| Titanium alloys are of great interest due to their excellent properties such as high strength,high toughness,corrosion resistance and light weight.The surfaces of many equipment components are subjected to higher stresses than the core.For example,the track plates of an unmanned combat vehicle require a working surface with high strength,hardness and wear resistance.In addition,the composite core is sufficiently malleable and ductile to withstand certain impact loads.Therefore,in this paper,three kinds of Ti C/Ti composites were prepared by in-situ synthesis technology using vacuum induction suspension melting furnace with Cr3C2 powder as carbon source and sponge titanium as matrix.The Cr3C2 content was3wt.%（XGTC3）,6wt.%（XGTC6）and 9wt.%（XGTC9）respectively.The effects of different Cr3C2contents on the microstructural evolution and room temperature mechanical properties（tensile strength,Rockwell hardness and wear resistance）of the composites were investigated.The relationship between the microstructure and room temperature mechanical properties of the composites and the strengthening mechanism were discussed.On this basis,the surface hardness and wear resistance of Ti C/Ti composites were enhanced by high frequency induction heating treatment.The effects of the high-frequency induction heating process（different output currents and heating times）on the microstructural evolution,hardened layer distribution,surface hardness and room temperature frictional wear properties of Ti C/Ti composites as well as the frictional wear mechanism have also been systematically investigated.Besides,by controlling different process parameters of high-frequency induction heating treatment,the aim of improving the surface hardness and wear resistance of Ti C/Ti composites without changing the core properties was achieved.This paper is important for the development of wear-resistant titanium alloys to meet the lightweight requirements of advanced weaponry and to extend the service life of titanium alloys as components.This topic firstly reveals the effect and mechanism of Cr3C2addition content on the microstructure and mechanical properties of pure Ti.The results show that Cr3C2reacts with Ti solution to form Ti C at high temperature.Therefore,the room temperature microstructure of titanium matrix composites with different Cr3C2addition content consists of Ti C of different sizes and shapes,α’phase and a small amount ofβphase andα-Ti.With the increase of Cr3C2addition（3～6～9 wt.%）,theα-Ti grains were significantly refined,from 1.7×103μm to 97.5μm;Ti C transformed from slender strips,short rods to spherical and then dendritic;the width ofα’phase decreased significantly（5.9～0.074μm）and the cluster size decreased（26.8～15.4μm）.With the increase of Cr3C2 addition,the tensile strength and plasticity of titanium matrix composites increased first and then decreased,the Rockwell hardness increased gradually（12.1～40.2 HRC）,the friction coefficient decreased gradually（0.669～0.538）,the wear rate decreased gradually(3.27×10-2～1.97×10-2 mg/m),and the wear mechanism changed from severe adhesive wear to mild adhesive wear and abrasive wear.When the addition amount of Cr3C2 is 6 wt.%,the tensile strength and plasticity of titanium matrix composites reach the maximum（1026.1 MPa,6.7%）,which are increased by 322.1 MPa,712.5 MPa and 609.5MPa,respectively.The main reason for the decrease of mechanical properties of XGTC9composites is that the Ti C with dendritic characteristics has a certain number of sharp edges and corners,which is easy to crack under tensile stress.As the addition of Cr3C2 increases,the C content of the titanium matrix composite increases,the Ti C size becomes larger,the Ti C contact surface area increases,and the hardness of the composite increases;the increase in Ti C content refines the grain size,increases the grain boundary area,improves the load-bearing capacity of the composite,and thus increases the hardness of the composite.In addition,as the addition of Cr3C2 increases,the Ti C content and Cr content increase,reducing the size of theα-lamellae andα-cluster in the composite,improving the strength and toughness of the composite.After high-frequency induction treatment of the as-cast Ti C/Ti composite,the titanium matrix composite consists of a surface hardened zone,a heat affected zone and an unaffected zone in the core.The microstructure of the surface hardening zone consists of a very fine layeredα-phase,Ti C and a small amount ofβ-phase.The heat affected zone extends from the surface towards the core with a fine layeredα-phase decomposed by theα’-phase,which gradually decreases in volume,and a gradient distribution of theα’-phase.In addition,the variation in Cr3C2 addition did not affect the microstructure of the titanium matrix composite after high-frequency induction treatment.With the increase of output current and heating time of high-frequency induction treatment,the decomposition volume ofα’phase in surface hardened zone and heat affected zone increases gradually,the content of fine layeredα-phase increases,and surface hardened areas extends.The test results of Rockwell hardness and friction and wear properties show that the Rockwell hardness of the hardened layers of the three composites is significantly improved compared with the wear resistance of the composite core.The friction coefficient,wear rate and surface roughness decreased significantly,the plough groove of the hardened layer became shallow,and the spalling phenomenon was reduced.The wear mechanism changes from severe abrasive wear and adhesive wear to slight abrasive wear and adhesive wear.In addition,by controlling the output current and heating time,the decomposition degree and distribution of theα’phase in the hardened zone can be affected,and a finer equilibriumαphase（fine grain strengthening）can be precipitated.The resulting equilibrium（α+β）phase is dispersed（dispersion strengthening）,and a higher temperature causes a small amount of C and Cr elements to solid solution into the matrix（solid solution strengthening）. |
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