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Enhancing the interface bonding in carbon nanotubes reinforced Al matrix composites by the in situ formation of TiAl3 and TiC
Accepted Manuscript Enhancing the interface bonding in carbon nanotubes reinforced Al matrix composites by the in situ formation of TiAl3 and TiC X.Q. Liu, C.J. Li, J.H. Yi, K.G. Prashanth, N. Chawake, J.M. Tao, X. You, Y.C. Liu, J. Eckert PII:
S0925-8388(18)32291-6
DOI:
10.1016/j.jallcom.2018.06.170
Reference:
JALCOM 46506
To appear in:
Journal of Alloys and Compounds
Received Date: 25 February 2018 Revised Date:
14 June 2018
Accepted Date: 16 June 2018
Please cite this article as: X.Q. Liu, C.J. Li, J.H. Yi, K.G. Prashanth, N. Chawake, J.M. Tao, X. You, Y.C. Liu, J. Eckert, Enhancing the interface bonding in carbon nanotubes reinforced Al matrix composites by the in situ formation of TiAl3 and TiC, Journal of Alloys and Compounds (2018), doi: 10.1016/ j.jallcom.2018.06.170. 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 proof before it is published in its final 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.
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Enhancing the interface bonding in carbon nanotubes reinforced Al matrix composites by the in situ formation of TiAl3 and TiC X.Q. Liu a, C.J. Li a,*, J.H. Yi a,†, K.G. Prashanth b,c, N. Chawake b, J.M. Tao a, X. You a,
Faculty of Materials Science and Engineering, Kunming University of Science and
Technology, Kunming 650093, China b
Erich Schmid Institute of Materials Science, Austrian Academy of Sciences,
c
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Jahnstraße 12, A-8700 Leoben, Austria
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a
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Y.C. Liua, J. Eckert b,d
Department of Manufacturing and Civil Engineering, Norwegian University of
Science and Technology, Teknologivegen 22, 2815,Gjovik, Norway d
Department of Materials Physics, Montanuniversität Leoben, Jahnstraße 12, A-8700
Abstract
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Leoben, Austria
Achieving effective load transfer at the interface between carbon nanotubes (CNTs)
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and aluminum (Al) is a crucial issue for fabricating high-performance CNTs
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reinforced Al matrix (CNT/Al) composites. In this work, CNT/Al composites with different Ti additions and the compared materials were prepared by powder metallurgy. Micro-sized Ti particles in which CNTs are well-dispersed firstly circumvent the difficulty of CNT dispersion, and subsequently act as nucleation site for sandwiched TiAl3 layers that lock the dispersed CNTs in place and improve the CNT-Al interface bonding. Additionally, Ti addition not only allows modification of * †
Corresponding author. E-mail: [email protected] (C.J. Li) Corresponding author. E-mail: [email protected] (J.H. Yi) 1
ACCEPTED MANUSCRIPT the dispersed CNTs but also enhances the strength of the composites by enhancing the load-bearing capacity of the CNTs through in situ formation of nano-sized titanium carbide (TiC). This work provides a new approach to improve the load transfer
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efficiency of CNTs by strengthening the interface bonding for fabricating high strength CNT/Al composites.
Keywords: Carbon nanotubes; Al matrix composites; Interfacial bonding; TiAl3
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layers; Nano titanium carbides
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1. Introduction
The outstanding overall properties of carbon nanotubes (CNTs) combined with their low density make them the ideal reinforcements for producing metal composites with excellent mechanical properties [1]. In order to meet the increasing demands of
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light weight, high strength and corrosion resistance in materials for automobile and aerospace applications, extensive investigations have been dedicated to develop CNT/Al composites in the past two decades [2, 3]. However, CNT/Al composites still
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face challenges like the uniform dispersion of CNTs into the matrix. Achieving strong
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interfacial bonding is another major issue, which directly relates to the load transfer efficiency, etc., restricting the production and application of CNT/Al composites. So far, methods such as molecular level mixing [4], nanoscale dispersion [5], in situ
synthesis [6], flake powder metallurgy [7], friction stirring [8], high energy ball milling [9] and solution coating [10] have been employed to fabricate CNT/Al composites. Among them, high energy ball milling has proved to be one of the effective techniques to disperse CNTs into a metal matrix [11, 12]. The approaches 2
ACCEPTED MANUSCRIPT mentioned above mainly focused on eliminating the agglomeration of the CNTs. However, enhancing the ultimate strength of the CNT/Al composites mainly depends on strong interfacial bonding to fully exploit the endurance limit of the high-strength
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CNTs and to facilitate effective load transfer at the interface [13]. To address this problem, previous investigations have adopted electro-deposition and chemical plating techniques to modify the surface of the CNTs with Ni, Cu or Ag, etc., which
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lead to a better interfacial bonding between the matrix and the CNTs by improving the
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wettability of the CNTs with the targeted matrix [14-16]. Nevertheless, achieving intact and uniform interfacial layers on CNTs was found to be complicated and required to optimize many deposition parameters. The formation of chemical bonds at the CNT-Al interface by favorable and controllable interfacial reaction is another a
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promising way to achieve efficient load transfer [17]. Zhou et al. [3, 18] precisely controlled the temperature of sintering and heat treatment to determine the optimized amount of aluminum carbide (Al4C3) as an interfacial product for improving the
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strength of CNT/Al composites. However, the quantity and morphology of Al4C3 were
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hardly controllable. Additionally, a transition phase such as Al3Ni sandwiched at the CNT-Al interface was also proposed to improve the interfacial bonding and enhancing the strength of the composites [14]. Titanium is a strong carbide forming element, and Gibb’s free energy for the
formation of Ti carbide is about -187 kJ/mol at 300 K and -215 kJ/mol at 893 K (sintering temperature) respectively [19]. Recently, Saba et al. [20] fabricated CNT/Al composites with nano-sized titanium carbides within the CNTs. Such arrangement 3
ACCEPTED MANUSCRIPT improved the load-bearing contribution of the nanotubes due to the linking effect of the in situ formed TiC in their defects. Moreover, Ti is also prone to react with Al and form titanium aluminide, and such hard micro-sized particles may help to strengthen
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metal matrix composites due to Orowan looping mechanism [21]. Guo et al. already fabricated Ti-Al3Ti core-shell structured particle reinforced Al based composite and reported its promising tensile strength of 197 MPa and high ductility[22]. Thus, Ti
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theoretically can be used as a buffer media to enhance the strength of Al matrix
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composites by in situ formation of TiC and the formation of sandwiched Ti-Al intermetallic compounds at the interface. However, the potential role of the Ti-Al intermetallic compounds at the CNT-Al interface and the underlying strengthening mechanism for improved properties were not investigated previously.
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In the present study, 2 wt.% CNT/Al composites containing different amounts of micro-sized Ti particles (hereafter referred to as (CNTs-Ti)/Al composites) and the corresponding compared materials were fabricated. The typical microstructures and
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the effects of Ti addition on the mechanical properties of the composites were
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investigated in detail.
2. Experimental procedure 2.1 Raw materials
Pristine graphitized multi-walled carbon nanotubes (MWCNTs) (TNGM3,
Chengdu Organic Chemicals Co. Ltd., China) with a diameter of 10-20 nm and length of 5-30 µm were synthesized by chemical vapor deposition (CVD). Pure Ti powders (with a purity of 99.99% and an average particle size of about 50 µm) were purchased 4
ACCEPTED MANUSCRIPT from Shanghai Aladdin biochemical Polytron Technologies. Ganzhou Jingke Technology Co. Ltd of China provided pure Al powders, and the purity and the average particle size were 99.99% and 25 µm, respectively. The morphologies of the
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raw materials are shown in Fig.1: the shape of the Ti powders is irregular and the MWCNTs are severely entangled. 2.2 Preparation of composites
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The preparation of the (CNTs-Ti)/Al composites mainly included three steps.
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Firstly, to get homogeneous CNT-Ti power mixtures, different weight ratios (2:0.5, 2:1, 2:1.5, 2:2) of MWCNTs and pure Ti powders, 1.5 wt. % stearic acid acting as process control agent and some GCr15 steel balls were placed into stainless steel vials in a glove box with purified argon (purity of 99.999%). Ball milling was performed at
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room temperature using a planetary ball mill for 2 h with a ball-to-powder weight ratio (BPR) of 20:1 and a rotation speed (RS) of 200 rpm. Al powders were ball milled for 4 h at 10:1 BPR and a RS of 300 rpm, respectively, in order to obtain Al
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particles in the form of flakes with the increased specific surface area providing many
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sites for CNT adhesion. Subsequently, the CNT-Ti power mixtures were added to the flake Al powders, where the content of CNTs and Ti particles were controlled to be 2 wt.% and 0 - 2 wt.%, respectively. The powder mixtures were ball milled for 4 h at a BPR of 10:1 and RS of 300 rpm, respectively. In the next step, the milled powders were heat-treated in a vacuum furnace at 723 K for 1 h to remove the remaining stearic acid. The second step was consolidation of the as-milled powder mixtures by cold compaction into cylinders of Ø 28 mm and height 21 mm using a pressure of 5
ACCEPTED MANUSCRIPT about 250 MPa. The compacted cylinders were sintered at 893 K for 4 h in flowing argon gas. Finally, the sintered bulks were extruded at 873 K to obtain Ø 5 mm composite rods. In addition, 2 wt. % CNT/Al composites without Ti addition, Al-2wt. %
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Ti materials and pure Al were processed under the same route for comparison purpose. 2.3 Characterization
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The microstructures of the as-milled powders and hot-extruded composites were
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characterized by field emission scanning electron microscopy (FESEM, Nova Nano 450) coupled with energy-dispersive analysis (OXFORD, X-Max) and transmission electron microscopy (TEM, FEI G2 F30 S-TWIN). The densities of the hot-extruded samples were measured by the Archimedes method. For tensile tests, the samples
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were cut and polished into dog-bone-shape (indicated in the insert image of Fig. 7a). The gauge length of tensile samples was 16.5 mm, the thickness 2 mm and the width 3 mm, respectively. The tensile tests were carried out using a universal testing
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machine (AG-X 100 kN, Shimadzu) at ambient temperature and a strain rate of 5 ×
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10−4 s−1. At least three specimens were tested for each condition and the average values are reported.
3. Results and discussion 3.1 Morphology of the CNT distribution within the Ti particles Fig. 2 shows FESEM images revealing the morphology of the CNTs-Ti powders. Fig. 2a shows that most of the Ti particles have an irregular shape, and few Ti rods and flake-liked Ti sheets are observed. The very uniform CNTs dispersion within a 6
ACCEPTED MANUSCRIPT flake-like Ti sheet is depicted in Fig. 2b. After 2 h of ball milling, a drastic deformation caused by the constant shearing force of the balls happened, which gradually flatted the Ti particles into sheets. The flake-liked sheets such as Al or Cu
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sheets were proved to be well contacted with CNTs which greatly helps the dispersion [23]. Meanwhile, as a result of cold welding, some small-sized Ti particles with sizes
2b.
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3.2 In situ formed TiC at the CNT-Ti interface
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below 1 µm are attached to the lateral side of the large Ti particles, as shown in Fig.
A uniform dispersion of CNTs that improve the load transfer at the interface has been proven to be a prerequisite for fabricating high-strength composites [10]. To further check the dispersion state of the CNTs within the Ti powders and possible
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formation of TiC on the CNTs, TEM studies were carried out. Fig. 3 presents TEM images of the CNTs-Ti mixed powders. As the result of ball milling, the Ti powder particles were broken into micro-sized (Fig. 3a) or even nano-sized particles (Fig. 3c).
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Fig. 3a shows a typical view of CNTs (marked by the white arrows) incorporated in
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the micro-sized Ti particles, suggesting a good dispersion of CNTs in the Ti particles. These micro-sized Ti particles with good CNTs dispersion were further used as media to link the CNT-Al interface. A TiC nano-layer between the CNTs and the Ti matrix was observed by TEM (Figs. 3b and c), which was confirmed by the Fast Fourier Transform (FFT) patterns displayed in Fig. 3d. The newly in situ formed TiC bonds the CNTs to Ti and further helps to disperse the CNTs. The distance of the lattice fringes (indicated by the yellow dashed lines in Figs. 3e and f) reveals the formation 7
ACCEPTED MANUSCRIPT of two typical TiC carbides with nano-lump (Fig. 3e) and nano-block (Fig. 3f) appearance. The formation mechanism of these two kinds of TiC carbides which is mainly related to the reacted carbon sources has been reported by Saba et. al [20].
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During the milling process, the impacting balls destroy the structure of the CNTs partially and induce defects on the outside walls or tips of the MWCNTs. Due to the severe deformation and fracture of these Ti particles, the atoms on the Ti particles
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become activated and can infiltrate into the CNTs through their surface defects to
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react with the carbon atoms in the defects of the CNTs, which finally generates TiC carbides. These in situ formed TiC nano-carbides can link the closed walls of the CNTs and improve the load-bearing capacity of the inner-walls of the CNTs. 3.3 Morphology of the（CNTs-Ti）/Al composite powders
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The morphology of the as-milled (CNTs-Ti)/Al powders with 2 wt.% Ti is shown in Fig. 4. The homogeneous distribution of CNTs in the Ti particles (indicated by the pale green arrows in Fig. 4b and c) was retained in the (CNTs-Ti)/Al composite
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powders, implying that adding Ti particles can improve the dispersion of the CNTs.
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Fig. 4d reveals that some Al powders also exhibit a relatively good CNT dispersion, indicating that high energy ball milling is effective to break up the residually entangled CNTs. At the same time, the observed Al sheets can also assist uniform dispersion of CNTs.
3.4 Microstructure of bulk composites Combining cold pressing, sintering and hot extrusion yields high relative densities of over 98% for all samples. Fig. 5 shows the microstructure of the extruded 8
ACCEPTED MANUSCRIPT composite rods with 2 wt. % Ti addition. Micro-sized second phases ranging from about 1 to 50 µm with a uniform distribution are clearly observed in Fig. 5a. The EDS line scanning result (Fig. 5d) of the representative area of the particle marked in Fig.
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5b, reveals that the Al content decreases from the matrix (1500 counts per second, cps) to the center of the particle (0 cps), whereas the Ti content increases from 0 cps to about 500 cps at the same time, indicating that the center of the particle consists of Ti.
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Additionally, the characteristic X-ray intensity ratio of Ti and Al is about 1:3 at the
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edge of the particle, which reveals that the edge of this particle may contain TiAl3 phase. In general, the Ti particles are completely surrounded by an in situ formed 2 - 5 µm thick TiAl3 layer with a strong Al-TiAl3 interface (see Fig. 5b). The formation of this TiAl3 layer is attributed to the relatively high temperature (893 K) and long
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sintering time (4 h), which facilitates atomic diffusion and promotes reaction of Ti atoms and Al matrix. Meanwhile, the severe deformation during high energy ball milling can create a large density of dislocations and fine grains at the Ti-Al interface,
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which can further accelerate atomic diffusion and interface reaction. According to the
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Ti-Al phase diagram, it is possible that Ti and Al react with each other and form several intermetallic compounds like TiAl3, TiAl2, TiAl and Ti3Al [24]. Among them, the energy for the nucleation of TiAl3 was demonstrated to be the lowest [25, 26]. The EDS scanning results of S2 (Fig. 5c) also confirm the formation of TiAl3 phase. In addition, as a result of solid state reaction, some of the small-sized Ti particles were totally dissolved. Thus, the well-dispersed CNTs at the Ti-Al interface are wrapped into the TiAl3 phase, and the in situ formed TiAl3 can lock the well-dispersed CNTs in 9
ACCEPTED MANUSCRIPT place by the stable covalent bonds of Ti-Al. This scenario is depicted in the schematic illustration of the Al-CNT-Ti interface in Figs. 5e and f. TiAl3 is well known as a potential thermo-structure composite material due to its low density (3.3 g cm-3)，high
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melting temperature (1614 K)，high hardness (6 GPa) [27]，high Young modulus (about 220 GPa) [28] and good oxidation resistance. As an interfacial product, TiAl3
the CNTs by its strong interfacial bonding.
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3.5 TEM analysis
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sandwiched between CNTs and Al can assist the load transfer from the Al matrix to
To further investigate the strengthening behavior, the microstructure of CNT/Al composites with 2 wt. % Ti addition was examined in more detailed by TEM. Fig. 6a presents a microstructure view of fine Al grains with a size ranging from about 400
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nm to 1.9 µm. The average size calculated from the scattered grains in Fig. 6a is about 1.1 µm, which can enhance the strength of the composites by the effect of fine grain strengthening. Distinct CNT zone along the Al grain boundaries (GB) and some
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modified CNTs (indicated by the white arrow) in the Al matrix were observed in Figs.
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6b and g, respectively. At the same time, Fig. 6f presents the evidence of the CNT defects (indicated by the white arrows and dash lines). Both walls and tips of some CNTs have lost their original intact shapes and present some defects. Those defects were proved to greatly facilitate the formation of TiCs. CNTs modified by two kinds of TiC carbides, i.e. nanoblock and nanolump carbides, are identified by the inserted selected area electron diffraction (SAED) patterns of high-resolution (HR) TEM images in Figs. 6d and h. Besides, both the lattice spacing of the (111) and (200) plane 10
ACCEPTED MANUSCRIPT of TiC were measured and marked around the CNTs, which further indicates the existence of TiCs. To demonstrate the possible shape of the in situ formed TiC, Figs. 6e and i display two corresponding schematic illustrations for the TiCs observed in
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Figs. 6d and h. It is interesting to note that both kinds of TiC play a role in linking the walls of the CNTs, indicating that the modified CNTs retained from the powders greatly enhance the load-bearing capacity of CNTs. Hence, dispersing these CNTs
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modified by nano TiC into the Al matrix contributes to the enhanced strength of the
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CNT/Al composites. Fig. 6c depict a typical formation of Al4C3 around the GB, confirmed by the lattice spacing of 0.83 nm for the (003) atom plane on the bottom left image of Fig. 6c and the SAED patterns (indicated by the area A) inserted to the upper right corner of Fig. 6c. The crucial role of in situ formed Al4C3 is locking the
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CNTs in place by transforming the bonding nature from mechanical bonding to chemical bonding [12]. This improved CNT/Al interfacial bonding due to the formation of Al4C3 significantly enhances the strength of the CNT/Al composites.
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Therefore, one can conclude that the enhanced strength of all the CNT/Al composites
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with or without Ti addition comes from the fine grains, the uniformly dispersed CNTs and the in situ formed Al4C3. With the Ti addition, composites in which the CNTs modified with nano-sized TiC are dispersed can reach superior properties compared to composites without Ti addition. 3.6 Mechanical properties The mechanical properties results of the fabricated materials are shown in Fig. 7. Fig. 7a presents the tensile engineering stress - engineering strain curves of 11
ACCEPTED MANUSCRIPT as-prepared materials. Compared with pure Al, all materials are strengthened with the addition of CNTs or Ti or CNTs and Ti. The yield strength (YS), ultimate tensile strength (UTS) and elongation of the CNT/Al composite without Ti addition were
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measured to be 131 MPa, 167 MPa and 9.4%, respectively. Introducing Ti into the CNT/Al composites improves both the strength and the ductility of the composites. The YS, UTS and the elongation of the composite with 2 wt.% Ti addition are 170
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MPa, 222 MPa and 15.5%, respectively. Fig. 7b shows the reinforced effect of CNTs
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and Ti addition respectively. Calculated from the as-measured results of tensile strength, an improvement of 43 MPa (31%) and 55 MPa (33%) in UTS was acquired while adding Ti into pure Al and CNT/Al composite (compared the curve b to a and curve g to c). Thus, significant enhancement in the materials with or without CNTs
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addition is achieved. Similarly, an improvement of 28 MPa (20%) and 40 MPa (22%) in UTS was acquired while adding CNTs into pure Al and Al-2wt.%Ti (compared the curve c to a and curve g to b). It also indicates that the reinforced effect of CNTs is
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significant with or without Ti addition, which have been reported by numerous studies
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[13, 29, 30]. The UTS increment is 83 MPa when CNTs and Ti were simultaneously added into pure Al, which is higher than the totality of UTS increment (71 MPa) attributed by sole CNTs or Ti addition. Hence, there exists extra synergistic enhancement (12 MPa) resulting from the modified effect of TiCs and the in situ formation of TiAl3 layers at the CNT-Al interfaces. According to the load transfer model for MMCs developed by Kelly and Tyson [31], fibrous CNTs are loaded by the plastically deformed matrix via an interfacial shear 12
ACCEPTED MANUSCRIPT stress (τ) generated along the CNT surface under tension. Compared with CNT/Al composites, the strength enhancement of the (CNTs-Ti)/Al composites in the present study mainly comes from two aspects: (1) introducing TiAl3 layers at the Al-CNT
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interface can lock the well-dispersed CNTs in place, which significantly improves the load transfer ability of CNTs at the interface; (2) the partial formation of TiC on the MWCNTs during ball milling improves the load-bearing contribution of the
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inner-walls of CNTs. Actually, every micro-sized Ti particle first acts as a medium to
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carry the CNTs, and subsequently reacts with the Al matrix and forms an in situ TiAl3 layer wrapping the CNTs at the interface. The key role of these in situ TiAl3 layers is to enwrap the CNTs instead of simple physical bonding of CNTs to Al. Therefore, these Al-CNT/(TiAl3)-Ti hybrid structures can facilitate the load transfer from the
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matrix to the CNTs at the improved CNT-Al interface. Besides, these micro-sized particles including Ti and TiAl3 (see Fig. 5a) can also enhance the strength due to Orowan strengthening. At the same time, the in situ formed nano-sized TiC on the
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CNTs (see Figs. 3e and f, and Figs. 6d and h) exploits the load-bearing of the
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inner-wall of the CNTs, which are dispersed into the Al matrix and assist the load transfer at the CNT-Al interface. With increasing amount of Ti addition, it is expected that the load transfer efficiency also increases with increasing the amounts of CNT-Al interfaces sandwiched by TiAl3 layers and the CNTs modified with TiCs. 4. Conclusions 2 wt.% CNT/Al composites with different weight fractions of Ti addition were fabricated by powder metallurgy. A homogenous distribution of CNTs on the Ti 13
ACCEPTED MANUSCRIPT particles was achieved, suggesting that the addition of Ti can assist CNTs dispersion. Sandwiching in situ formed TiAl3 layers at the Al-Ti interface significantly improves the CNT-Al interfacial bonding by locking the dispersed CNTs in place, thus leading
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to efficient load transfer. Additionally, the in situ formed TiC on CNTs also contributes to the strength enhancement by exerting the load-bearing capacity of the CNTs. Therefore, the addition of Ti can improve the mechanical properties of CNT/Al
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composites. And with the participation of Ti, the strengthening effect of CNTs is more
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significant. The current work provides a new idea to help designing high-performance CNT reinforced metal based composites. Acknowledgments
This work was supported by the National Natural Science Foundation of China
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(grant 51361017, 51401098 and 51301079), the Science Foundation of the Yunnan Provincial Science and Technology Department (grant 2014FC001 and 2017HC033), the Science Foundation of the Yunnan Provincial Education Department (grant
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2016CYH08), the Natural Science Foundation of Kunming University of Science and
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Technology (grant KKSY201551036) and the Science Foundation of Rare and Precious Metals Advanced Materials Collaborative Innovation Center of Yunnan Province (grant KKPT201551005). Additional support through the European Research
Council
under
the
ERC
Advanced
Grant
INTELHYB
(grant
ERC-2013-ADG-340025) is gratefully acknowledged. References [1] B. Guo, M. Song, J. Yi, S. Ni, T. Shen, Y. Du, Improving the mechanical 14
ACCEPTED MANUSCRIPT properties of carbon nanotubes einforced pure aluminum matrix composites by achieving non-equilibrium interface, Materials & Design, 120 (2017) 56-65. [2] C. Li, X. Liu, J. Yi, L. Teng, R. Bao, J. Tan, C. Yang, Z. Zou, Effects of sintering
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parameters on the microstructure and mechanical properties of carbon nanotubes reinforced aluminum matrix composites, Journal of Materials Research, 31 (2016) 3757-3765.
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[3] W. Zhou, T. Yamaguchi, K. Kikuchi, N. Nomura, A. Kawasaki, Effectively
M AN U
enhanced load transfer by interfacial reactions in multi-walled carbon nanotube reinforced Al matrix composites, Acta Materialia, 125 (2017) 369-376. [4] A. Maqbool, M.A. Hussain, F.A. Khalid, N. Bakhsh, A. Hussain, M.H. Kim, Mechanical characterization of copper coated carbon nanotubes reinforced
TE D
aluminum matrix composites, Materials Characterization, 86 (2013) 39-48. [5] H. Kwon, M. Estili, K. Takagi, T. Miyazaki, A. Kawasaki, Combination of hot extrusion and spark plasma sintering for producing carbon nanotube reinforced
EP
aluminum matrix composites, Carbon, 47 (2009) 570-577.
AC C
[6] C. He, N. Zhao, C. Shi, X. Du, J. Li, H. Li, Q. Cui, An Approach to Obtaining Homogeneously Dispersed Carbon Nanotubes in Al Powders for Preparing Reinforced Al-Matrix Composites, Advanced Materials, 19 (2007) 1128-1132.
[7] L. Jiang, Z. Li, G. Fan, L. Cao, D. Zhang, The use of flake powder metallurgy to produce carbon nanotube (CNT)/aluminum composites with a homogenous CNT distribution, Carbon, 50 (2012) 1993-1998. [8] Z.Y. Liu, B.L. Xiao, W.G. Wang, Z.Y. Ma, Developing high-performance 15
ACCEPTED MANUSCRIPT aluminum matrix composites with directionally aligned carbon nanotubes by combining friction stir processing and subsequent rolling, Carbon, 62 (2013) 35-42.
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[9] J. Liu, T. Gottschall, K.P. Skokov, J.D. Moore, O. Gutfleisch, Giant magnetocaloric effect driven by structural transitions, Nature materials, 11 (2012) 620-626.
SC
[10] B. Chen, K. Kondoh, H. Imai, J. Umeda, M. Takahashi, Simultaneously
M AN U
enhancing strength and ductility of carbon nanotube/aluminum composites by improving bonding conditions, Scripta Materialia, 113 (2016) 158-162. [11] S.J. Yoo, S.H. Han, W.J. Kim, Strength and strain hardening of aluminum matrix composites with randomly dispersed nanometer-length fragmented carbon
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nanotubes, Scripta Materialia, 68 (2013) 711-714.
[12] X. Liu, C. Li, J. Eckert, K.G. Prashanth, O. Renk, L. Teng, Y. Liu, R. Bao, J. Tao, T. Shen, J. Yi, Microstructure evolution and mechanical properties of carbon
EP
nanotubes reinforced Al matrix composites, Materials Characterization, 133
AC C
(2017) 122-132.
[13] J.G. Park, D.H. Keum, Y.H. Lee, Strengthening mechanisms in carbon nanotube-reinforced aluminum composites, Carbon, 95 (2015) 690-698.
[14] O. Carvalho, M. Buciumeanu, S. Madeira, G. Miranda, F.S. Silva, Interface analysis on an eutectic AlSi alloy reinforced with Ni coated MWCNT, Composites Part B: Engineering, 93 (2016) 229-235. [15] M. Jagannatham, S. Sankaran, P. Haridoss, Microstructure and mechanical 16
ACCEPTED MANUSCRIPT behavior of copper coated multiwall carbon nanotubes reinforced aluminum composites, Materials Science and Engineering: A, 638 (2015) 197-207. [16] Q. Zhao, M. Xie, Y. Liu, J. Yi, Improved electroless plating method through
RI PT
ultrasonic spray atomization for depositing silver nanoparticles on multi-walled carbon nanotubes, Applied Surface Science, 409 (2017) 164-168.
[17] B. Chen, J. Shen, X. Ye, H. Imai, J. Umeda, M. Takahashi, K. Kondoh,
SC
Solid-state interfacial reaction and load transfer efficiency in carbon nanotubes
M AN U
(CNTs)-reinforced aluminum matrix composites, Carbon, 114 (2017) 198-208. [18] W.W. Zhou, S. Bang, H. Kurita, T. Miyazaki, Y. Fan, A. Kawasaki, Interface and interfacial reactions in multi-walled carbon nanotube-reinforced aluminum matrix composites, Carbon, 96 (2016) 919-928.
(2003) 367-373.
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[19] K. Frisk, A revised thermodynamic description of the Ti–C system, Calphad, 27
[20] F. Saba, S.A. Sajjadi, M. Haddad-Sabzevar, F. Zhang, Formation mechanism of
EP
nano titanium carbide on multi-walled carbon nanotube and influence of the
AC C
nanocarbides on the load-bearing contribution of the nanotubes inner-walls in aluminum-matrix composites, Carbon, 115 (2017) 720-729.
[21] Y. Su, Q. Ouyang, W. Zhang, Z. Li, Q. Guo, G. Fan, D. Zhang, Composite structure modeling and mechanical behavior of particle reinforced metal matrix composites, Materials Science & Engineering A, 597 (2014) 359-369. [22] B. Guo, S. Ni, R. Shen, M. Song, Fabrication of Ti–Al3Ti core–shell structured particle reinforced Al based composite with promising mechanical properties, 17
ACCEPTED MANUSCRIPT Materials Science and Engineering: A, 639 (2015) 269-273. [23] L. Jiang, G. Fan, Z. Li, X. Kai, D. Zhang, Z. Chen, S. Humphries, G. Heness, W.Y. Yeung, An approach to the uniform dispersion of a high volume fraction of
RI PT
carbon nanotubes in aluminum powder, Carbon, 49 (2011) 1965-1971. [24] X.Y. Nie, J.C. Liu, H.X. Li, Q. Du, J.S. Zhang, L.Z. Zhuang, An investigation on bonding mechanism and mechanical properties of Al/Ti compound materials
SC
prepared by insert moulding, Materials & Design, 63 (2014) 142-150.
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[25] U.R. Kattner, J.C. Lin, Y.A. Chang, Thermodynamic Assessment and Calculation of the Ti-Al System, Metallurgical and Materials Transactions A, 23 (1992) 2081-2090.
[26] J. Liu, Y. Su, Y. Xu, L. Luo, J. Guo, H. Fu, First Phase Selection in Solid Ti/Al
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Diffusion Couple, Rare Metal Materials & Engineering, 40 (2011) 753-756. [27] S.s. He, X.w. Yin, L.t. Zhang, X.m. Li, L.f. Cheng, Ti3AlC2-Al2O3-TiAl3 composite fabricated by reactive melt infiltration, Transactions of Nonferrous
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Metals Society of China, 19 (2009) 1215-1221.
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[28] S.H. Wang, P.W. Kao, C.P. Chang, Stress-strain behavior of fine-grained Al Al3Ti alloys, Scripta Metallurgica Et Materialia, 29 (1993) 323-328.
[29] S.I. Cha, K.T. Kim, S.N. Arshad, C.B. Mo, S.H. Hong, Extraordinary Strengthening Effect of Carbon Nanotubes in Metal‐Matrix Nanocomposites Processed by Molecular ‐ Level Mixing, Advanced Materials, 17 (2005) 1377-1381. [30] S.R. Bakshi, A. Agarwal, An analysis of the factors affecting strengthening in 18
ACCEPTED MANUSCRIPT carbon nanotube reinforced aluminum composites, Carbon, 49 (2011) 533-544. [31] A. Kelly, W.R. Tyson, Tensile properties of fibre reinforced metals—II. Creep of silver-tungsten, Journal of the Mechanics & Physics of Solids, 14 (1966)
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177-184.
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List of figure captions Fig. 1. Morphology of the as-received raw materials: (a) pure Ti particles, and (b) pristine MWCNTs.
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Fig. 2. Morphology of the CNTs-Ti powders after ball milling: (a) shows an overview of CNTs dispersed upon the Ti particles; (b) show a uniform distribution of CNTs dispersed in Ti particles at high magnification.
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Fig. 3. TEM images of the CNTs-Ti powders. (a-c) shows their morphologies. (d) FFT
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pattern of the select area marked in (b). (e) and (f) show two typical in situ formed TiC nanoparticles in the selected areas e and f marked in (c), respectively. Fig. 4. Morphology of the as-milled (CNTs-Ti)/Al powders with 2 wt.% Ti: (a) shows an overview of composite powders; (b-d) show the CNTs dispersed in the composite
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powders at high magnification.
Fig. 5. Microstructure of the extruded CNT/Al composite with 2 wt.% Ti addition. (a) and (c) Two views of the microstructure at different magnification. (b) A
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backscattered electron image. (d) Results of EDS line scanning. (e) and (f) Schematic
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illustration of the Al-Ti interface sandwiched by the TiAl3 layer. Fig. 6. TEM images and schematic illustrations of the extruded CNT/Al composite with 2 wt.% Ti addition. (a) A microstructure view of fine grains. (b) (d) and (g-h) CNTs modified by two kinds of nano-sized TiC dispersed in the Al matrix. (c) A view of Al4C3 formed around the Al grain boundaries with a HRTEM image of the selected area and the SAED pattern of Al4C3. (f) TEM image shows the defects in the walls and at the tips of CNTs. (e) Schematic illustration of the TiC nanoblock formed on the 20
ACCEPTED MANUSCRIPT CNTs. (i) Schematic illustration of the TiC nanolump formed on the CNTs. Fig. 7. Mechanical properties results of the fabricated materials. (a) Tensile engineering stress - engineering strain curves of as-prepared materials. (b) The
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Highlights (CNTs-Ti)/Al composites with a homogenous distribution of CNTs were fabricated.
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Interface bonding was enhanced by sandwiching TiAl3 layers at CNT-Al interface.
The modification with in situ TiC can improve the load-bearing capacity of
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Ti introduction helps to enhance the mechanical properties of CNT/Al
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S0925-8388(18)32291-6
DOI:
10.1016/j.jallcom.2018.06.170
Reference:
JALCOM 46506
To appear in:
Journal of Alloys and Compounds
Received Date: 25 February 2018 Revised Date:
14 June 2018
Accepted Date: 16 June 2018
Please cite this article as: X.Q. Liu, C.J. Li, J.H. Yi, K.G. Prashanth, N. Chawake, J.M. Tao, X. You, Y.C. Liu, J. Eckert, Enhancing the interface bonding in carbon nanotubes reinforced Al matrix composites by the in situ formation of TiAl3 and TiC, Journal of Alloys and Compounds (2018), doi: 10.1016/ j.jallcom.2018.06.170. 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 proof before it is published in its final 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.
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Enhancing the interface bonding in carbon nanotubes reinforced Al matrix composites by the in situ formation of TiAl3 and TiC X.Q. Liu a, C.J. Li a,*, J.H. Yi a,†, K.G. Prashanth b,c, N. Chawake b, J.M. Tao a, X. You a,
Faculty of Materials Science and Engineering, Kunming University of Science and
Technology, Kunming 650093, China b
Erich Schmid Institute of Materials Science, Austrian Academy of Sciences,
c
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Jahnstraße 12, A-8700 Leoben, Austria
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a
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Y.C. Liua, J. Eckert b,d
Department of Manufacturing and Civil Engineering, Norwegian University of
Science and Technology, Teknologivegen 22, 2815,Gjovik, Norway d
Department of Materials Physics, Montanuniversität Leoben, Jahnstraße 12, A-8700
Abstract
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Leoben, Austria
Achieving effective load transfer at the interface between carbon nanotubes (CNTs)
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and aluminum (Al) is a crucial issue for fabricating high-performance CNTs
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reinforced Al matrix (CNT/Al) composites. In this work, CNT/Al composites with different Ti additions and the compared materials were prepared by powder metallurgy. Micro-sized Ti particles in which CNTs are well-dispersed firstly circumvent the difficulty of CNT dispersion, and subsequently act as nucleation site for sandwiched TiAl3 layers that lock the dispersed CNTs in place and improve the CNT-Al interface bonding. Additionally, Ti addition not only allows modification of * †
Corresponding author. E-mail: [email protected] (C.J. Li) Corresponding author. E-mail: [email protected] (J.H. Yi) 1
ACCEPTED MANUSCRIPT the dispersed CNTs but also enhances the strength of the composites by enhancing the load-bearing capacity of the CNTs through in situ formation of nano-sized titanium carbide (TiC). This work provides a new approach to improve the load transfer
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efficiency of CNTs by strengthening the interface bonding for fabricating high strength CNT/Al composites.
Keywords: Carbon nanotubes; Al matrix composites; Interfacial bonding; TiAl3
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layers; Nano titanium carbides
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1. Introduction
The outstanding overall properties of carbon nanotubes (CNTs) combined with their low density make them the ideal reinforcements for producing metal composites with excellent mechanical properties [1]. In order to meet the increasing demands of
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light weight, high strength and corrosion resistance in materials for automobile and aerospace applications, extensive investigations have been dedicated to develop CNT/Al composites in the past two decades [2, 3]. However, CNT/Al composites still
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face challenges like the uniform dispersion of CNTs into the matrix. Achieving strong
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interfacial bonding is another major issue, which directly relates to the load transfer efficiency, etc., restricting the production and application of CNT/Al composites. So far, methods such as molecular level mixing [4], nanoscale dispersion [5], in situ
synthesis [6], flake powder metallurgy [7], friction stirring [8], high energy ball milling [9] and solution coating [10] have been employed to fabricate CNT/Al composites. Among them, high energy ball milling has proved to be one of the effective techniques to disperse CNTs into a metal matrix [11, 12]. The approaches 2
ACCEPTED MANUSCRIPT mentioned above mainly focused on eliminating the agglomeration of the CNTs. However, enhancing the ultimate strength of the CNT/Al composites mainly depends on strong interfacial bonding to fully exploit the endurance limit of the high-strength
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CNTs and to facilitate effective load transfer at the interface [13]. To address this problem, previous investigations have adopted electro-deposition and chemical plating techniques to modify the surface of the CNTs with Ni, Cu or Ag, etc., which
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lead to a better interfacial bonding between the matrix and the CNTs by improving the
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wettability of the CNTs with the targeted matrix [14-16]. Nevertheless, achieving intact and uniform interfacial layers on CNTs was found to be complicated and required to optimize many deposition parameters. The formation of chemical bonds at the CNT-Al interface by favorable and controllable interfacial reaction is another a
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promising way to achieve efficient load transfer [17]. Zhou et al. [3, 18] precisely controlled the temperature of sintering and heat treatment to determine the optimized amount of aluminum carbide (Al4C3) as an interfacial product for improving the
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strength of CNT/Al composites. However, the quantity and morphology of Al4C3 were
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hardly controllable. Additionally, a transition phase such as Al3Ni sandwiched at the CNT-Al interface was also proposed to improve the interfacial bonding and enhancing the strength of the composites [14]. Titanium is a strong carbide forming element, and Gibb’s free energy for the
formation of Ti carbide is about -187 kJ/mol at 300 K and -215 kJ/mol at 893 K (sintering temperature) respectively [19]. Recently, Saba et al. [20] fabricated CNT/Al composites with nano-sized titanium carbides within the CNTs. Such arrangement 3
ACCEPTED MANUSCRIPT improved the load-bearing contribution of the nanotubes due to the linking effect of the in situ formed TiC in their defects. Moreover, Ti is also prone to react with Al and form titanium aluminide, and such hard micro-sized particles may help to strengthen
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metal matrix composites due to Orowan looping mechanism [21]. Guo et al. already fabricated Ti-Al3Ti core-shell structured particle reinforced Al based composite and reported its promising tensile strength of 197 MPa and high ductility[22]. Thus, Ti
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theoretically can be used as a buffer media to enhance the strength of Al matrix
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composites by in situ formation of TiC and the formation of sandwiched Ti-Al intermetallic compounds at the interface. However, the potential role of the Ti-Al intermetallic compounds at the CNT-Al interface and the underlying strengthening mechanism for improved properties were not investigated previously.
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In the present study, 2 wt.% CNT/Al composites containing different amounts of micro-sized Ti particles (hereafter referred to as (CNTs-Ti)/Al composites) and the corresponding compared materials were fabricated. The typical microstructures and
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the effects of Ti addition on the mechanical properties of the composites were
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investigated in detail.
2. Experimental procedure 2.1 Raw materials
Pristine graphitized multi-walled carbon nanotubes (MWCNTs) (TNGM3,
Chengdu Organic Chemicals Co. Ltd., China) with a diameter of 10-20 nm and length of 5-30 µm were synthesized by chemical vapor deposition (CVD). Pure Ti powders (with a purity of 99.99% and an average particle size of about 50 µm) were purchased 4
ACCEPTED MANUSCRIPT from Shanghai Aladdin biochemical Polytron Technologies. Ganzhou Jingke Technology Co. Ltd of China provided pure Al powders, and the purity and the average particle size were 99.99% and 25 µm, respectively. The morphologies of the
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raw materials are shown in Fig.1: the shape of the Ti powders is irregular and the MWCNTs are severely entangled. 2.2 Preparation of composites
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The preparation of the (CNTs-Ti)/Al composites mainly included three steps.
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Firstly, to get homogeneous CNT-Ti power mixtures, different weight ratios (2:0.5, 2:1, 2:1.5, 2:2) of MWCNTs and pure Ti powders, 1.5 wt. % stearic acid acting as process control agent and some GCr15 steel balls were placed into stainless steel vials in a glove box with purified argon (purity of 99.999%). Ball milling was performed at
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room temperature using a planetary ball mill for 2 h with a ball-to-powder weight ratio (BPR) of 20:1 and a rotation speed (RS) of 200 rpm. Al powders were ball milled for 4 h at 10:1 BPR and a RS of 300 rpm, respectively, in order to obtain Al
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particles in the form of flakes with the increased specific surface area providing many
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sites for CNT adhesion. Subsequently, the CNT-Ti power mixtures were added to the flake Al powders, where the content of CNTs and Ti particles were controlled to be 2 wt.% and 0 - 2 wt.%, respectively. The powder mixtures were ball milled for 4 h at a BPR of 10:1 and RS of 300 rpm, respectively. In the next step, the milled powders were heat-treated in a vacuum furnace at 723 K for 1 h to remove the remaining stearic acid. The second step was consolidation of the as-milled powder mixtures by cold compaction into cylinders of Ø 28 mm and height 21 mm using a pressure of 5
ACCEPTED MANUSCRIPT about 250 MPa. The compacted cylinders were sintered at 893 K for 4 h in flowing argon gas. Finally, the sintered bulks were extruded at 873 K to obtain Ø 5 mm composite rods. In addition, 2 wt. % CNT/Al composites without Ti addition, Al-2wt. %
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Ti materials and pure Al were processed under the same route for comparison purpose. 2.3 Characterization
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The microstructures of the as-milled powders and hot-extruded composites were
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characterized by field emission scanning electron microscopy (FESEM, Nova Nano 450) coupled with energy-dispersive analysis (OXFORD, X-Max) and transmission electron microscopy (TEM, FEI G2 F30 S-TWIN). The densities of the hot-extruded samples were measured by the Archimedes method. For tensile tests, the samples
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were cut and polished into dog-bone-shape (indicated in the insert image of Fig. 7a). The gauge length of tensile samples was 16.5 mm, the thickness 2 mm and the width 3 mm, respectively. The tensile tests were carried out using a universal testing
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machine (AG-X 100 kN, Shimadzu) at ambient temperature and a strain rate of 5 ×
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10−4 s−1. At least three specimens were tested for each condition and the average values are reported.
3. Results and discussion 3.1 Morphology of the CNT distribution within the Ti particles Fig. 2 shows FESEM images revealing the morphology of the CNTs-Ti powders. Fig. 2a shows that most of the Ti particles have an irregular shape, and few Ti rods and flake-liked Ti sheets are observed. The very uniform CNTs dispersion within a 6
ACCEPTED MANUSCRIPT flake-like Ti sheet is depicted in Fig. 2b. After 2 h of ball milling, a drastic deformation caused by the constant shearing force of the balls happened, which gradually flatted the Ti particles into sheets. The flake-liked sheets such as Al or Cu
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sheets were proved to be well contacted with CNTs which greatly helps the dispersion [23]. Meanwhile, as a result of cold welding, some small-sized Ti particles with sizes
2b.
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3.2 In situ formed TiC at the CNT-Ti interface
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below 1 µm are attached to the lateral side of the large Ti particles, as shown in Fig.
A uniform dispersion of CNTs that improve the load transfer at the interface has been proven to be a prerequisite for fabricating high-strength composites [10]. To further check the dispersion state of the CNTs within the Ti powders and possible
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formation of TiC on the CNTs, TEM studies were carried out. Fig. 3 presents TEM images of the CNTs-Ti mixed powders. As the result of ball milling, the Ti powder particles were broken into micro-sized (Fig. 3a) or even nano-sized particles (Fig. 3c).
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Fig. 3a shows a typical view of CNTs (marked by the white arrows) incorporated in
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the micro-sized Ti particles, suggesting a good dispersion of CNTs in the Ti particles. These micro-sized Ti particles with good CNTs dispersion were further used as media to link the CNT-Al interface. A TiC nano-layer between the CNTs and the Ti matrix was observed by TEM (Figs. 3b and c), which was confirmed by the Fast Fourier Transform (FFT) patterns displayed in Fig. 3d. The newly in situ formed TiC bonds the CNTs to Ti and further helps to disperse the CNTs. The distance of the lattice fringes (indicated by the yellow dashed lines in Figs. 3e and f) reveals the formation 7
ACCEPTED MANUSCRIPT of two typical TiC carbides with nano-lump (Fig. 3e) and nano-block (Fig. 3f) appearance. The formation mechanism of these two kinds of TiC carbides which is mainly related to the reacted carbon sources has been reported by Saba et. al [20].
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During the milling process, the impacting balls destroy the structure of the CNTs partially and induce defects on the outside walls or tips of the MWCNTs. Due to the severe deformation and fracture of these Ti particles, the atoms on the Ti particles
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become activated and can infiltrate into the CNTs through their surface defects to
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react with the carbon atoms in the defects of the CNTs, which finally generates TiC carbides. These in situ formed TiC nano-carbides can link the closed walls of the CNTs and improve the load-bearing capacity of the inner-walls of the CNTs. 3.3 Morphology of the（CNTs-Ti）/Al composite powders
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The morphology of the as-milled (CNTs-Ti)/Al powders with 2 wt.% Ti is shown in Fig. 4. The homogeneous distribution of CNTs in the Ti particles (indicated by the pale green arrows in Fig. 4b and c) was retained in the (CNTs-Ti)/Al composite
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powders, implying that adding Ti particles can improve the dispersion of the CNTs.
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Fig. 4d reveals that some Al powders also exhibit a relatively good CNT dispersion, indicating that high energy ball milling is effective to break up the residually entangled CNTs. At the same time, the observed Al sheets can also assist uniform dispersion of CNTs.
3.4 Microstructure of bulk composites Combining cold pressing, sintering and hot extrusion yields high relative densities of over 98% for all samples. Fig. 5 shows the microstructure of the extruded 8
ACCEPTED MANUSCRIPT composite rods with 2 wt. % Ti addition. Micro-sized second phases ranging from about 1 to 50 µm with a uniform distribution are clearly observed in Fig. 5a. The EDS line scanning result (Fig. 5d) of the representative area of the particle marked in Fig.
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5b, reveals that the Al content decreases from the matrix (1500 counts per second, cps) to the center of the particle (0 cps), whereas the Ti content increases from 0 cps to about 500 cps at the same time, indicating that the center of the particle consists of Ti.
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Additionally, the characteristic X-ray intensity ratio of Ti and Al is about 1:3 at the
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edge of the particle, which reveals that the edge of this particle may contain TiAl3 phase. In general, the Ti particles are completely surrounded by an in situ formed 2 - 5 µm thick TiAl3 layer with a strong Al-TiAl3 interface (see Fig. 5b). The formation of this TiAl3 layer is attributed to the relatively high temperature (893 K) and long
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sintering time (4 h), which facilitates atomic diffusion and promotes reaction of Ti atoms and Al matrix. Meanwhile, the severe deformation during high energy ball milling can create a large density of dislocations and fine grains at the Ti-Al interface,
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which can further accelerate atomic diffusion and interface reaction. According to the
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Ti-Al phase diagram, it is possible that Ti and Al react with each other and form several intermetallic compounds like TiAl3, TiAl2, TiAl and Ti3Al [24]. Among them, the energy for the nucleation of TiAl3 was demonstrated to be the lowest [25, 26]. The EDS scanning results of S2 (Fig. 5c) also confirm the formation of TiAl3 phase. In addition, as a result of solid state reaction, some of the small-sized Ti particles were totally dissolved. Thus, the well-dispersed CNTs at the Ti-Al interface are wrapped into the TiAl3 phase, and the in situ formed TiAl3 can lock the well-dispersed CNTs in 9
ACCEPTED MANUSCRIPT place by the stable covalent bonds of Ti-Al. This scenario is depicted in the schematic illustration of the Al-CNT-Ti interface in Figs. 5e and f. TiAl3 is well known as a potential thermo-structure composite material due to its low density (3.3 g cm-3)，high
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melting temperature (1614 K)，high hardness (6 GPa) [27]，high Young modulus (about 220 GPa) [28] and good oxidation resistance. As an interfacial product, TiAl3
the CNTs by its strong interfacial bonding.
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3.5 TEM analysis
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sandwiched between CNTs and Al can assist the load transfer from the Al matrix to
To further investigate the strengthening behavior, the microstructure of CNT/Al composites with 2 wt. % Ti addition was examined in more detailed by TEM. Fig. 6a presents a microstructure view of fine Al grains with a size ranging from about 400
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nm to 1.9 µm. The average size calculated from the scattered grains in Fig. 6a is about 1.1 µm, which can enhance the strength of the composites by the effect of fine grain strengthening. Distinct CNT zone along the Al grain boundaries (GB) and some
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modified CNTs (indicated by the white arrow) in the Al matrix were observed in Figs.
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6b and g, respectively. At the same time, Fig. 6f presents the evidence of the CNT defects (indicated by the white arrows and dash lines). Both walls and tips of some CNTs have lost their original intact shapes and present some defects. Those defects were proved to greatly facilitate the formation of TiCs. CNTs modified by two kinds of TiC carbides, i.e. nanoblock and nanolump carbides, are identified by the inserted selected area electron diffraction (SAED) patterns of high-resolution (HR) TEM images in Figs. 6d and h. Besides, both the lattice spacing of the (111) and (200) plane 10
ACCEPTED MANUSCRIPT of TiC were measured and marked around the CNTs, which further indicates the existence of TiCs. To demonstrate the possible shape of the in situ formed TiC, Figs. 6e and i display two corresponding schematic illustrations for the TiCs observed in
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Figs. 6d and h. It is interesting to note that both kinds of TiC play a role in linking the walls of the CNTs, indicating that the modified CNTs retained from the powders greatly enhance the load-bearing capacity of CNTs. Hence, dispersing these CNTs
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modified by nano TiC into the Al matrix contributes to the enhanced strength of the
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CNT/Al composites. Fig. 6c depict a typical formation of Al4C3 around the GB, confirmed by the lattice spacing of 0.83 nm for the (003) atom plane on the bottom left image of Fig. 6c and the SAED patterns (indicated by the area A) inserted to the upper right corner of Fig. 6c. The crucial role of in situ formed Al4C3 is locking the
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CNTs in place by transforming the bonding nature from mechanical bonding to chemical bonding [12]. This improved CNT/Al interfacial bonding due to the formation of Al4C3 significantly enhances the strength of the CNT/Al composites.
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Therefore, one can conclude that the enhanced strength of all the CNT/Al composites
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with or without Ti addition comes from the fine grains, the uniformly dispersed CNTs and the in situ formed Al4C3. With the Ti addition, composites in which the CNTs modified with nano-sized TiC are dispersed can reach superior properties compared to composites without Ti addition. 3.6 Mechanical properties The mechanical properties results of the fabricated materials are shown in Fig. 7. Fig. 7a presents the tensile engineering stress - engineering strain curves of 11
ACCEPTED MANUSCRIPT as-prepared materials. Compared with pure Al, all materials are strengthened with the addition of CNTs or Ti or CNTs and Ti. The yield strength (YS), ultimate tensile strength (UTS) and elongation of the CNT/Al composite without Ti addition were
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measured to be 131 MPa, 167 MPa and 9.4%, respectively. Introducing Ti into the CNT/Al composites improves both the strength and the ductility of the composites. The YS, UTS and the elongation of the composite with 2 wt.% Ti addition are 170
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MPa, 222 MPa and 15.5%, respectively. Fig. 7b shows the reinforced effect of CNTs
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and Ti addition respectively. Calculated from the as-measured results of tensile strength, an improvement of 43 MPa (31%) and 55 MPa (33%) in UTS was acquired while adding Ti into pure Al and CNT/Al composite (compared the curve b to a and curve g to c). Thus, significant enhancement in the materials with or without CNTs
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addition is achieved. Similarly, an improvement of 28 MPa (20%) and 40 MPa (22%) in UTS was acquired while adding CNTs into pure Al and Al-2wt.%Ti (compared the curve c to a and curve g to b). It also indicates that the reinforced effect of CNTs is
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significant with or without Ti addition, which have been reported by numerous studies
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[13, 29, 30]. The UTS increment is 83 MPa when CNTs and Ti were simultaneously added into pure Al, which is higher than the totality of UTS increment (71 MPa) attributed by sole CNTs or Ti addition. Hence, there exists extra synergistic enhancement (12 MPa) resulting from the modified effect of TiCs and the in situ formation of TiAl3 layers at the CNT-Al interfaces. According to the load transfer model for MMCs developed by Kelly and Tyson [31], fibrous CNTs are loaded by the plastically deformed matrix via an interfacial shear 12
ACCEPTED MANUSCRIPT stress (τ) generated along the CNT surface under tension. Compared with CNT/Al composites, the strength enhancement of the (CNTs-Ti)/Al composites in the present study mainly comes from two aspects: (1) introducing TiAl3 layers at the Al-CNT
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interface can lock the well-dispersed CNTs in place, which significantly improves the load transfer ability of CNTs at the interface; (2) the partial formation of TiC on the MWCNTs during ball milling improves the load-bearing contribution of the
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inner-walls of CNTs. Actually, every micro-sized Ti particle first acts as a medium to
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carry the CNTs, and subsequently reacts with the Al matrix and forms an in situ TiAl3 layer wrapping the CNTs at the interface. The key role of these in situ TiAl3 layers is to enwrap the CNTs instead of simple physical bonding of CNTs to Al. Therefore, these Al-CNT/(TiAl3)-Ti hybrid structures can facilitate the load transfer from the
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matrix to the CNTs at the improved CNT-Al interface. Besides, these micro-sized particles including Ti and TiAl3 (see Fig. 5a) can also enhance the strength due to Orowan strengthening. At the same time, the in situ formed nano-sized TiC on the
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CNTs (see Figs. 3e and f, and Figs. 6d and h) exploits the load-bearing of the
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inner-wall of the CNTs, which are dispersed into the Al matrix and assist the load transfer at the CNT-Al interface. With increasing amount of Ti addition, it is expected that the load transfer efficiency also increases with increasing the amounts of CNT-Al interfaces sandwiched by TiAl3 layers and the CNTs modified with TiCs. 4. Conclusions 2 wt.% CNT/Al composites with different weight fractions of Ti addition were fabricated by powder metallurgy. A homogenous distribution of CNTs on the Ti 13
ACCEPTED MANUSCRIPT particles was achieved, suggesting that the addition of Ti can assist CNTs dispersion. Sandwiching in situ formed TiAl3 layers at the Al-Ti interface significantly improves the CNT-Al interfacial bonding by locking the dispersed CNTs in place, thus leading
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to efficient load transfer. Additionally, the in situ formed TiC on CNTs also contributes to the strength enhancement by exerting the load-bearing capacity of the CNTs. Therefore, the addition of Ti can improve the mechanical properties of CNT/Al
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composites. And with the participation of Ti, the strengthening effect of CNTs is more
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significant. The current work provides a new idea to help designing high-performance CNT reinforced metal based composites. Acknowledgments
This work was supported by the National Natural Science Foundation of China
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(grant 51361017, 51401098 and 51301079), the Science Foundation of the Yunnan Provincial Science and Technology Department (grant 2014FC001 and 2017HC033), the Science Foundation of the Yunnan Provincial Education Department (grant
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2016CYH08), the Natural Science Foundation of Kunming University of Science and
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Technology (grant KKSY201551036) and the Science Foundation of Rare and Precious Metals Advanced Materials Collaborative Innovation Center of Yunnan Province (grant KKPT201551005). Additional support through the European Research
Council
under
the
ERC
Advanced
Grant
INTELHYB
(grant
ERC-2013-ADG-340025) is gratefully acknowledged. References [1] B. Guo, M. Song, J. Yi, S. Ni, T. Shen, Y. Du, Improving the mechanical 14
ACCEPTED MANUSCRIPT properties of carbon nanotubes einforced pure aluminum matrix composites by achieving non-equilibrium interface, Materials & Design, 120 (2017) 56-65. [2] C. Li, X. Liu, J. Yi, L. Teng, R. Bao, J. Tan, C. Yang, Z. Zou, Effects of sintering
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parameters on the microstructure and mechanical properties of carbon nanotubes reinforced aluminum matrix composites, Journal of Materials Research, 31 (2016) 3757-3765.
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[3] W. Zhou, T. Yamaguchi, K. Kikuchi, N. Nomura, A. Kawasaki, Effectively
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enhanced load transfer by interfacial reactions in multi-walled carbon nanotube reinforced Al matrix composites, Acta Materialia, 125 (2017) 369-376. [4] A. Maqbool, M.A. Hussain, F.A. Khalid, N. Bakhsh, A. Hussain, M.H. Kim, Mechanical characterization of copper coated carbon nanotubes reinforced
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aluminum matrix composites, Materials Characterization, 86 (2013) 39-48. [5] H. Kwon, M. Estili, K. Takagi, T. Miyazaki, A. Kawasaki, Combination of hot extrusion and spark plasma sintering for producing carbon nanotube reinforced
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aluminum matrix composites, Carbon, 47 (2009) 570-577.
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[6] C. He, N. Zhao, C. Shi, X. Du, J. Li, H. Li, Q. Cui, An Approach to Obtaining Homogeneously Dispersed Carbon Nanotubes in Al Powders for Preparing Reinforced Al-Matrix Composites, Advanced Materials, 19 (2007) 1128-1132.
[7] L. Jiang, Z. Li, G. Fan, L. Cao, D. Zhang, The use of flake powder metallurgy to produce carbon nanotube (CNT)/aluminum composites with a homogenous CNT distribution, Carbon, 50 (2012) 1993-1998. [8] Z.Y. Liu, B.L. Xiao, W.G. Wang, Z.Y. Ma, Developing high-performance 15
ACCEPTED MANUSCRIPT aluminum matrix composites with directionally aligned carbon nanotubes by combining friction stir processing and subsequent rolling, Carbon, 62 (2013) 35-42.
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[9] J. Liu, T. Gottschall, K.P. Skokov, J.D. Moore, O. Gutfleisch, Giant magnetocaloric effect driven by structural transitions, Nature materials, 11 (2012) 620-626.
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[10] B. Chen, K. Kondoh, H. Imai, J. Umeda, M. Takahashi, Simultaneously
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enhancing strength and ductility of carbon nanotube/aluminum composites by improving bonding conditions, Scripta Materialia, 113 (2016) 158-162. [11] S.J. Yoo, S.H. Han, W.J. Kim, Strength and strain hardening of aluminum matrix composites with randomly dispersed nanometer-length fragmented carbon
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nanotubes, Scripta Materialia, 68 (2013) 711-714.
[12] X. Liu, C. Li, J. Eckert, K.G. Prashanth, O. Renk, L. Teng, Y. Liu, R. Bao, J. Tao, T. Shen, J. Yi, Microstructure evolution and mechanical properties of carbon
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nanotubes reinforced Al matrix composites, Materials Characterization, 133
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(2017) 122-132.
[13] J.G. Park, D.H. Keum, Y.H. Lee, Strengthening mechanisms in carbon nanotube-reinforced aluminum composites, Carbon, 95 (2015) 690-698.
[14] O. Carvalho, M. Buciumeanu, S. Madeira, G. Miranda, F.S. Silva, Interface analysis on an eutectic AlSi alloy reinforced with Ni coated MWCNT, Composites Part B: Engineering, 93 (2016) 229-235. [15] M. Jagannatham, S. Sankaran, P. Haridoss, Microstructure and mechanical 16
ACCEPTED MANUSCRIPT behavior of copper coated multiwall carbon nanotubes reinforced aluminum composites, Materials Science and Engineering: A, 638 (2015) 197-207. [16] Q. Zhao, M. Xie, Y. Liu, J. Yi, Improved electroless plating method through
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ultrasonic spray atomization for depositing silver nanoparticles on multi-walled carbon nanotubes, Applied Surface Science, 409 (2017) 164-168.
[17] B. Chen, J. Shen, X. Ye, H. Imai, J. Umeda, M. Takahashi, K. Kondoh,
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Solid-state interfacial reaction and load transfer efficiency in carbon nanotubes
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(CNTs)-reinforced aluminum matrix composites, Carbon, 114 (2017) 198-208. [18] W.W. Zhou, S. Bang, H. Kurita, T. Miyazaki, Y. Fan, A. Kawasaki, Interface and interfacial reactions in multi-walled carbon nanotube-reinforced aluminum matrix composites, Carbon, 96 (2016) 919-928.
(2003) 367-373.
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[19] K. Frisk, A revised thermodynamic description of the Ti–C system, Calphad, 27
[20] F. Saba, S.A. Sajjadi, M. Haddad-Sabzevar, F. Zhang, Formation mechanism of
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nano titanium carbide on multi-walled carbon nanotube and influence of the
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nanocarbides on the load-bearing contribution of the nanotubes inner-walls in aluminum-matrix composites, Carbon, 115 (2017) 720-729.
[21] Y. Su, Q. Ouyang, W. Zhang, Z. Li, Q. Guo, G. Fan, D. Zhang, Composite structure modeling and mechanical behavior of particle reinforced metal matrix composites, Materials Science & Engineering A, 597 (2014) 359-369. [22] B. Guo, S. Ni, R. Shen, M. Song, Fabrication of Ti–Al3Ti core–shell structured particle reinforced Al based composite with promising mechanical properties, 17
ACCEPTED MANUSCRIPT Materials Science and Engineering: A, 639 (2015) 269-273. [23] L. Jiang, G. Fan, Z. Li, X. Kai, D. Zhang, Z. Chen, S. Humphries, G. Heness, W.Y. Yeung, An approach to the uniform dispersion of a high volume fraction of
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carbon nanotubes in aluminum powder, Carbon, 49 (2011) 1965-1971. [24] X.Y. Nie, J.C. Liu, H.X. Li, Q. Du, J.S. Zhang, L.Z. Zhuang, An investigation on bonding mechanism and mechanical properties of Al/Ti compound materials
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prepared by insert moulding, Materials & Design, 63 (2014) 142-150.
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[25] U.R. Kattner, J.C. Lin, Y.A. Chang, Thermodynamic Assessment and Calculation of the Ti-Al System, Metallurgical and Materials Transactions A, 23 (1992) 2081-2090.
[26] J. Liu, Y. Su, Y. Xu, L. Luo, J. Guo, H. Fu, First Phase Selection in Solid Ti/Al
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Diffusion Couple, Rare Metal Materials & Engineering, 40 (2011) 753-756. [27] S.s. He, X.w. Yin, L.t. Zhang, X.m. Li, L.f. Cheng, Ti3AlC2-Al2O3-TiAl3 composite fabricated by reactive melt infiltration, Transactions of Nonferrous
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Metals Society of China, 19 (2009) 1215-1221.
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[28] S.H. Wang, P.W. Kao, C.P. Chang, Stress-strain behavior of fine-grained Al Al3Ti alloys, Scripta Metallurgica Et Materialia, 29 (1993) 323-328.
[29] S.I. Cha, K.T. Kim, S.N. Arshad, C.B. Mo, S.H. Hong, Extraordinary Strengthening Effect of Carbon Nanotubes in Metal‐Matrix Nanocomposites Processed by Molecular ‐ Level Mixing, Advanced Materials, 17 (2005) 1377-1381. [30] S.R. Bakshi, A. Agarwal, An analysis of the factors affecting strengthening in 18
ACCEPTED MANUSCRIPT carbon nanotube reinforced aluminum composites, Carbon, 49 (2011) 533-544. [31] A. Kelly, W.R. Tyson, Tensile properties of fibre reinforced metals—II. Creep of silver-tungsten, Journal of the Mechanics & Physics of Solids, 14 (1966)
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List of figure captions Fig. 1. Morphology of the as-received raw materials: (a) pure Ti particles, and (b) pristine MWCNTs.
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Fig. 2. Morphology of the CNTs-Ti powders after ball milling: (a) shows an overview of CNTs dispersed upon the Ti particles; (b) show a uniform distribution of CNTs dispersed in Ti particles at high magnification.
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Fig. 3. TEM images of the CNTs-Ti powders. (a-c) shows their morphologies. (d) FFT
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pattern of the select area marked in (b). (e) and (f) show two typical in situ formed TiC nanoparticles in the selected areas e and f marked in (c), respectively. Fig. 4. Morphology of the as-milled (CNTs-Ti)/Al powders with 2 wt.% Ti: (a) shows an overview of composite powders; (b-d) show the CNTs dispersed in the composite
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Fig. 5. Microstructure of the extruded CNT/Al composite with 2 wt.% Ti addition. (a) and (c) Two views of the microstructure at different magnification. (b) A
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backscattered electron image. (d) Results of EDS line scanning. (e) and (f) Schematic
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illustration of the Al-Ti interface sandwiched by the TiAl3 layer. Fig. 6. TEM images and schematic illustrations of the extruded CNT/Al composite with 2 wt.% Ti addition. (a) A microstructure view of fine grains. (b) (d) and (g-h) CNTs modified by two kinds of nano-sized TiC dispersed in the Al matrix. (c) A view of Al4C3 formed around the Al grain boundaries with a HRTEM image of the selected area and the SAED pattern of Al4C3. (f) TEM image shows the defects in the walls and at the tips of CNTs. (e) Schematic illustration of the TiC nanoblock formed on the 20
ACCEPTED MANUSCRIPT CNTs. (i) Schematic illustration of the TiC nanolump formed on the CNTs. Fig. 7. Mechanical properties results of the fabricated materials. (a) Tensile engineering stress - engineering strain curves of as-prepared materials. (b) The
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Highlights (CNTs-Ti)/Al composites with a homogenous distribution of CNTs were fabricated.
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Interface bonding was enhanced by sandwiching TiAl3 layers at CNT-Al interface.
The modification with in situ TiC can improve the load-bearing capacity of
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Ti introduction helps to enhance the mechanical properties of CNT/Al
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composites.