Reaction Precipitation Behavior And Mechanical Properties Of (Ti,W)C-based Composites Induced By Ti/Zr

In order to improve the sinterability and mechanical properties of refractory metal carbide ceramics,this paper proposes a new method of a second phase selfprecipitating in(Ti,W)C with adding Ti/Zr active metal inducer to prepare highperformance(Ti,W)C-based composites,based on changing trend of solid solubility of W in(Ti,W)C solid solution with C content.(Ti,W)C and Ti H2/Zr H2 powder were used as raw materials.The composition design and process optimization were carried out according to the thermodynamic calculation results of C-W-Ti-Zr system.The(Ti,W)C-W and(Ti,Zr,W)C-(Zr,Ti)C-W composites with nano-precipitate phase were successfully fabricated by hot-pressing at low temperature.The influence of sintering temperature,W content in(Ti,W)C solid solution and Ti content on the microstructure of(Ti,W)C matrix composites were investigated by XRD,SEM,TEM and EDS.Simultaneously,the mechanical properties(Vickers haardness,flexctural strength and fracture toughness)were also measured by indentation method and three-point bending.Microstructure evolution in interfacial diffusion layers between(Ti,W)C and Ti/Zr was studied under different temperature and holding time.The reaction process of W,C and Ti/Zr reveals the precipitation mechanism of the W(Ti)phase in the composite.The results show that the(Ti,W)C-W composites have been prepared with 30 mol.% Ti H2 and 70 mol.%(Ti0.76,W0.24)C by hot pressing at 1300?1800°C for 1h under 30 MPa.Only two phases of(Ti,W)C and W(Ti)are detected in(Ti,W)C-W composites.The relative density and grain size increase with increasing sintering temperature and the holding time.Furthermore,the content of W(Ti)increases first and then decreases with increasing temperature,but the position gradually changes from intragranul/intercrystalline to intergranul precipitation.The optimization of sintering process is 1500°C/30MPa/1h,and its hardness,flexural strength and fracture toughness are 16.2 GPa,380 MPa and 4.43 MPa·m1/2,respectively.The density and microstructural homogeneity of(Ti,W)C-W composites decrease with the increase of W content in(Ti,W)C under 1500°C/30MPa/1h.Simutaneously,the content of W(Ti)phase reduced and the W(Ti)phase transforms from a sphere to a long rod gradually in shape,the hardness and fracture toughness of the composites decrease.With the increase of Ti addition,the hardness of the composite decreased and the fracture toughness increased.The(Ti,W)C-based composites prepared by using Zr as inducer into(Ti,W)C are composed of(Ti,Zr,W)C,(Zr,Ti)C and W(Ti)solid solution.With the increase of sintering temperature,the content of(Zr,Ti)C and W(Ti)phases decreased,the hardness increased but the fracture toughness decreased slightly.The optimization of sintering process is 1800°C/30MPa/1h,and its Vickers hardness,elastic modulus,flexural strength and fracture toughness are 18.9GPa,397 GPa,474MPa and 4.05MPa·m1/2,respectively.The relative density of the composite decreases with the increasing the W content in the(Ti,W)C.The particle size of the precipitated phase decreases,and the precipitation position changes from intergranular to intracrystalline site.The Vickers hardness and fracture toughness are also reduced.The results of diffusion couples show that(Ti,W)Cx+W(Ti),?-Ti(W)and Ti Cx intermediate zones are formed in the diffusion layer of(Ti,W)C and Ti.However,only(Ti,W)Cx+W(Ti)and Zr Cx phase can be detected in(Ti,W)C-Zr interface diffusion layer.The results of diffusion kinetics show that all the growth process of generating layer is controlled by diffusion mecahnism.The reaction process of(Ti,W)C with adding Ti or Zr as inducer is the same.They all originate from the decarburization of(Ti,W)C solid solution,and then C and Ti/Zr react to form Ti Cx/Zr Cx.The W(Ti)phase is generated from the decarburization of(Ti,W)C.With the reaction temperature increasing,the new phase(Ti,W)C further synthesizes by Ti Cx and unstabilized(Ti,W)C,and even the secondary intragranular precipitation occurs during the cooling process.The reaction between Zr Cx and the unstabilized(Ti,W)C is mainly deponded on the mass transfer between Ti and Zr,and the corresponding phases,(Zr,Ti)C and(Ti,Zr,W)C,are formed simutaneously.