9], carbon nanotubes (CNTs) have been regarded as the most attractive reinforcement to fabricate light weight and high strength metal-matrix composites. However, because of the high tendency of CNTs to form clusters, poor wettability between CNTs and metal, it is challenging to fabricate the CNT-reinforced metal matrix composites with prominent properties. Previous efforts have focused on the fabrication and properties of CNT-reinforced Al matrix dense composites [10-12]. High energy ball milling (HEM) technique [13, 14] and the pre-treatment of CNTs [15] route are often used to solve the above-mentioned problems. Whereas, the properties of CNT-reinforced Al composite foams have rarely been reported up to date [16, 17], and the field of which is still in the infancy. The major hurdle is how to foam molding while maintaining the well dispersion of CNT reinforcement. Duarte et al. [16] reported an
approach of combining colloidal-processing and powder metallurgy to fabricate CNT/Al composite foams, but did not show the mechanical properties of the foams. Zhang et al. [17] developed a closed-cell Al foam reinforced with different contents of CNT by using a modified melt foaming method. Whereas, the strength of the composite foams decreased as the MWCNTs content exceeded only 0.5 vol.%, owing to the clustering of the CNTs. Our group has firstly reported that in-situ chemical vapor deposition (CVD) method is a highly effective approach to make the CNTs uniformly dispersed in Al powders [18-20]. Recently, the CNT/Al composite foams with 2.0 wt.% CNT content were successfully fabricated by a combination of CVD, a short time ball milling and the space holder method, with carbamide particles as the space-holder materials [21]. The CNT/Al composite foams exhibited an obvious improved compressive property at room temperature, which makes it possible to further evaluate mechanical properties and the energy absorption capacity of CNT/Al composite foams under elevated temperatures. In the present study, the compressive properties of in-situ grown CNT-reinforced Al composite foams with different CNT contents, were investigated at ambient temperature between 25 and 250 oC. The strengthening mechanism of the composite foams under high temperature environment has also been discussed. 1. Experimental works 1.1 Raw materials and fabrication of CNT/Al composite foams Pure Al powders with an average particle size of 70 μm (99% purity, Tianjin
Weichen Co., Ltd ) and spherical carbamide particles with an average diameter of 1.5 mm (99% purity, Tianjin Fengchuan Chemical Reagent Technology Co., Ltd ) were applied as the matrix and space-holder materials, respectively (as shown in Fig. 1a-b). The impregnation method [19] was employed to produce Co/Al catalyst containing 0.5 wt.% Co. By introducing a mixture flow of C2H2/Ar (20/240 mL/min) into the tubular furnace, CNTs were synthesized at 600 oC for 0-60 min using Co/Al catalyst. Finally, the system was cooled down to room temperature under Ar atmosphere and the CNT/Al composite powders were obtained. The as-obtained CNT/Al composite powders with different CNT contents were transferred into the 250 mL stainless steel jars containing 6 mm stainless steel balls. The mixing jars were filled with Ar and then were agitated in the planetary ball mill (QM-3SP4, Nanjing Nanda Instrument Plant, China) at 400 rpm for 90 min. Here, the ball to powder weight ratio was 10:1. The ball-milled CNT/Al composite powders and spherical carbamide particles were cold-compacted in a cylinder die under the pressure of 500 MPa. Then, the as-pressed green compact was immersed in a 200 mL beaker containing 80 oC hot water for 6 h. Finally, the CNT/Al composite foams with a porosity (P) of 60% were achieved through a sintering process at 650 oC for 3 h. In this investigation, the P is calculated by: 𝑃 = (1 −
𝜌∗ 𝜌𝑠
) ∗ 100%
where ρ* is the density of Al foam and ρs is the density of the pore wall, respectively. 1.2 Characterization
(1)
Field emission scanning electron microscope (SEM, Hitachi S4800) and high-resolution transmission electron microscope (TEM, Philips Tecnai G2 F20, 200kV) were employed to characterize the sizes, distributions and structural integrity of CNTs in the matrix. Raman spectroscopy of the CNT/Al composite powders was performed by using the 532 nm line of Ar+ laser as the excitation source to validate the quality of CNTs. Compressive specimens were prepared with a diameter of 20 mm and a height of 20 mm, which were held at each measure temperature for 30 min and then tested at temperatures between 25 and 250 oC under a crosshead speed of 1mm/min on an Instron 5982 tester. To ensure the reproducibility and reliability of the results, three samples were tested under the same condition. 2. Results and discussion 2.1 Microstructure Fig. 1c shows that the pores of CNT/Al composite foams with an average size of 1.5 mm are uniformly distributed in the composite foams, which well replicate the shape of original spherical carbamide particles. The average pore wall thickness of the composite foams is about 200 μm, and the pore wall also shows a smooth surface (seen in Fig. 1d). Actually, compared with other space-holder materials such as NaCl [22], the spherical-shaped pores can help reduce the surface roughness with less edges and corners, which benefits to lessen the local stress concentrations and inhomogeneous deformation during compression, and thus, improves the strength of CNT/Al composite
foams. Besides, the pore size and porosity as well as the morphology of the composite foams could be easily tailored by the spacer method, which has been confirmed by our previous works [21, 23].
Fig. 1. (a) SEM image of pure Al powders. Photographs of (b) original carbamide particles and (c) 3.0 wt.%-CNT/Al composite foams. (d) is the magnified image of (c). Fig. 2 shows the SEM images of CNT distributions in CNT/Al composite powders obtained by CVD. With the reaction time increasing, the surface of Al particles is gradually homogeneously covered by CNTs of several microns. When the reaction time increases to 60 min, the CNT content reaches 3.0 wt.%, and the Al matrix is fully covered by CNTs (seen in Fig. 2c). Thus, the CNT content could be well controlled by adjusting the reaction time. Even though the in-situ CVD method alleviates the
agglomeration of CNTs to a certain extent, some CNTs still present an entangled morphology (Fig. 2d-f) due to the large aspect ratio of CNTs and the large van der Waals force among the CNTs. Moreover, the high percentage of CNTs distributed on the Al surface will greatly hinder the effective combination of Al particles during the forming process of composite foams, finally becoming the failure initiation sources and weakening the strength of CNT/Al composite foams. Herein, the short-time ball-milling technique is introduced to further improve the CNT dispersion and the interfacial bonding between CNTs and Al matrix.
Fig. 2. SEM images of (a,d) 2.0 wt.%, (b,e) 2.5 wt.% and (c, f) 3.0 wt.% CNT reinforced Al composite powders synthesized by 15, 30 and 60 min. (d), (e) and (f) are the magnification of (a), (b) and (c), respectively. SEM images in Fig. 3 display that the average size of 3.0 wt.%-CNT/Al composite powders is about 120 μm after ball milling of 90 min, a little larger than that of the original Al powders (~70 μm, see Fig. 1a) because of the repetitive process of fracturing
and welding [24]. Meanwhile, the separately dispersed CNTs are deeply embeded inside the Al matrix instead of covering on the surface of Al particles (Fig. 3b). In practice, the final distribution of CNTs in the composite foams depends on its initial distribution in the original CNT/Al composite powders, because the dispersion of CNTs in the Al matrix can not be affected and improved during the next process, including the cold compaction and sintering [25]. Therefore, the as-ball-milled CNT/Al composite powders can be very effective to ensure the good dispersion of CNTs in the finally formed composite foams.
Fig. 3. (a) SEM image of 3.0 wt.%-CNT/Al composite powders after 90 min ball milling. (b) is the magnified image of (a). Fig. 4 shows the typical TEM images of CNTs. The synthesized CNTs with multi-walled hollow tubular structures and outer diameters of 10-20 nm present relatively clean and smooth wall surfaces (Fig. 4a). The apparent graphitic sheets of CNTs with 0.34 nm of the interlayer spacing are observed in Fig. 4b, which is consistent with the ideal graphitic interlayer space (0.34 nm). This suggests that the purity and quality of CNTs is sufficient to act as an excellent reinforcement in CNT/Al composite
foams without an additional treatment. After ball milling of 90 min, the CNTs are implanted into the Al matrix with mild structure damage (Fig. 4c-d), forming a strong interfacial bonding between CNTs and Al matrix. Fig. 4e-f shows the TEM images of composite foam wall after compressive test at room temperature. Although the CNTs are still embedded in Al matrix, it merits the attention that some defects, such as the broken carbon layers (pointed by the red arrow), occur around the interface between CNTs and Al matrix, indicating that the structure integrity of CNTs is injured during the compressive tests. It also confirms that CNTs are subjected to the heavy loads by the means of interfacial load transfer.
Fig. 4. TEM images of (a,b) raw CNTs, 3.0 wt.%-CNT/Al composite powders, (c, d) after 90 min ball milling, and (e, f) after being compacted at 25 oC. (b), (d) and (f) are the magnification of (a), (c) and (e), respectively. To further assess the quality variations of CNTs during the experiment, Raman spectrum measurements are carried out. The relative density between the two peaks (G-line and D-line) is known to provide information about the quality of internal CNTs [18]. The relative intensity ratio of D to G peak (ID/IG) (Fig. 5) increases from 0.74 (raw CNTs) to 0.89 (after ball milling), indicating that there is negligible damage to the CNTs
graphite structure during the 90 min of ball milling. Nevertheless, the ID/IG value of CNTs after being compacted reaches 1.02, demonstrating that the structure of CNTs is injured, which is in agreement with the observations in Fig. 4e-f. But overall, the feasibility of keeping balance between uniform dispersion and structural integrity of CNTs, makes the in-situ CVD method followed by a short time ball milling technique as a promising route to realize the potential of CNTs as the reinforcement in Al composite foams.
Fig. 5. Raman spectrum of the CNT/Al composite foams. 2.2 Compressive property Fig. 6 shows the compressive stress-strain curves of pure Al foam and CNT/Al composite foams at different ambient temperatures ranging from 25 to 250 oC. The specific parameters of compressive properties are also summarized in Fig. 7. The stress-strain curves at both room temperature and elevated temperatures all exhibit three distinct regions, just as described in previous literatures [4-6]: a linear elastic region in the initial stage of deformation, then a plateau region with nearly constant stress over a
wide range of strain, and finally a densification region where the stress increases rapidly with the strain increasing. Generally, two types of metallic foams can be distinguished from the compressive stress-strain curves [26]. One is ductile metallic foams whose curve is characterized by smooth and steadily rising stress. The failure is mainly controlled by the buckling of the pore walls. On the contrary, another is brittle foams which exbihit severe fluctuations during the plateau region. The failure of these foams is mainly dominated by fracture and collapse of the pore walls. According to Fig. 6a, the stress-strain curves of pure Al foam show an extremely smooth tendency, because of the strong plastic deformation ability of pure Al matrix. However, Fig. 6b-d shows that the stress-strain curves of CNT/Al composite foams display some slight fluctuations during the plateau region. Nevertheness, by comparison with other ceramic phases reinforced Al composite foams [5, 27], the fluctuations in our present work are comparatively small. This is mainly ascribed to the high intrinsic toughness of CNTs.
Fig. 6. Compressive stress-strain curves of (a) pure Al foam, (b) 2.0 wt.%, (c) 2.5 wt.% and (d) 3.0 wt.% CNT reinforced Al composite foams at 25-250 oC. Fig. 7 shows the significant parameters of the metallic foams including the compressive yield stress, plateau stress and densification strain of the as-fabricated Al foams at different temperatures. In general, the yield stress is regarded as the first peak stress of metallic foams, and the average stress in the plateau region is the plateau stress. As Ramamury [28] suggested, the densification strain is the intersection of tangents to stress-strain curve for the plateau region and densification region. It is obvious that both the yield stress and plateau stress decrease along with the temperature elevation, showing an significant temperature softening effect. But the densification strains of both pure Al foam and CNT/Al composite foams are almost invariant to the temperture
( shown in Fig. 7c). This may be determined by the nature of metallic foams themselves. Among temperatures of 25-250 oC, the properties of 3.0 wt.%-CNT/Al composite foams are always the most pronounced. At 25 oC, the yield stress and plateau stress of 3.0 wt.%-CNT/Al composite foams are 22.8 and 33.1 MPa, which are ~1.6 and ~0.8 times higher than that of the pure Al foam, respectively. Though the stresses of CNT/Al composite foams decrease with the increase of temperature, the improvement is still evident compared to the pure Al foam. Especially at 150 oC, the yield stress and plateau stress of 3.0 wt.%-CNT/Al composite foams are 16.8 and 20.2 MPa, respectively, which are much higher than the corresponding yield stress (5.7 MPa) and plateau stress (8.6 MPa) of the pure Al foam.
Fig. 7. Graphs of (a) yield stress, (b) plateau stress and (c) densification strain varying with the temperature and the content of CNT.
2.3 Energy absorption capacity of the as-prepared Al foams One of the most prominent features of metallic foams is that they can absorb energy because of the bending, yielding and collapse of pore walls as well as the friction between pore walls when they contact with each other during the compression [1]. The energy absorption capacity (W) of metallic foams is usually characterized by: 𝜀
𝑊 = ∫0 𝜎𝑑𝜀
(2)
where W is defined as the energy absorbed per unit volume, ε is the compressive strain and σ is the corresponding compressive stress. Here, the maximum value of ε is set as the densification strain. The energy absorption capacity (W) evolutions of Al foam with different CNT contents at 25, 150 and 250 oC are shown in Fig. 8a-c, respectively. It can be seen that increasing the testing temperature can lead to a decreased W level, for both pure Al foam and composite foams. However, the value of W increases with the increae of CNT content. Meanwhile, the W values of CNT/Al composite foams are dramatically higher that of the pure Al foam at temperature 25-250 oC, which is due to higher yield stress and plateau stress of the composite foams. When it is compressed to the densification strain, according to Eq. (1), the values of W for the 3.0 wt.%-CNT/Al composite foams at 150 and 250 oC are calculated to be 19.8 and 11.6 MJ/m3, respectively. Whereas, for the pure Al foam, the values of W at 150 and 250 oC decrease to be only 5.6 and 3.9 MJ/m3.
Fig. 8. (a), (b) and (c) are the diagrams of energy absorption capacity (W) at 25, 150 and 250 oC, respectively. For the engineering applications, the Al foam as an energy-absorbing material has a promising prospect. Considering the different preparation processes of the Al composite foams, a dimensionless quantity named enhanced coefficient (η) is introduced to quantitatively evaluate the reinforcement effects on the energy absorption capacity of the composite foams. The expression of η is given as follows: 𝜂=
𝑊1−𝑊2 𝑊2
∗ 100%
(3)
where η is the enhanced coefficient, W1 and W2 are the energy absorption capacities of the composite foams and matrix, respectively. For comparison, the values of η for Al composite foams in other literatures are calculated using Eq. (2) and the results are
listed in Table 1. The intuitive statistical data suggests that the contribution of CNTs to the enhancement of energy absorption capacity in this work is higher than most of the referenced reinforcements [29-33]. Especially when the temperature increases 150 oC, the value of η even reaches 253.6%. This demonstrates that the improved powder metallurgy approach we used can bring a significant improvement in energy absorption of the Al composite foams, attributing to the uniform dispersion of reinforcement in the matrix. Table 1 Enhanced coefficient of Al composite foams fabricated by different methods Composite
foams
(Reinforcement/Mat
Preparation
Reinforcement
processing
content
Porosity (%)
Pore (mm)
size
Testing
Enhanced
temperature
coefficient (η)
o
rix)
( C)
CNT/Al [this work]
In-situ
CVD
+
3.0 wt.%
~60
~1.5
25
117.3%
150
253.6%
250
197.4%
space holder
CNT/Al [17]
Melt foaming
0.5 vol.%
~85.0
~2.4
25
73.3%
SiCp/Al [29]
Powder metallurgy
1.0 vol.%
~60.1
~0.5
25
69.4%
Melt foaming
2.5 vol.%
~73.5
2.0~4.0
25
142.9%
In-situ synthesis +
10.0 wt.%
~60.0
~0.5
25
198.2%
+ foaming Ceramic microsphere/Al [30] MgAl2O4/Al [31]
space holder Al2O3/Al [32]
Melt foaming
15.0 vol.%
~77.0
~3.0
25
100.0%
SiC/Al-Si [33]
Stir casting
18.0 wt.%
12.2~26.3
~2.9
25
181.3%
12.5~26.7
~2.9
200
160.0%
10.7~26.9
~2.9
400
25.4%
2.4 Fractograph and strengthening mechanism Fig. 9a-c is the fractographs of pure Al foam at 25 oC, showing that there are many dimples on the fracture surface, which present a typical ductile failure mode. Moreover,
when the temperature is elevated to 250 oC, lots of tearing ridges instead of the dimples occurr on the fracture surface (Fig. 9d-f), which implies that the Al matrix is markedly softened under the high temperature environment. The result is consistent with the experimental result observed by Aly et al. [4], which also verified the softening effect of Al matrix at high temperatures. On the other hand, Fig. 10a and b show that the fracture of 3.0 wt.%-CNT/Al composite foams is not completely caved and partly keeps the original morphology of the pore at 25 oC. Compared with the fractograph of pure Al foam, the dimple number of composite foams is obviously reduced and the depth becomes shallow. Besides, some notches also are observed. It should be noted that, when the testing temperature increases to 250 oC, the composite foams are almost pressed into a pie shape, as indicated in Fig. 10e. Some cleavage facets as well as scratches with varying depths are exposed to the fracture surface, showing an intergranular fracture characteristic. In addition, CNTs are not pulled out in the fracture surface by observation (Fig. 10c-d and Fig. 10g-h), manifesting the strong interfacial bonding between CNTs and Al matrix. Combined with the above observation, it can be inferred that the failure characteristics of the Al foam change from a ductile type to the combination of brittle and ductile type, resulting from the CNT addition. It is generally accepted that the CNTs are thermodynamically stable and do not react with Al matrix even at high temperatures, as long as they keep the perfect cylindrical structure which consists of several rolled up graphitic basal planes [34]. Similarly, no interfacial reactant (e.g. Al4C3) is formed in the
observation (Fig. 4), which also has been confirmed by our previous study [35]. Thereby, the brittle behavior of the CNT/Al composite foams may mainly be caused by the incompatible deformation between CNTs and the Al matrix under the condition of compression.
Fig. 9. Fractographs of pure Al foam at (a-c) 25 and (d-f) 250 oC.
Fig. 10. Fractographs of 3.0 wt.%-CNT/Al composite foams at (a-d) 25 oC and (e-h) 250 oC. The strengthening mechansim of CNT/Al composite foams in present work can be
summarized to several factors. Firstly, like the case of CNT-reinforced Al matrix composites [36, 37], the fiber strengthening is the dominating mechanism. Due to the well interfacial bonding between CNTs and the Al matrix, an applied force can be effectively transmitted from the matrix to the CNTs by the means of shear stresses. Liu et al. [38] studied the tensile properties of CNT/2009Al composites at temperatures of 20-300 oC and found that the load transfer mechanism play an important role at temperatures elevated up to 300
o
C, thus the yield strength of the 1.5
vol.%-CNT/2009Al composites at 150-300 oC, was improved compared with the 2009Al matrix. Secondly, a three-dimensional network of uniformly distributed nano-reinforcement (CNTs) are formed in the matrix of composite foams, causing a dispersion strengthening effect [10]. Furthermore, when the temperature is elevated from 25 to 250 oC, the soften of Al foam matrix can be effectively impeded by CNTs, because of the extremely low thermal expansion coefficient (CET ≈ 0) [12, 20, 38] and the excellent high temperature mechanical properties of CNTs [38]. Certainly, the above mentioned strengthening mechanisms do not exist in isolation and the intercoupling of these strengthening effects finally improve the elevated temperature compressive properties and the energy absorption capacity of CNT/Al composite foams. 3. Conclusions In this investigation, 60% porosity of CNT/Al composite foams with an average pore size of 1.5 mm were successfully fabricated. The in-situ grown CNTs with 10-20 nm diameters and several microns length are homogeneously dispersed and embedded
in the Al foam matrix after ball-milling of 90 min. The composite foams with uniformly distributed pores replicate the size and shape of carbamide particles. Both the compressive properties and energy absorption capacity decrease with the temperature rising, but increase with the increment of CNT content. The failure mode of the Al foam changes from ductile type to brittle type combined with ductile type, as a result of the CNT addition in the matrix. The interfacial load transfer and the excellent high temperature mechanical properties of CNTs are supposed to be the strengthening mechanisms for CNT/Al composite foams. In addition, the high temperature performance of the CNT/Al composite foams is expected to be further improved with the optimization of CNT content and composite microstructure. Such research is still in progress. Acknowledgements This work is supported by the Key Program of National Natural Science Foundation of China (No. 51531004), the National Natural Science Foundation of China for Young Scholars (No. 51301198). References [1] L.J. Gibson, M.F. Ashby, Cellular Solids: structure and properties. Cambridge University Press: Cambridge, UK, 1997. [2] M.F. Ashby, T. Evans, N.A. Fleck, L.J. Gibson, J.W. Hutchinson, H.N.G. Wadley, Metal Foams: A Design Guide., Butterworth Heinemann: Elsevier, 2000. [3] J. Banhart, Manufacture, characterization and application of cellular metals and
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