| TiAl intermetallic compound has excellent performance such as high melting point, low density, outstanding heat conductivity, good oxidation resistance under high temperature, which is considered as promising structural materials for high temperature applications and has been the research hotspot at present. However, the engineering application of TiAl is severely restricted due to its low fracture toughness,poor compressive strength and high friction coefficient, as well as poor high temperature oxidation resistance above 800?. In recent years, MAX phase with layered ternary has been extensively studied. As a member of MAX, Ti2AlC has metal properties of good thermal,electrical conductivity, workability and thermal shock resistance. At same times, it also has ceramic properties of high melting point, high strength, thermal stability and high temperature oxidation resistance. Made full use of advantages of intermetallics and reinforcements, the composite of Ti2AlC strengthening phase in TiAl matrix can be expected to a new type of composite material, which simultaneously has high strength, wear resistance, antifriction, thermal stabilities.In this paper,TiAl-Ti2AlC-G and TiAl-Ti2AlC-TiB2 were fabricated via spark plasma sintering (SPS) and vacuum arc melting,with Ti,Al,graphite (G) and boron carbide (B4C) as raw powders. G and B4C can be used as different sources of carbon.Meanwhile, G play a role in solid lubrication function and B4C can provide boron for TiB2. The phase constitution, microstructure and composition distribution of composites were investigated. Friction and wear properties, mechanical properties,and oxidation resistance properties of composites were analyzed. Synergy strengthen and toughen mechanism of Ti2AlC, G,TiB2 was put forward. The wear resistance,anti-friction properties and oxidation resistance of the composites were studied. The results are as follows:(1) TiAl-Ti2AlC-G composites were synthesized by SPS using graphite as the source of carbon. With the increasing of graphite contents, TiAl grains are refined,Ti2AlC and TiC synthesized in-situ enhance the microhardness of composites. Because graphite dissever the matrix, the compressive strength and plasticity of composites are reduced. Ti2AlC-G multilayer sheet-shape grains improve the wear performance by the effection of increasing hardness, bearing load, stretching, solid lubrication, pinning.The wear mechanism of materials are abrasion,oxidation and adhesion wear mechanism. Material transfer occurs between the composites and grinding ball.(2) TiAl-Ti2AlC-TiB2 composites were synthesized by vacuum arc melting using B4C powders as the sources of carbon and boron. With the increasing of B4C contents,the morphologies and sizes of Ti2AIC and TiB2 particles synthesized in-situ are changed. TiAl grains are refined greatly. Ti2AIC is considered to be synthesized from the peritectic reaction between TiC and Ti-Al melt. The reinforcing mechanisms of the Ti2AlC and TiB2 include crack deflection,pinning and pull-out,which is benefit for the strengthening and ductility of TiAl-Ti2AlC-TiB2 composites. Meanwhile, the friction coefficient of composites are reduced by stress transfer and crack propagation between reinforcements and matrix. However, third-body wear caused by hard particles increased the materials' wear loss in the wear process. Compared to TiAl alloy,the oxidation resistance of TiAl-Ti2AlC-TiB2 composites have been improved significantly. Composites show good protective,dense and successive Al2O3-rich layers by selective oxidation among grain boundary diffusion under 800?-900?.(3) TiAl-Ti2AlC-TiB2 composites were synthesized by SPS using B4C powders as sources of carbon and boron. The phase transformation process and structural evolution mechanism from 900? to 1200? of TiAl-Ti2AlC-TiB2 are studied. B4C gradually decompose under 1000?, and reacts with Ti,Al liquid to form Ti2AlC and TiB2. B4C is completely consumed at 1200?. The phases of composites are mainly composed of TiAl,Ti3Al,TiB2 and Ti2AlC. The size of TiB2 is very small,mixed within the flake of Ti2AlC. The sizes of ceramic particles grow up with heat preservation time extended to 30 min at 1200?, and reaction mechanism is mainly controlled by the reaction diffusion. With the increasing of B4C contents at 1200?,the amount of Ti2AlC and TiB2 particles increases in composites, and TiAl matrix grains are refined greatly. When the addition amount of B4C in raw material is 6 wt%,particles take place aggregation and coupling, and cracks appear in matrix. Due to the formation of Ti2AlC-TiB2,the average hardness, ductility and compressive strength of TiAl-Ti2AlC-TiB2 composites are increased obviously. When the addition amount of B4C is 2 wt%, the fracture strain of composites is biggest, while the composites show the highest compression strength of 1556 MPa when the addition amount of B4C in raw material is 4 wt%. The mechanisms including crack deflection, pinning and pull-out of Ti2AIC and TiB2 is benefit for strength and ductility of composites. |
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