Theoretical Calculations And Experimental Study On The Spinodal Decomposition Behavior Of Ti-Zr-C-N-Ni System Cermets

Cermets have gradually shown a good application prospect in high-speed cutting,geological exploration and Marine mining as their high hardness,high wear resistance and corrosion resistance.However,due to their insufficient strength and toughness,the wide application of cermets is still limited.Spinodal decomposition can generate nanometer structure,thus enhancing the properties of cermets.Therefore,the application of spinodal decomposition to cermets is an effective way to improve strength and toughness of cermets.Based on the thermodynamic calculation and main experiments,the effects of composition and temperature on the spinodal decomposition of Ti-Zr-C-N based cermets were researched.In addition,the mechanisms of spinodal decomposition on the mechanical properties and tribological properties of cermets were systematically studied.In this thesis,the isothermal section of phase diagram of Ti-Zr-C-N-Ni system are calculated via the CALPHAD method.Sequentially,the influence of Ni and N elements on the miscibility gap and spinodal decomposition regions of Ti-Zr-C-N-Ni system was studied.The results show that trace Ni can enlarge the miscibility gap of the system,but narrow the spinodal decomposition regions.While the area ratio curves of the miscibility gap and the spinodal region exhibited a bimodal distribution.The peaks occurred when the N content were 15 at.%and 90 at.%.And then,the synergistic effect of Ni and N elements was further studied,and it was found that a subtle increase in Ni（by 0.5 at.%）counteracted the effect of 20 at.%N addition on the spinodal region in the multicomponent Ti-Zr-C-N-Ni cermet system.（Ti₈,Zr₂）（C,N）-x Ni（x=0,3,9,18 wt.%）cermets were prepared by powder metallurgy method and aged at 1500°C.The existence of sideband peak next to Fcc phase peak in（Ti₈,Zr₂）（C,N）-x Ni sample was observed by XRD characterization,while not observed in the XRD patterns of（Ti₈,Zr₂）（C,N）-18Ni cermets.Combined with the SEM,lamellar spinodal structures are observed in both alloys and cermets,but the extent of spinodal decomposition in cermtes is significantly lower than that in alloys.Consequently,it shows that Ni elements inhibit the spinodal decomposition in cermets.In addition,the effect of ageing treatment on microstructure and properties of（Ti₈,Zr₂）（C,N）-Ni cermets was further studied.It found that the hardness and fracture toughness of cermets gradually increased with the increase of ageing time at 1500°C.This is because ageing treatment promoted the spinodal decomposition in cermets.However,the fracture toughness decreased after ageing 2 h,which was caused by the coarsening of hard particles and the reduction of crack nucleation and propagation resistance.Spinodal decomposition cermets for industrial applications and a control group also were prepared by powder metallurgy method,and the effects of spinodal decomposition on properties were studied.It is found that the spinodal decomposition occurs in Ti（C₅,N₅）-（Ti₅,Zr₅）C-WC-Mo₂C-Co/Ni cermets,which makes the difference microstructure and properties between Ti（C₅,N₅）-（Ti₅,Zr₅）C-WC-Mo₂C-Co/Ni and Ti（C₅,N₅）-WC-Mo₂C-Co/Ni.To sum up,Ti（C₅,N₅）-（Ti₅,Zr₅）C-WC-Mo₂C-Co/Ni cermets show better strength and toughness matching performance.At 25°C,Ti（C₅,N₅）-WC-Mo₂C-Co/Ni cermets has better tribological properties due to its slightly higher hardness.Interestingly,with the increase of friction temperature,the spinodal decomposition of Ti（C₅,N₅）-（Ti₅,Zr₅）C-WC-Mo₂C-Co/Ni cermets obtains favorable conditions.At400°C and 700°C,the friction coefficient and wear rate are gradually lower than that of Ti（C₅,N₅）-WC-Mo₂C-Co/Ni cermets,and shows better tribological properties at high temperature.