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Fabrication and mechanical properties of network structured titanium alloy matrix composites reinforced with Ti2AlC particulates
Materials Science & Engineering A 776 (2020) 139065
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Materials Science & Engineering A journal homepage: http://www.elsevier.com/locate/msea
Fabrication and mechanical properties of network structured titanium alloy matrix composites reinforced with Ti2AlC particulates Faming Zhang *, Maolong Du, Kuowei Fan, Can Ye, Bin Zhang Jiangsu Key Laboratory for Advanced Metallic Materials, School of Materials Science and Engineering, Southeast University, 211189, Nanjing, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Titanium matrix composites Network microstructures MAX phase ceramics Spark plasma sintering Mechanical properties
Network structured Ti-6.5Al–2Zr–1Mo–1V (TA15) matrix composites reinforced with Ti2AlC particles were fabricated by low energy ball milling and subsequent spark plasma sintering (SPS). Microstructures, mechanical properties, and wear resistance properties of the composites were evaluated as a function of Ti2AlC weight fraction. Experimental results showed that the fine Ti2AlC powders were adhered on the surface of the spherical TA15 powders after low-energy milling process and Widmanstatten αþβ microstructure with a network archi tecture was formed after SPS. The Ti2AlC reinforcements were reacted with Ti matrix and formed a Ti3AlC phase at the interface. The hardness, tensile, and compressive strength of the TA15/Ti3AlC composites show significant improvement with less ductility loss. The TA15/1.0 wt% Ti2AlC exhibited tensile yield strength of 929 MPa and ultimate tensile strength of 1013 MPa with ductility of 13.06%. The TA15/Ti3AlC composites had a lower friction coefficient and decreased wear rate with delamination and abrasive wear modes.
1. Introduction A significant amount of attention has been paid to titanium (Ti) metal matrix composites (TMC) due to their high mechanical strength, low density, and corrosion resistance [1–6]. Based on these excellent properties, TMCs are often used in aerospace, petrochemical, and mili tary industries [7–9]. In the early stages of TMC research, the main preparation method was to select reinforcements to strengthen the Ti matrix alloys. It should be noted that different types of Ti matricies have significant effects on the properties of TMCs, according to previous studies [8,9]. TA15 alloy (Ti-6.5Al–2Zr–1Mo–1V) is similar to α-Ti, which is also similar to BT20-Ti first fabricated in Russia. It displays high specific strength, creep resistance, good weldability, and is usually used to fabricate significant components of aero-engines [6,10–12]. Howev er, the conventional TA15 alloys have some disadvantages, including low hardness, poor wear resistance, and thermal-conductivity proper ties. In the past few decades, the hard particulate-reinforced TMCs are one of the significant candidates for fabricating Ti matrix composites to improve their mechanical properties. In previous studies, the re inforcements used usually in TMC are ceramic materials and carbon nanomaterials, such as TiB2 [12–14], TiC [6,10], SiC [15], carbon nanotubes [16], graphene [17,18] and nanodiamonds [19,20] etc. TMCs reinforced with these materials exhibit higher strength, higher elastic
modulus, and wear resistance compared with the Ti matrix. However, it should be pointed out that uniform distribution of reinforcements in the matrix is the most common in the above-mentioned TMCs. Although its hardness, strength, and wear resistance have been greatly improved, the composites with a uniform distribution structure exhibit terrible room temperature damage tolerance. As a new idea of design materials in recent years, network-structured distribution of an inhomogeneous structure can give better ductility than homogeneous structure [21–23]. Typically, the Ti/TiBw composite was fabricated by powder metallurgy and the strength was improved with less ductility loss by the network reinforcement distribution [24]. This is attributed to the in-situ systhesis of TiB whiskers network architecture along the “grain boundaries” of the Ti matrix particles. In this structure, the stronger network boundary phase can strength the composite and the softer matrix can toughen the composites. Mnþ1AXn phases, namely the MAX phases, are novel ternary layered carbides or nitrides. Where M is an early transition metal, A is an A group element, X is C and/or N, and n ¼ 1–3 [25–28]. The MAX phases are often referred as cermets because they combine the excellent prop erties of both metals and ceramics. Therefore, MAX phases have good machinability, excellent electrical and thermal conductivity, and a low friction coefficient [29–32].The MAX phase ceramics have a unique advantage being metal matrix composites reinforcement compared with
* Corresponding author. E-mail address: [email protected] (F. Zhang). https://doi.org/10.1016/j.msea.2020.139065 Received 30 November 2019; Received in revised form 3 February 2020; Accepted 4 February 2020 Available online 5 February 2020 0921-5093/© 2020 Elsevier B.V. All rights reserved.
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Fig. 1. SEM micrographs of the spherical TA15 particles (a), Ti2AlC powders (b), TA15/2%Ti2AlC powder mixtures (c) and EDS point inspection result of the powder mixture (d).
Fig. 2. XRD patterns of the Ti2AlC powders and TA15/Ti2AlC powder mixtures (a), and formation of the TA15/Ti3AlC composites after sintering via SPS.
traditional ceramic particles. MAX phase can form good bonding in terfaces with matrix, which improve the mechanical properties and wear resistance properties [33–41]. Recently, MAX phase has been added into metal matrix composites such as TiAl [35–37], Mg [38,39], Cu [33,40] and other metal matrix composites. Ti2AlC, one kind of the MAX phase, has good self-lubricing machinability, lower vickers hardness, and a higher Elastic modulus [27–31]. Mg/Ti2AlC composite was fabricated by stir casting and hot extrusion methods and the compressive and tensile strength were both improved [38].In addition to above, another example TiAl/Ti2AlC composite has been prepared successfully by spark plasma sintering (SPS). The Ti2AlC phase formed a continuous inter penetrating network in the matrix, which greatly improves the hardness, yield strength and fracture strength of the composite [37]. Nevertheless,
as far as we know, researches on the use of Ti2AlC particle reinforced TA15 alloy matrix composites have never been reported. In general, powder metallurgy is a common method to prepare metal matrix composites [18]. However, compared to the use of hot-press sintering and hot isostatic pressing to consolidate the powders [41, 42], SPS has many advantages for the preparation of composite mate rials with ultra-fine structure, high purity, and high relative density [43]. This is due to the SPS has the characteristics of high heating rate � (up to 600 C/min), high sintering efficiency (effectively destroying the oxide surface layers on powder particles), and short holding time. Therefore, in this study, we utilized a combination of low energy ball milling and the SPS to synthesize bulk TA15/Ti2AlC composites with a novel microstructure consisting of an interpenetrating network of the 2
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Fig. 3. Metallography images of the sintered pure TA15 (a) and TA15/Ti3AlC composites with various Ti2AlC addition: TA15/0.5%Ti2AlC (b), TA15/1.0%Ti2AlC (c), TA15/1.5%Ti2AlC (d), TA15/2.0%Ti2AlC (e) and TA15/15%Ti2AlC (f).
Fig. 4. SEM micrographs of the TA15/2%Ti2AlC (a) with EDS line inspections (b, c) and the TA15/15%Ti2AlC composites (d) with EDS line and point inspections (e-i).
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Fig. 5. TEM micrographs of the TA15/2%Ti2AlC composites at low (a) and high magnifications with inserted SAD pattern confirming the formation of Ti3AlC phase (b).
Ti2AlC phase. Effects of the Ti2AlC particles content on the microstruc tures, mechanical properties and wear resistance properties of the TA15/Ti2AlC composites were investigated systematically.
cylindrical graphite mold with a diameter of 20 mm or 30 mm, and a suitable thickness of boron nitride (h-BN) was sprayed around the mold. Then, the powders were sintered in a vacuum via a spark plasma sin tering system (FCT HP-D5, FCT Systeme GmbH, Germany). The sintering parameters of SPS are kept at 1050 � C for a holding time of 10 min under a pressure of 60 MPa. The final samples were fabricated with the size of Φ20 � 8 mm and Φ30 � 5 mm.
2. Experimental 2.1. Materials
2.3. Characterization of samples
Spherical TA15 (Ti-6.5Al–2Zr–1Mo–1V) powders (99.5% purity) were used as starting material with particle sizes ranging from 100 to 200 μm, which were fabricated by vacuum gas atomization method and purchased from Jiangsu Vilory New Materials Technology Co. Ltd., China. Fig. 1a shows the morphology of the raw TA15 particles. From the figure, we can see that these particles have regular spherical struc ture with an average particle size of 150 μm. For the preparation of Ti2AlC, a mixture of Ti, Al and TiC powders with a molar ratio of 1:1:0.75 is prepared by a 3D mixer [30,44]. Then the powder mixtures were sintered by the SPS under Ar gas atmosphere at a temperature of 1350 � C for 10 min in a pressureless condition. The synthesized bulk Ti2AlC with low relative density was smashed and milled by a high energy planetary ball milling machine to get the powders to the size of 5–40 μm. The SEM images of the synthetic Ti2AlC powders are shown in Fig. 1b. In the magnified image, layered structure of the MAX phase ceramic powders can be seen apparently in the Ti2AlC powders.
The relative densities of the sintered samples were measured ac cording to Archimedes principle. The relative densities of these samples were all above 99%, indicating the sintering parameters selected in the experiment can produce relatively fully dense composite materials. The phase compositions of these samples were identified by X-ray diffraction (XRD, D8 discover, Bruker) with Cu kα monochromatic radiation. To facilitate observation of the microstructure, the polished samples were etched by Kroll’s solution (5%HFþ15% concentrated HNO3þ80%H2O) for 6–8 s. The microstructure of these samples was characterized by optical microscopy (OM, Olympus, BX60 M), field emission scanning electron microscopy (FESEM, Sirion, FEI) equipped with energy dispersive spectrometer (EDS). In order to better understand the microstructure of sintered samples, transmission electron microscopy (TEM, Tecnai, FEI) with selected area electron diffractions (SAD) was also utilized. Hardness of the sintered samples was tested by a micro hardness tester (FM-700, Future-Tech, Japan) with a load of 300 gf and duration of 10 s. A total of 20 indents per samples were made to obtain the average value. The compressive and tensile tests at ambient tem perature were carried out to evaluate the mechanical properties of sin tered samples. The tests were conducted using a universal testing machine (CMT5105, MTS) with a constant strain rate of 0.5 mm/min. Here, the compressive specimens with the size of Φ6 � 8 mm were machined from the sintered samples of Φ20 � 8 mm. Similarly, the tensile specimens with the size of cross section of 2 � 1.5 mm and a gauge length of 10 mm were machined from the sintered sample of Φ30 � 5 mm. In order to ensure the accuracy of the results, compressive and tensile tests of each group specimen were tested five times for statistical analysis of the mechanical properties, and finally the average values were reported. Wear tests were measured via ball pin friction wear testing machine (MPX-3G). Wearing specimens with the size of Φ30 � 2 mm were rubbed by Si3N4 ball whose ball radius is 6.35 mm in 15 N press at the speed of 200 r/min for 20 min with friction radius of 3 mm.
2.2. Fabrication procedure In order to achieve uniform dispersion of the reinforcements onto the spherical Ti alloy particles, the Ti2AlC with various weight fractions of 0.5 wt%, 1.0 wt%, 1.5 wt% and 2.0 wt% was dispersed in alcohol so lution for 15 min using ultrasonication method. Afterwards, the TA15 powders were added into the above solution and dispersed for another 15 min. Finally, the mixture was ball milled by a low energy planetary ball milling machine (QM-3SP2) with ball to powder ratio (BPR) of 10:1 for 5 h at 150 RPM. The powder mixtures were dried in a vacuum oven at 80 � C for 6 h. The weight fractions of 10 wt% and 15 wt% Ti2AlC were chosen to further demonstrate the microstructure of TA15 composites since the reinforcements with low weight fractions are not easy to demonstrate. Fig. 1c exhibits the SEM micrograph of TA15/2 wt% Ti2AlC powder mixture after the ball milling process. It can be observed that the Ti2AlC powders are dispersed uniformly on the surface of TA15 particles. Additionally, the EDS point inspection shows that the TA15 particle is surrounded by the Ti2AlC powders (Fig. 1d). In order to facilitate sintering, the mixed powder prepared above was put into a 4
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TA15/Ti2AlC powder mixtures. A small amount of TiC could be identi fied by the XRD in the Ti2AlC powders. TiC phase is formed during the synthesis process of the Ti2AlC MAX phase ceramic powders and it is difficult to be avoided as reported in previous investigations [27,31,44]. From these XRD pattern of the TA15/Ti2AlC powder mixtures, only Ti phase can be seen when the addition amount of Ti2AlC is less than 2 wt %. However, Ti2AlC and TiC phase can be detected when the addition amount of Ti2AlC exceeding 10 wt%. It is because of the concentration of the Ti2AlC and TiC phases are too small to be detected by the XRD method when the Ti2AlC is present in small amounts. Fig. 2b presents the XRD patterns of the TA15 composites sintered via SPS with various contents of the Ti2AlC. As it can be seen, all the samples after sintering contained two distinct phase structures: α-Ti (HCP) and β-Ti (BCC). It should be noted that the Ti3AlC and the TiC phase in the matrix can be detected when the addition amount of Ti2AlC exceeds 10 wt%. There fore, this indicates that the Ti3AlC phase can be in-situ formed under the sintering parameters selected in the experiments, which also has both metallic and ceramic characteristics, similar to Ti2AlC. The Ti3AlC phase (Ti2AlC þ Ti⇌Ti3AlC) is derived from the in-situ reaction of the Ti2AlC particles with the Ti in the matrix during sintering. The composites can be named as TA15/Ti3AlC composites. The TiC phase in the samples after sintering is more related to the residue during the synthesis of Ti2AlC.Due to the fact that these phases cannot be detected when the addition amount of Ti2AlC is less than 2 wt%, we speculate that the content of Ti3AlC and TiC phases are too small to be detected by the XRD method. Fig. 3 exhibits the metallography images of the spark plasma sintered pure TA15 and TA15/Ti3AlC composites after etching. As shown in Fig. 3a, interlaced distribution of the flake-shaped α-Ti phase and typical €tten [45] structured β-Ti phase can be observed in the sin Widmansta tered samples. The network boundary with reinforcements can be found after adding the Ti2AlC powders into the TA15 matrix (Fig. 3b-f). After increasing the reinforcements, the network boundary becomes thicker and clearer, which shows the interpenetrating network structure of the reinforcements (Fig. 3b-f). According to the results of XRD, it was confirmed that the reinforcements located at the network boundary are Ti3AlC phase which was formed due to the in-situ reaction. The homo geneous network reinforcements in the sintered composites indirectly reflect the uniform distribution of the Ti2AlC on the spherical TA15 particles. When the content of the Ti2AlC increases to 15 wt%, the network boundary becomes extremely thick (Fig. 3f). The composites form a microstructure of quasi-continuous reinforcing skeleton with a network structure decorated in a soft matrix. Therefore, we speculate that this kind of structure will be beneficial to the improvement of the mechanical properties of the composites. In order to further characterize the microstructure of the sintered samples, SEM observation was carried out. Fig. 4 shows the SEM mi crographs of the TA15/2% Ti2AlC and TA15/15% Ti2AlC with EDS point and line inspections. In Fig. 4a, it can be seen that a network-like rein forcement is uniformly distributed around the TA15 matrix which is similar to the results of metallography. However, the composition of matrix and reinforcement phase cannot be clearly identified from the EDS line inspection results (Fig. 4b, c). Therefore, the composites with higher ratio of the Ti2AlC addition are observed to clarify the micro structure. As shown in Fig. 4d, e, the light-colored spherical particles are surrounded by dark-colored network structure and the average diameter of the network is about 150 μm. The EDS line inspection across the matrix and brain boundary looks almost unchanged in Fig. 4b, c because of the low magnifcation with a long distance of line inspection. In ac tuality, the Ti content decreased and the contents of Al and C increased on the grain boundary as indicated in Fig. 4e, f. The EDS point inspection results of corresponding positions are shown in Fig. 4g-i. The EDS analysis (Fig. 4g) reveals that the network reinforcements contained about 34.75 at% of C, 10.32 at% of Al and 54.93 at% of Ti. Similarly, the EDS point inspection results of the matrix (Fig. 4i) contained about 10.51 at% of Al, 1.16 at% of Mo and 88.34 at% of Ti, which can be
Fig. 6. Vickers Micro-hardness (a), tensile stress-strain curves (b) and compressive stress-strain curves (c) of the TA15/Ti3AlC composites with various Ti2AlC addition.
3. Results and discussion 3.1. Microstructures of the TA15 composites with various Ti2AlC addition Fig. 2a shows the XRD patterns of the synthetic Ti2AlC powders and 5
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Table 1 Tensile and compressive properties of the TA15/Ti3AlC composites with various Ti2AlC addition: yield strength (YS, σ0.2), ultimate tensile strength (UTS, σs), ultimate compressive strength (UCS, σcs). Samples
Tensile properties
Pure TA15 TA15–0.5 wt%Ti2AlC TA15–1.0 wt%Ti2AlC TA15–1.5 wt%Ti2AlC TA15–2.0 wt%Ti2AlC
Compressive properties
YS (σ0.2,MPa)
UTS (σs, MPa)
Elongation (ε, %)
YS (σ0.2,MPa)
830 � 908 � 929 � 930 � 951 �
965 � 9 994 � 12 1013 � 3 1026 � 19 1016 � 12
14.023 � 0.995 13.996 � 0.746 13.063 � 0.316 9.390 � 0.462 9.220 � 0.407
1053 � 5 1107 � 17 1147 � 6 1175 � 13 1185 � 11
16 16 10 20 10
UCS (σcs, MPa) 1709 � 13 1764 � 19 1763 � 10 1827 � 5 1830 � 18
Compressive strain (%) 31.146 30.482 30.808 26.624 26.128
� 0.135 � 0.36 � 0.38 � 0.4 � 0.3
Fig. 7. SEM micrographs of the fracture surfaces for the tensile specimens: pure TA15 (a), TA15/0.5%Ti2AlC (b) and TA15/1.5%Ti2AlC (c, d).
determined as the TA15 matrix. The result indicates the higher Al and C concentrations but lower Ti concentrations in the network boundary than those in the matrix because of the presence of MAX phase reinforcements. In order to examine the bonding state between the reinforcement and the matrix, TEM was used as shown in Fig. 5., which shows the TEM micrographs of the TA15/2%Ti2AlC with SAD pattern. It can be seen that the reinforcements between the TA15 matrix are equiaxed grains with sizes in the range of several hundred nanometers in Fig. 5a. In the magnified TEM image of Fig. 5b, it is worth noting that the interfaces between in-situ formed Ti3AlC and TA15 matrix are clean, which indi cating that they formed a tight bond at the interface. In addition, crystal twinned structure is evidenced in the Ti3AlC crystal. The inserted SAD pattern showed that the Ti3AlC phase is a crystal twinned structure. Crystal twinning occurs when two separate crystals share some of the same crystal lattice points in a symmetrical manner. It confirms that the reinforcement produced by the in-situ reaction is Ti3AlC phase. Fig. 6a shows the average micro-hardness of the TA15/Ti3AlC com posites with different content of the Ti2AlC. It can be seen that the hardness has a rising trend with increasing Ti2AlC amount. Compared with the pure TA15, the composites with larger amounts of Ti2AlC powders have higher hardness values. The hardness values raise from
333 HV (pure TA15) to 372 HV (TA15/1.5%Ti2AlC). The reason for this phenomenon can be attributed to the non-uniform distribution of the reinforcements in the composites. That is to say, the hardness value of the reinforcement at the boundary of the matrix particle will be signif icantly higher than that of the matrix. However, it is undeniable that the hardness values of the composites is significantly improved by the Ti3AlC reinforcement formed at the boundary of the matrix grains. This suggests that the addition of Ti2AlC has a great influence on the improvement of hardness. Fig. 6b illustrates the tensile stress-strain curves of the TA15/Ti3AlC composites. It can be seen that the composites can maintain a high plasticity level while significantly improving the yield strength when the addition amount of the Ti2AlC is less than 1.0 wt%. The data of the YS (yield strength, σ0.2), UTS (ultimate tensile strength, σs) and tensile ductility (ε) with increasing of the Ti2AlC content are summarized in Table 1. The YS and UTS of all the composites show an increasing and then decreasing trend with the increase of Ti2AlC content. It should be noted that the YS of the TA15/2.0%Ti2AlC is increased by 14.5% compared with the pure TA15 but its elongation is reduced to 9.22%, which is far less than that of pure TA15 (14.02%). Based on the above analysis, the TA15/Ti3AlC composite exhibits significantly improved strength and substantially maintained plasticity when the addition of 6
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tearing ridge lines are similar to the pure TA15. So the TA15/0.5% Ti2AlC mainly contains ductile fracture and the elongation does not decrease much compared to the pure TA15. However, some tiny rein forced particles can be found in cracks without propagation. Stepped structures can be seen notably in the fractured surface of the TA15/1.5% Ti2AlC (Fig. 7c) and dimples become smaller and shallower, indicating the reduced ductility. Many micro-cracks can be observed in Fig. 7c and these cracks are propagated along the network boundaries. In the magnified SEM image of the crack (Fig. 7d), some tiny particles can be found in the fracture surfaces. Fig. 8 presents friction coefficient and wear rate of the TA15/Ti3AlC composites with different amounts of Ti2AlC. As shown in Fig. 8a, the friction coefficients of the TA15/Ti3AlC composites are all lower than that of pure TA15. This is due to the Ti3AlC MAX phase having selflubriciting properties and a low friction coefficient. The amount of Ti2AlC addition in the TA15 composites is raised up to 2.0 wt% so that the amount of in-situ formed Ti3AlC phase is not overabundant Because of this, the friction coefficients have not decreasedto a detrimental point. Fig. 8b indicates wear rate of the TA15/Ti3AlC composites with various Ti2AlC addition. It was noticed that the wear rate has first a falling and then a rising trend with the increase of Ti2AlC (Fig. 8b). This trend is similar to the hardness curve of the TA15/Ti3AlC composites. The hardness and strength of the TA15 composites with Ti2AlC addition have been increased compared with the pure TA15. This trend corresponds to the improvement of wear resistance.Fig. 9 shows SEM micrographs of the worn surfaces of the pure TA15 and TA15/Ti3AlC composites with various contents of the Ti2AlC. In Fig. 9a, c, e, furrowy shapes and grains are observable indicating that delamination wear and abrasive wear are the main wear modes for the TA15/Ti3AlC composites. The amount of worn particles increased with the rise of Ti2AlC content. The hardness and strength values of the pure TA15 are lower and the ductility is better, which indicates plastic deformation happened in the wearing process. This is confirmed byless worn particles existing in the pure TA15 (Fig. 9b). The hardness and strength of TA15/0.5, 1.0% Ti2AlC is better than the pure TA15, but the ductility is reduced. The worn par ticles and delamination are observable in Fig. 9c-f. The worn particles shown are mainly the MAX phase ceramic reinforcements. The MAX phase particles have good self-lubricity, machinability, and can improve the wear resistance of the TA15 matrix. In this study, the added reinforcement Ti2AlC was reacted with the Ti alloy matrix and formed to be Ti3AlC phase at the interface after SPS processing. The Ti3AlC belongs to the large family of antiperovskite compounds. Wilhelmsson et al. [46] demonstrated that the hardness and Young’s modulus of Ti3AlC were in the same range of other two ternary carbides in the Ti–Al–C system, Ti3AlC2 and Ti2AlC. It is reported [47] that at room temperature, the Vickers hardness of the Ti3AlC is 7.8 GPa; flexural, compressive strength and fracture toughness are 182 MPa, 708 MPa and 2.6 MPa�m1/2; Young’s modulus, shear modulus, bulk modulus and Poisson’s ratio are 209 MPa, 83 MPa, 140 GPa, and 0.25, respec tively. The Ti3AlC has been used as reinforcement for the Ti alloys,e.g., Wu et al. [48] fabricated the (Ti2AlC þ Ti3AlC)–TiAl ceramic-intermetallic laminate (CIL) composite sheets by hot pressing and reaction annealing exhibiting spervior high-temperature tensile properties. In the present study, the hardness, strength and wear resis tance of the TA15/Ti3AlC composites have been improved with acceptable ductility. For the TA15/1.0% Ti2AlC composite, the tensile yield strength (YS) is 929 MPa and the ultimate tensile strength (UTS) is 1013 MPa, which are much higher than those of the pure TA15 (YS: 830 MPa, UTS: 965 MPa). Additionally, its tensile ductility (elongation) of 13.06% is comparable to that of the pure TA15 (14.02%), just a slightly decreasing. The TA15/1.5 wt%Ti2AlC composites show YS of 930 MPa, UTS of 1026 MPa and duclility of 9.39%. As-cast 10 vol% TA15/TiC composites showed UTS of 1048 MPa and ductility of 3.9% and the UTS was increased to be 1120 MPa, but the duclity was reduced to 2.2% after heat treatment [10]. Feng et al. [12,42] reported that the TA15/2.5 vol % TiBw composites fabricated by vacuum hot-pressing sintering had
Fig. 8. Friction coefficient (a) and wear rate (b) of the TA15/Ti3AlC composites with various Ti2AlC addition.
Ti2AlC is less than 1.0 wt%. The compressive stress-strain curves of the TA15/Ti3AlC composites with different content of Ti2AlC are presented in Fig. 6c. Pure TA15 and four groups of TA15/Ti2AlC (0.5%, 1.0%, 1.5%, 2.0%) were analyzed. The compressive yield strengths of the TA15/0.5–2.0% Ti2AlC samples are much higher than that of pure TA15. The relevant compression performance results are summarized in Table 1. It can be seen from Table 1 that the YS of the composites increases gradually while its plasticity decreases gradually with the increase of Ti2AlC content. In particular, the YS of the TA15/2.0%Ti2AlC is 1180 MPa, which is 12.5% higher than that of pure TA15. However, the compressive strain of the TA15/2.0% Ti2AlC is 26.13%, which is 14.52% decrement than that of pure TA15 (31.15%). Therefore, too much reinforcement has a detri mental influence on the ductility of the matrix. Huang et al. [23,24] reported that the Ti/TiBw and Ti6Al4V/TiBw composites with a network structure can improve the strength with less ductility loss in the composites, which exhibited strengthening effect of the novel network structured composites. Accordingly, the good mechanical properties of the TA15/Ti3AlC composites are attributed to the novel network struc ture and the damagetolerance properties of the Ti3AlC reinforcing phase. Fig. 7 exhibits the tensile fractured surfaces of the TA15/Ti3AlC composites with different fractions of Ti2AlC. From fracture surfaces of the pure TA15 (Fig. 7a), the dimples and tearing ridge lines are observed on the surface, which indicates the ductile fracture mode. Fig. 7b shows the fractured surfaces of the TA15/0.5% Ti2AlC. The dimples and 7
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Fig. 9. SEM micrographs of worn surfaces of the pure TA15 (a, b) and the TA15/Ti3AlC composites with various Ti2AlC addition: TA15/1.0%Ti2AlC (c, d) and TA15/ 2.0%Ti2AlC (e, f).
UTS of 1050 MPa with ductility of 5.6% and the same composites fabricated by pre-sintering and canned extrusion showed UTS of 1240 MPa with ductility of 9.7%. The TA15/3.5 vol% TiBw composite with network reinforcement architecture exhibits room temperature UTS of 1100 MPa and ductility of 4.5% [41]. In a comparison with other re inforcements of TiC and TiBW in the TA15 matrix, the TA15/Ti3AlC composites show a smaller UTS but higher ductility due to the MAX phase ceramics combining advantages of metals and ceramics. The Ti3AlC phase has lower hardness (H: 7.8 GPa [47]) and Young’s modulus (E: 140 GPa [47]) than those of the TiC (H: 32 GPa E: 450 GPa [49]) and TiBw (H:18 GPa E:371 GPa [49]) ceramics, but has better fracture toughness and superior thermal shock resistance, as well as high thermal and electrical conductivity. The MAX phase ceramic particulates have promising future for the application of reinforcement in Ti metal matrix composites.
and β-Ti phases, but Ti3AlC and TiC could be detected when the fraction of Ti2AlC is more than 10 wt%. The results of metallography images, SEM micrographs, and TEM micrographs show that Ti3AlC network structured boundary formed is distributed in the interface among TA15 particles. The hardness and strength of the TA15/Ti3AlC composites have a significant increase compared to the pure TA15. When the content of Ti2AlC is lower than 1.0 wt%, the compressive and tensile strength of the TA15/Ti3AlC composites are higher than that of pure TA15 with similar compressive and tensile strains. Low ratio of Ti2AlC can improve the strength without losing of too much ductility. When the content of reinforcement is more than 1.0 wt%, the elongation decreases remark ably. Ti3AlC network structured interface can make the stress transfer from the matrix to the reinforcements with improved hardness, strength and less ductility loss in the Ti composites. Wearing resistance property of the TA15/Ti3AlC composites improves due to their higher hardness, strength, and good self-lubricity and machinability of the MAX phase.
4. Conclusions
Credit author statement
TA15/Ti3AlC composites with network structures were fabricated by ball milling of TA15 with Ti2AlC particles and subsequently sintered by SPS at 1050 � C and 60 MPa for 10 min in vacuum. Ti2AlC phase was reacted with Ti and in-situ formed to be Ti3AlC phase during the sin tering process. XRD patterns of the TA15/Ti3AlC composites present α-Ti
Zhang Faming: Funding acquisition, writing- reviewing and editing. Du Maolong: Investigation, Writing - original draft, Data curation; Formal analysis; Validation. 8
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Fan Kuowei: Methodology. Ye Can & Zhang Bin: Investigation.
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[38] W.B. Yu, H.B. Zhao, X.S. Hu, Anisotropic mechanical and physical properties in textured Ti2AlC reinforced AZ91D magnesium composite, J. Alloys Compd. 732 (2018) 894–901. [39] W.B. Yu, X.J. Wang, H.B. Zhao, C. Ding, Z.Y. Huang, H.X. Zhai, Z.P. Guo, S. M. Xiong, Microstructure, mechanical properties and fracture mechanism of Ti2AlC reinforced AZ91D composites fabricated by stir casting, J. Alloys Compd. 702 (2017) 199–208. [40] W.J. Wang, H.X. Zhai, L.L. Chen, Y. Zhou, Z.Y. Huang, G.P. Bei, P. Greil, Sintering and properties of mechanical alloyed Ti3AlC2-Cu composites, Mater. Sci. Eng., A 685 (2017) 154–158. [41] R. Zhang, D.J. Wang, L.J. Huang, S.J. Yuan, Effects of heat treatment on microstructure and high temperature tensile properties of TiBw/TA15 composite billet with network architecture, Mater. Sci. Eng., A 679 (2017) 314–322. [42] Y.J. Feng, W.C. Zhang, L. Zeng, G.R. Cui, W.Z. Chen, Room-Temperature and hightemperature tensile mechanical properties of TA15 titanium alloy and TiB whiskerreinforced TA15 matrix composites fabricated by vacuum hot-pressing sintering, Materials 10 (2017) 424. [43] O. Guillon, J. Gonzalez-Julian, B. Dargatz, T. Kessel, G. Schierning, J. RaThel, M. Herrmann, Field-assisted sintering technology/spark plasma sintering: mechanisms, materials, and Technology developments, Adv. Eng. Mater. 16 (2014) 830–849. [44] W.B. Zhou, B.C. Mei, J.Q. Zhu, X.L. Hong, Rapid synthesis of Ti2AlC by spark plasma sintering technique, Mater. Lett. 59 (2005) 131–134. [45] J.H. Wang, X.L. Guo, J.N. Qin, D. Zhang, W.J. Lu, Microstructure and mechanical properties of investment casted titanium matrix composites with B4C additions, Mater. Sci. Eng., A 628 (2015) 366–373. [46] O. Wilhelmsson, J.P. Palmquist, E. Lewin, J. Emmerlich, P. Eklund, P.O.A. Persson, H. Hogberg, S. Li, R. Ahujia, O. Eriksson, L. Hultman, U. 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Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The authors gratefully acknowledge the financial support from Na tional Natural Science Foundation of China (No. U1737103), Scientific Research Foundation for the Returned Overseas Chinese Scholars at State Education Ministry (No. 2015-1098), Jiangsu Key Laboratory for Advanced Metallic Materials (No. BM2007204) at Southeast University. We would like to thank Mr. Logan Winston in USA for his English revision of the manuscript. References [1] S.C. Tjong, Y.W. Mai, Processing-structure-property aspects of particulate- and whisker-reinforced titanium matrix composites, Compos. Sci. Technol. 68 (2008) 583–601. [2] F.C. Wang, Z.H. Zhang, Y.J. Sun, Y. Liu, Z.Y. Hu, H. Wang, A.V. Korznikov, E. Korznikova, Z.F. Liu, S. Osamu, Rapid and low temperature spark plasma sintering synthesis of novel carbon nanotube reinforced titanium matrix composites, Carbon 95 (2015) 396–407. [3] Y. Sun, J. Zhang, G.Q. Luo, Q. Shen, L.M. Zhang, Microstructure and mechanical behaviors of titanium matrix composites containing in-situ whiskers synthesized via plasma activated sintering, Materials 11 (2018) 1–11. [4] L. Geng, L.J. Huang, High temperature properties of discontinuously reinforced titanium matrix composites: a review, Acta Metall. Sin. 27 (2014) 787–797. [5] L. Xiao, W.J. Lu, Z.F. Yang, J.N. Qin, D. Zhang, M.M. Wang, F. Zhu, B. Ji, Effect of reinforcements on high temperature mechanical properties of in situ synthesized titanium matrix composites, Mater. Sci. Eng.: Struct. Mater. Prop. Microstruct. Process. 491 (2008) 192–198. [6] D. Liu, S.Q. Zhang, A. Li, Microstructure and tensile properties of laser melting deposited TiC/TA15 titanium matrix composites, J. Alloys Compd. 485 (2009) 156–162. [7] K. Kondoh, T. Threrujirapapong, J. Umeda, B. Fugetsu, High-temperature properties of extruded titanium composites fabricated from carbon nanotubes coated titanium powder by spark plasma sintering and hot extrusion, Compos. Sci. Technol. 72 (2012) 1291–1297. [8] M.D. Hayat, H. Singh, Z. He, P. Cao, Titanium metal matrix composites: an overview, Composites Part A 121 (2019) 418–438. [9] Y. Liu, L.F. Chen, H.P. Tang, C.T. Liu, B. Liu, B.Y. Huang, Design of powder metallurgy titanium alloys and composites, Mater. Sci. Eng. 418 (2006) 25–35. [10] J.Q. Qi, H.W. Wang, C.M. Zou, Z.J. Wei, Influence of matrix characteristics on tensile properties of in situ synthesized TiC/TA15 composite, Mater. Sci. Eng. 553 (2012) 59–66. [11] Q.J. Sun, G.C. Wang, Microstructure and superplasticity of TA15 alloy, Mater. Sci. Eng., A 606 (2014) 401–408. [12] Y.J. Feng, W.C. Zhang, G.R. Cui, W.Z. Chen, Y. Yu, Room temperature tensile fracture characteristics of the oriented TiB whisker reinforced TA15 matrix composites fabricated by pre-sintering and canned extrusion, Mater. Sci. Eng. 707 (2017) 40–43. [13] W.C. Zhang, M.M. Wang, W.Z. Chen, Y.J. Feng, Y. Yu, Evolution of inhomogeneous reinforced structure in TiBw/Ti-6Al-4V composite prepared by pre-sintering and canned extrusion, Mater. Des. 88 (2015) 471–477. [14] L.J. Huang, C.J. Lu, B. Yuan, S.L. Wei, X.P. Cui, L. Geng, Comparative study on superplastic tensile behaviors of the as-extruded Ti6Al4V alloys and TiBw/Ti6Al4V composites with tailored architecture, Mater. Des. 93 (2016) 81–90. [15] S.K. Choi, M. Chandrasekaran, M.J. Brabers, Interaction between titanium and SiC, J. Mater. Sci. 25 (1990) 1957–1964. [16] I.M. Melendez, E. Neubauer, P. Angerer, H. Danninger, J.M. Torralba, Influence of nano-reinforcements on the mechanical properties and microstructure of titanium matrix composites, Compos. Sci. Technol. 71 (2011) 1154–1162. [17] X.J. Zhang, F. Song, Z.P. Wei, W.C. Yang, Z.K. Dai, Microstructural and mechanical characterization of in-situ TiC/Ti titanium matrix composites fabricated by graphene/Ti sintering reaction, Mater. Sci. Eng., A 705 (2017) 153–159. [18] S.C. Tjong, Recent progress in the development and properties of novel metal matrix nanocomposites reinforced with carbon nanotubes and graphene nanosheets, Mater. Sci. Eng. R 74 (2013) 281–350. [19] F.M. Zhang, S.L. Liu, P.P. Zhao, T.F. Liu, J. Sun, Titanium/nanodiamond nanocomposites: effect of nanodiamond on microstructure and mechanical properties of titanium, Mater. Des. (2017) 144–155. [20] Faming Zhang, Tengfei Liu, Nanodiamonds reinforced titanium matrix nanocomposites with network architecture, Composites B 165 (2019) 143–154.
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Materials Science & Engineering A journal homepage: http://www.elsevier.com/locate/msea
Fabrication and mechanical properties of network structured titanium alloy matrix composites reinforced with Ti2AlC particulates Faming Zhang *, Maolong Du, Kuowei Fan, Can Ye, Bin Zhang Jiangsu Key Laboratory for Advanced Metallic Materials, School of Materials Science and Engineering, Southeast University, 211189, Nanjing, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Titanium matrix composites Network microstructures MAX phase ceramics Spark plasma sintering Mechanical properties
Network structured Ti-6.5Al–2Zr–1Mo–1V (TA15) matrix composites reinforced with Ti2AlC particles were fabricated by low energy ball milling and subsequent spark plasma sintering (SPS). Microstructures, mechanical properties, and wear resistance properties of the composites were evaluated as a function of Ti2AlC weight fraction. Experimental results showed that the fine Ti2AlC powders were adhered on the surface of the spherical TA15 powders after low-energy milling process and Widmanstatten αþβ microstructure with a network archi tecture was formed after SPS. The Ti2AlC reinforcements were reacted with Ti matrix and formed a Ti3AlC phase at the interface. The hardness, tensile, and compressive strength of the TA15/Ti3AlC composites show significant improvement with less ductility loss. The TA15/1.0 wt% Ti2AlC exhibited tensile yield strength of 929 MPa and ultimate tensile strength of 1013 MPa with ductility of 13.06%. The TA15/Ti3AlC composites had a lower friction coefficient and decreased wear rate with delamination and abrasive wear modes.
1. Introduction A significant amount of attention has been paid to titanium (Ti) metal matrix composites (TMC) due to their high mechanical strength, low density, and corrosion resistance [1–6]. Based on these excellent properties, TMCs are often used in aerospace, petrochemical, and mili tary industries [7–9]. In the early stages of TMC research, the main preparation method was to select reinforcements to strengthen the Ti matrix alloys. It should be noted that different types of Ti matricies have significant effects on the properties of TMCs, according to previous studies [8,9]. TA15 alloy (Ti-6.5Al–2Zr–1Mo–1V) is similar to α-Ti, which is also similar to BT20-Ti first fabricated in Russia. It displays high specific strength, creep resistance, good weldability, and is usually used to fabricate significant components of aero-engines [6,10–12]. Howev er, the conventional TA15 alloys have some disadvantages, including low hardness, poor wear resistance, and thermal-conductivity proper ties. In the past few decades, the hard particulate-reinforced TMCs are one of the significant candidates for fabricating Ti matrix composites to improve their mechanical properties. In previous studies, the re inforcements used usually in TMC are ceramic materials and carbon nanomaterials, such as TiB2 [12–14], TiC [6,10], SiC [15], carbon nanotubes [16], graphene [17,18] and nanodiamonds [19,20] etc. TMCs reinforced with these materials exhibit higher strength, higher elastic
modulus, and wear resistance compared with the Ti matrix. However, it should be pointed out that uniform distribution of reinforcements in the matrix is the most common in the above-mentioned TMCs. Although its hardness, strength, and wear resistance have been greatly improved, the composites with a uniform distribution structure exhibit terrible room temperature damage tolerance. As a new idea of design materials in recent years, network-structured distribution of an inhomogeneous structure can give better ductility than homogeneous structure [21–23]. Typically, the Ti/TiBw composite was fabricated by powder metallurgy and the strength was improved with less ductility loss by the network reinforcement distribution [24]. This is attributed to the in-situ systhesis of TiB whiskers network architecture along the “grain boundaries” of the Ti matrix particles. In this structure, the stronger network boundary phase can strength the composite and the softer matrix can toughen the composites. Mnþ1AXn phases, namely the MAX phases, are novel ternary layered carbides or nitrides. Where M is an early transition metal, A is an A group element, X is C and/or N, and n ¼ 1–3 [25–28]. The MAX phases are often referred as cermets because they combine the excellent prop erties of both metals and ceramics. Therefore, MAX phases have good machinability, excellent electrical and thermal conductivity, and a low friction coefficient [29–32].The MAX phase ceramics have a unique advantage being metal matrix composites reinforcement compared with
* Corresponding author. E-mail address: [email protected] (F. Zhang). https://doi.org/10.1016/j.msea.2020.139065 Received 30 November 2019; Received in revised form 3 February 2020; Accepted 4 February 2020 Available online 5 February 2020 0921-5093/© 2020 Elsevier B.V. All rights reserved.
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Fig. 1. SEM micrographs of the spherical TA15 particles (a), Ti2AlC powders (b), TA15/2%Ti2AlC powder mixtures (c) and EDS point inspection result of the powder mixture (d).
Fig. 2. XRD patterns of the Ti2AlC powders and TA15/Ti2AlC powder mixtures (a), and formation of the TA15/Ti3AlC composites after sintering via SPS.
traditional ceramic particles. MAX phase can form good bonding in terfaces with matrix, which improve the mechanical properties and wear resistance properties [33–41]. Recently, MAX phase has been added into metal matrix composites such as TiAl [35–37], Mg [38,39], Cu [33,40] and other metal matrix composites. Ti2AlC, one kind of the MAX phase, has good self-lubricing machinability, lower vickers hardness, and a higher Elastic modulus [27–31]. Mg/Ti2AlC composite was fabricated by stir casting and hot extrusion methods and the compressive and tensile strength were both improved [38].In addition to above, another example TiAl/Ti2AlC composite has been prepared successfully by spark plasma sintering (SPS). The Ti2AlC phase formed a continuous inter penetrating network in the matrix, which greatly improves the hardness, yield strength and fracture strength of the composite [37]. Nevertheless,
as far as we know, researches on the use of Ti2AlC particle reinforced TA15 alloy matrix composites have never been reported. In general, powder metallurgy is a common method to prepare metal matrix composites [18]. However, compared to the use of hot-press sintering and hot isostatic pressing to consolidate the powders [41, 42], SPS has many advantages for the preparation of composite mate rials with ultra-fine structure, high purity, and high relative density [43]. This is due to the SPS has the characteristics of high heating rate � (up to 600 C/min), high sintering efficiency (effectively destroying the oxide surface layers on powder particles), and short holding time. Therefore, in this study, we utilized a combination of low energy ball milling and the SPS to synthesize bulk TA15/Ti2AlC composites with a novel microstructure consisting of an interpenetrating network of the 2
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Fig. 3. Metallography images of the sintered pure TA15 (a) and TA15/Ti3AlC composites with various Ti2AlC addition: TA15/0.5%Ti2AlC (b), TA15/1.0%Ti2AlC (c), TA15/1.5%Ti2AlC (d), TA15/2.0%Ti2AlC (e) and TA15/15%Ti2AlC (f).
Fig. 4. SEM micrographs of the TA15/2%Ti2AlC (a) with EDS line inspections (b, c) and the TA15/15%Ti2AlC composites (d) with EDS line and point inspections (e-i).
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Fig. 5. TEM micrographs of the TA15/2%Ti2AlC composites at low (a) and high magnifications with inserted SAD pattern confirming the formation of Ti3AlC phase (b).
Ti2AlC phase. Effects of the Ti2AlC particles content on the microstruc tures, mechanical properties and wear resistance properties of the TA15/Ti2AlC composites were investigated systematically.
cylindrical graphite mold with a diameter of 20 mm or 30 mm, and a suitable thickness of boron nitride (h-BN) was sprayed around the mold. Then, the powders were sintered in a vacuum via a spark plasma sin tering system (FCT HP-D5, FCT Systeme GmbH, Germany). The sintering parameters of SPS are kept at 1050 � C for a holding time of 10 min under a pressure of 60 MPa. The final samples were fabricated with the size of Φ20 � 8 mm and Φ30 � 5 mm.
2. Experimental 2.1. Materials
2.3. Characterization of samples
Spherical TA15 (Ti-6.5Al–2Zr–1Mo–1V) powders (99.5% purity) were used as starting material with particle sizes ranging from 100 to 200 μm, which were fabricated by vacuum gas atomization method and purchased from Jiangsu Vilory New Materials Technology Co. Ltd., China. Fig. 1a shows the morphology of the raw TA15 particles. From the figure, we can see that these particles have regular spherical struc ture with an average particle size of 150 μm. For the preparation of Ti2AlC, a mixture of Ti, Al and TiC powders with a molar ratio of 1:1:0.75 is prepared by a 3D mixer [30,44]. Then the powder mixtures were sintered by the SPS under Ar gas atmosphere at a temperature of 1350 � C for 10 min in a pressureless condition. The synthesized bulk Ti2AlC with low relative density was smashed and milled by a high energy planetary ball milling machine to get the powders to the size of 5–40 μm. The SEM images of the synthetic Ti2AlC powders are shown in Fig. 1b. In the magnified image, layered structure of the MAX phase ceramic powders can be seen apparently in the Ti2AlC powders.
The relative densities of the sintered samples were measured ac cording to Archimedes principle. The relative densities of these samples were all above 99%, indicating the sintering parameters selected in the experiment can produce relatively fully dense composite materials. The phase compositions of these samples were identified by X-ray diffraction (XRD, D8 discover, Bruker) with Cu kα monochromatic radiation. To facilitate observation of the microstructure, the polished samples were etched by Kroll’s solution (5%HFþ15% concentrated HNO3þ80%H2O) for 6–8 s. The microstructure of these samples was characterized by optical microscopy (OM, Olympus, BX60 M), field emission scanning electron microscopy (FESEM, Sirion, FEI) equipped with energy dispersive spectrometer (EDS). In order to better understand the microstructure of sintered samples, transmission electron microscopy (TEM, Tecnai, FEI) with selected area electron diffractions (SAD) was also utilized. Hardness of the sintered samples was tested by a micro hardness tester (FM-700, Future-Tech, Japan) with a load of 300 gf and duration of 10 s. A total of 20 indents per samples were made to obtain the average value. The compressive and tensile tests at ambient tem perature were carried out to evaluate the mechanical properties of sin tered samples. The tests were conducted using a universal testing machine (CMT5105, MTS) with a constant strain rate of 0.5 mm/min. Here, the compressive specimens with the size of Φ6 � 8 mm were machined from the sintered samples of Φ20 � 8 mm. Similarly, the tensile specimens with the size of cross section of 2 � 1.5 mm and a gauge length of 10 mm were machined from the sintered sample of Φ30 � 5 mm. In order to ensure the accuracy of the results, compressive and tensile tests of each group specimen were tested five times for statistical analysis of the mechanical properties, and finally the average values were reported. Wear tests were measured via ball pin friction wear testing machine (MPX-3G). Wearing specimens with the size of Φ30 � 2 mm were rubbed by Si3N4 ball whose ball radius is 6.35 mm in 15 N press at the speed of 200 r/min for 20 min with friction radius of 3 mm.
2.2. Fabrication procedure In order to achieve uniform dispersion of the reinforcements onto the spherical Ti alloy particles, the Ti2AlC with various weight fractions of 0.5 wt%, 1.0 wt%, 1.5 wt% and 2.0 wt% was dispersed in alcohol so lution for 15 min using ultrasonication method. Afterwards, the TA15 powders were added into the above solution and dispersed for another 15 min. Finally, the mixture was ball milled by a low energy planetary ball milling machine (QM-3SP2) with ball to powder ratio (BPR) of 10:1 for 5 h at 150 RPM. The powder mixtures were dried in a vacuum oven at 80 � C for 6 h. The weight fractions of 10 wt% and 15 wt% Ti2AlC were chosen to further demonstrate the microstructure of TA15 composites since the reinforcements with low weight fractions are not easy to demonstrate. Fig. 1c exhibits the SEM micrograph of TA15/2 wt% Ti2AlC powder mixture after the ball milling process. It can be observed that the Ti2AlC powders are dispersed uniformly on the surface of TA15 particles. Additionally, the EDS point inspection shows that the TA15 particle is surrounded by the Ti2AlC powders (Fig. 1d). In order to facilitate sintering, the mixed powder prepared above was put into a 4
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TA15/Ti2AlC powder mixtures. A small amount of TiC could be identi fied by the XRD in the Ti2AlC powders. TiC phase is formed during the synthesis process of the Ti2AlC MAX phase ceramic powders and it is difficult to be avoided as reported in previous investigations [27,31,44]. From these XRD pattern of the TA15/Ti2AlC powder mixtures, only Ti phase can be seen when the addition amount of Ti2AlC is less than 2 wt %. However, Ti2AlC and TiC phase can be detected when the addition amount of Ti2AlC exceeding 10 wt%. It is because of the concentration of the Ti2AlC and TiC phases are too small to be detected by the XRD method when the Ti2AlC is present in small amounts. Fig. 2b presents the XRD patterns of the TA15 composites sintered via SPS with various contents of the Ti2AlC. As it can be seen, all the samples after sintering contained two distinct phase structures: α-Ti (HCP) and β-Ti (BCC). It should be noted that the Ti3AlC and the TiC phase in the matrix can be detected when the addition amount of Ti2AlC exceeds 10 wt%. There fore, this indicates that the Ti3AlC phase can be in-situ formed under the sintering parameters selected in the experiments, which also has both metallic and ceramic characteristics, similar to Ti2AlC. The Ti3AlC phase (Ti2AlC þ Ti⇌Ti3AlC) is derived from the in-situ reaction of the Ti2AlC particles with the Ti in the matrix during sintering. The composites can be named as TA15/Ti3AlC composites. The TiC phase in the samples after sintering is more related to the residue during the synthesis of Ti2AlC.Due to the fact that these phases cannot be detected when the addition amount of Ti2AlC is less than 2 wt%, we speculate that the content of Ti3AlC and TiC phases are too small to be detected by the XRD method. Fig. 3 exhibits the metallography images of the spark plasma sintered pure TA15 and TA15/Ti3AlC composites after etching. As shown in Fig. 3a, interlaced distribution of the flake-shaped α-Ti phase and typical €tten [45] structured β-Ti phase can be observed in the sin Widmansta tered samples. The network boundary with reinforcements can be found after adding the Ti2AlC powders into the TA15 matrix (Fig. 3b-f). After increasing the reinforcements, the network boundary becomes thicker and clearer, which shows the interpenetrating network structure of the reinforcements (Fig. 3b-f). According to the results of XRD, it was confirmed that the reinforcements located at the network boundary are Ti3AlC phase which was formed due to the in-situ reaction. The homo geneous network reinforcements in the sintered composites indirectly reflect the uniform distribution of the Ti2AlC on the spherical TA15 particles. When the content of the Ti2AlC increases to 15 wt%, the network boundary becomes extremely thick (Fig. 3f). The composites form a microstructure of quasi-continuous reinforcing skeleton with a network structure decorated in a soft matrix. Therefore, we speculate that this kind of structure will be beneficial to the improvement of the mechanical properties of the composites. In order to further characterize the microstructure of the sintered samples, SEM observation was carried out. Fig. 4 shows the SEM mi crographs of the TA15/2% Ti2AlC and TA15/15% Ti2AlC with EDS point and line inspections. In Fig. 4a, it can be seen that a network-like rein forcement is uniformly distributed around the TA15 matrix which is similar to the results of metallography. However, the composition of matrix and reinforcement phase cannot be clearly identified from the EDS line inspection results (Fig. 4b, c). Therefore, the composites with higher ratio of the Ti2AlC addition are observed to clarify the micro structure. As shown in Fig. 4d, e, the light-colored spherical particles are surrounded by dark-colored network structure and the average diameter of the network is about 150 μm. The EDS line inspection across the matrix and brain boundary looks almost unchanged in Fig. 4b, c because of the low magnifcation with a long distance of line inspection. In ac tuality, the Ti content decreased and the contents of Al and C increased on the grain boundary as indicated in Fig. 4e, f. The EDS point inspection results of corresponding positions are shown in Fig. 4g-i. The EDS analysis (Fig. 4g) reveals that the network reinforcements contained about 34.75 at% of C, 10.32 at% of Al and 54.93 at% of Ti. Similarly, the EDS point inspection results of the matrix (Fig. 4i) contained about 10.51 at% of Al, 1.16 at% of Mo and 88.34 at% of Ti, which can be
Fig. 6. Vickers Micro-hardness (a), tensile stress-strain curves (b) and compressive stress-strain curves (c) of the TA15/Ti3AlC composites with various Ti2AlC addition.
3. Results and discussion 3.1. Microstructures of the TA15 composites with various Ti2AlC addition Fig. 2a shows the XRD patterns of the synthetic Ti2AlC powders and 5
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Table 1 Tensile and compressive properties of the TA15/Ti3AlC composites with various Ti2AlC addition: yield strength (YS, σ0.2), ultimate tensile strength (UTS, σs), ultimate compressive strength (UCS, σcs). Samples
Tensile properties
Pure TA15 TA15–0.5 wt%Ti2AlC TA15–1.0 wt%Ti2AlC TA15–1.5 wt%Ti2AlC TA15–2.0 wt%Ti2AlC
Compressive properties
YS (σ0.2,MPa)
UTS (σs, MPa)
Elongation (ε, %)
YS (σ0.2,MPa)
830 � 908 � 929 � 930 � 951 �
965 � 9 994 � 12 1013 � 3 1026 � 19 1016 � 12
14.023 � 0.995 13.996 � 0.746 13.063 � 0.316 9.390 � 0.462 9.220 � 0.407
1053 � 5 1107 � 17 1147 � 6 1175 � 13 1185 � 11
16 16 10 20 10
UCS (σcs, MPa) 1709 � 13 1764 � 19 1763 � 10 1827 � 5 1830 � 18
Compressive strain (%) 31.146 30.482 30.808 26.624 26.128
� 0.135 � 0.36 � 0.38 � 0.4 � 0.3
Fig. 7. SEM micrographs of the fracture surfaces for the tensile specimens: pure TA15 (a), TA15/0.5%Ti2AlC (b) and TA15/1.5%Ti2AlC (c, d).
determined as the TA15 matrix. The result indicates the higher Al and C concentrations but lower Ti concentrations in the network boundary than those in the matrix because of the presence of MAX phase reinforcements. In order to examine the bonding state between the reinforcement and the matrix, TEM was used as shown in Fig. 5., which shows the TEM micrographs of the TA15/2%Ti2AlC with SAD pattern. It can be seen that the reinforcements between the TA15 matrix are equiaxed grains with sizes in the range of several hundred nanometers in Fig. 5a. In the magnified TEM image of Fig. 5b, it is worth noting that the interfaces between in-situ formed Ti3AlC and TA15 matrix are clean, which indi cating that they formed a tight bond at the interface. In addition, crystal twinned structure is evidenced in the Ti3AlC crystal. The inserted SAD pattern showed that the Ti3AlC phase is a crystal twinned structure. Crystal twinning occurs when two separate crystals share some of the same crystal lattice points in a symmetrical manner. It confirms that the reinforcement produced by the in-situ reaction is Ti3AlC phase. Fig. 6a shows the average micro-hardness of the TA15/Ti3AlC com posites with different content of the Ti2AlC. It can be seen that the hardness has a rising trend with increasing Ti2AlC amount. Compared with the pure TA15, the composites with larger amounts of Ti2AlC powders have higher hardness values. The hardness values raise from
333 HV (pure TA15) to 372 HV (TA15/1.5%Ti2AlC). The reason for this phenomenon can be attributed to the non-uniform distribution of the reinforcements in the composites. That is to say, the hardness value of the reinforcement at the boundary of the matrix particle will be signif icantly higher than that of the matrix. However, it is undeniable that the hardness values of the composites is significantly improved by the Ti3AlC reinforcement formed at the boundary of the matrix grains. This suggests that the addition of Ti2AlC has a great influence on the improvement of hardness. Fig. 6b illustrates the tensile stress-strain curves of the TA15/Ti3AlC composites. It can be seen that the composites can maintain a high plasticity level while significantly improving the yield strength when the addition amount of the Ti2AlC is less than 1.0 wt%. The data of the YS (yield strength, σ0.2), UTS (ultimate tensile strength, σs) and tensile ductility (ε) with increasing of the Ti2AlC content are summarized in Table 1. The YS and UTS of all the composites show an increasing and then decreasing trend with the increase of Ti2AlC content. It should be noted that the YS of the TA15/2.0%Ti2AlC is increased by 14.5% compared with the pure TA15 but its elongation is reduced to 9.22%, which is far less than that of pure TA15 (14.02%). Based on the above analysis, the TA15/Ti3AlC composite exhibits significantly improved strength and substantially maintained plasticity when the addition of 6
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tearing ridge lines are similar to the pure TA15. So the TA15/0.5% Ti2AlC mainly contains ductile fracture and the elongation does not decrease much compared to the pure TA15. However, some tiny rein forced particles can be found in cracks without propagation. Stepped structures can be seen notably in the fractured surface of the TA15/1.5% Ti2AlC (Fig. 7c) and dimples become smaller and shallower, indicating the reduced ductility. Many micro-cracks can be observed in Fig. 7c and these cracks are propagated along the network boundaries. In the magnified SEM image of the crack (Fig. 7d), some tiny particles can be found in the fracture surfaces. Fig. 8 presents friction coefficient and wear rate of the TA15/Ti3AlC composites with different amounts of Ti2AlC. As shown in Fig. 8a, the friction coefficients of the TA15/Ti3AlC composites are all lower than that of pure TA15. This is due to the Ti3AlC MAX phase having selflubriciting properties and a low friction coefficient. The amount of Ti2AlC addition in the TA15 composites is raised up to 2.0 wt% so that the amount of in-situ formed Ti3AlC phase is not overabundant Because of this, the friction coefficients have not decreasedto a detrimental point. Fig. 8b indicates wear rate of the TA15/Ti3AlC composites with various Ti2AlC addition. It was noticed that the wear rate has first a falling and then a rising trend with the increase of Ti2AlC (Fig. 8b). This trend is similar to the hardness curve of the TA15/Ti3AlC composites. The hardness and strength of the TA15 composites with Ti2AlC addition have been increased compared with the pure TA15. This trend corresponds to the improvement of wear resistance.Fig. 9 shows SEM micrographs of the worn surfaces of the pure TA15 and TA15/Ti3AlC composites with various contents of the Ti2AlC. In Fig. 9a, c, e, furrowy shapes and grains are observable indicating that delamination wear and abrasive wear are the main wear modes for the TA15/Ti3AlC composites. The amount of worn particles increased with the rise of Ti2AlC content. The hardness and strength values of the pure TA15 are lower and the ductility is better, which indicates plastic deformation happened in the wearing process. This is confirmed byless worn particles existing in the pure TA15 (Fig. 9b). The hardness and strength of TA15/0.5, 1.0% Ti2AlC is better than the pure TA15, but the ductility is reduced. The worn par ticles and delamination are observable in Fig. 9c-f. The worn particles shown are mainly the MAX phase ceramic reinforcements. The MAX phase particles have good self-lubricity, machinability, and can improve the wear resistance of the TA15 matrix. In this study, the added reinforcement Ti2AlC was reacted with the Ti alloy matrix and formed to be Ti3AlC phase at the interface after SPS processing. The Ti3AlC belongs to the large family of antiperovskite compounds. Wilhelmsson et al. [46] demonstrated that the hardness and Young’s modulus of Ti3AlC were in the same range of other two ternary carbides in the Ti–Al–C system, Ti3AlC2 and Ti2AlC. It is reported [47] that at room temperature, the Vickers hardness of the Ti3AlC is 7.8 GPa; flexural, compressive strength and fracture toughness are 182 MPa, 708 MPa and 2.6 MPa�m1/2; Young’s modulus, shear modulus, bulk modulus and Poisson’s ratio are 209 MPa, 83 MPa, 140 GPa, and 0.25, respec tively. The Ti3AlC has been used as reinforcement for the Ti alloys,e.g., Wu et al. [48] fabricated the (Ti2AlC þ Ti3AlC)–TiAl ceramic-intermetallic laminate (CIL) composite sheets by hot pressing and reaction annealing exhibiting spervior high-temperature tensile properties. In the present study, the hardness, strength and wear resis tance of the TA15/Ti3AlC composites have been improved with acceptable ductility. For the TA15/1.0% Ti2AlC composite, the tensile yield strength (YS) is 929 MPa and the ultimate tensile strength (UTS) is 1013 MPa, which are much higher than those of the pure TA15 (YS: 830 MPa, UTS: 965 MPa). Additionally, its tensile ductility (elongation) of 13.06% is comparable to that of the pure TA15 (14.02%), just a slightly decreasing. The TA15/1.5 wt%Ti2AlC composites show YS of 930 MPa, UTS of 1026 MPa and duclility of 9.39%. As-cast 10 vol% TA15/TiC composites showed UTS of 1048 MPa and ductility of 3.9% and the UTS was increased to be 1120 MPa, but the duclity was reduced to 2.2% after heat treatment [10]. Feng et al. [12,42] reported that the TA15/2.5 vol % TiBw composites fabricated by vacuum hot-pressing sintering had
Fig. 8. Friction coefficient (a) and wear rate (b) of the TA15/Ti3AlC composites with various Ti2AlC addition.
Ti2AlC is less than 1.0 wt%. The compressive stress-strain curves of the TA15/Ti3AlC composites with different content of Ti2AlC are presented in Fig. 6c. Pure TA15 and four groups of TA15/Ti2AlC (0.5%, 1.0%, 1.5%, 2.0%) were analyzed. The compressive yield strengths of the TA15/0.5–2.0% Ti2AlC samples are much higher than that of pure TA15. The relevant compression performance results are summarized in Table 1. It can be seen from Table 1 that the YS of the composites increases gradually while its plasticity decreases gradually with the increase of Ti2AlC content. In particular, the YS of the TA15/2.0%Ti2AlC is 1180 MPa, which is 12.5% higher than that of pure TA15. However, the compressive strain of the TA15/2.0% Ti2AlC is 26.13%, which is 14.52% decrement than that of pure TA15 (31.15%). Therefore, too much reinforcement has a detri mental influence on the ductility of the matrix. Huang et al. [23,24] reported that the Ti/TiBw and Ti6Al4V/TiBw composites with a network structure can improve the strength with less ductility loss in the composites, which exhibited strengthening effect of the novel network structured composites. Accordingly, the good mechanical properties of the TA15/Ti3AlC composites are attributed to the novel network struc ture and the damagetolerance properties of the Ti3AlC reinforcing phase. Fig. 7 exhibits the tensile fractured surfaces of the TA15/Ti3AlC composites with different fractions of Ti2AlC. From fracture surfaces of the pure TA15 (Fig. 7a), the dimples and tearing ridge lines are observed on the surface, which indicates the ductile fracture mode. Fig. 7b shows the fractured surfaces of the TA15/0.5% Ti2AlC. The dimples and 7
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Fig. 9. SEM micrographs of worn surfaces of the pure TA15 (a, b) and the TA15/Ti3AlC composites with various Ti2AlC addition: TA15/1.0%Ti2AlC (c, d) and TA15/ 2.0%Ti2AlC (e, f).
UTS of 1050 MPa with ductility of 5.6% and the same composites fabricated by pre-sintering and canned extrusion showed UTS of 1240 MPa with ductility of 9.7%. The TA15/3.5 vol% TiBw composite with network reinforcement architecture exhibits room temperature UTS of 1100 MPa and ductility of 4.5% [41]. In a comparison with other re inforcements of TiC and TiBW in the TA15 matrix, the TA15/Ti3AlC composites show a smaller UTS but higher ductility due to the MAX phase ceramics combining advantages of metals and ceramics. The Ti3AlC phase has lower hardness (H: 7.8 GPa [47]) and Young’s modulus (E: 140 GPa [47]) than those of the TiC (H: 32 GPa E: 450 GPa [49]) and TiBw (H:18 GPa E:371 GPa [49]) ceramics, but has better fracture toughness and superior thermal shock resistance, as well as high thermal and electrical conductivity. The MAX phase ceramic particulates have promising future for the application of reinforcement in Ti metal matrix composites.
and β-Ti phases, but Ti3AlC and TiC could be detected when the fraction of Ti2AlC is more than 10 wt%. The results of metallography images, SEM micrographs, and TEM micrographs show that Ti3AlC network structured boundary formed is distributed in the interface among TA15 particles. The hardness and strength of the TA15/Ti3AlC composites have a significant increase compared to the pure TA15. When the content of Ti2AlC is lower than 1.0 wt%, the compressive and tensile strength of the TA15/Ti3AlC composites are higher than that of pure TA15 with similar compressive and tensile strains. Low ratio of Ti2AlC can improve the strength without losing of too much ductility. When the content of reinforcement is more than 1.0 wt%, the elongation decreases remark ably. Ti3AlC network structured interface can make the stress transfer from the matrix to the reinforcements with improved hardness, strength and less ductility loss in the Ti composites. Wearing resistance property of the TA15/Ti3AlC composites improves due to their higher hardness, strength, and good self-lubricity and machinability of the MAX phase.
4. Conclusions
Credit author statement
TA15/Ti3AlC composites with network structures were fabricated by ball milling of TA15 with Ti2AlC particles and subsequently sintered by SPS at 1050 � C and 60 MPa for 10 min in vacuum. Ti2AlC phase was reacted with Ti and in-situ formed to be Ti3AlC phase during the sin tering process. XRD patterns of the TA15/Ti3AlC composites present α-Ti
Zhang Faming: Funding acquisition, writing- reviewing and editing. Du Maolong: Investigation, Writing - original draft, Data curation; Formal analysis; Validation. 8
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Fan Kuowei: Methodology. Ye Can & Zhang Bin: Investigation.
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