Study On Control Of Microstructure And Properties And Mechanisms For Non-Magnetic Ti(C,N)–Ni-Based Cermets

Hard materials are one of the key basic materials to realize the strategic goal of China’s manufacturing powerful country.Ti（C,N）–Ni-based cermets possess many advantages,such as wear resistance,corrosion resistance and low cost,compared with the most widely used WC–Co cemented carbides.Ti（C,N）–Ni-based cermets will be suitable for production of non-magnetic cutting tools,forming tools and wear parts,etc.,if they possess excellent mechanical properties and room-temperature non-magnetism.Therefore,in this thesis,the variations of microstructure,magnetic properties and mechanical properties with TiN content for TiC–xTiN–15Mo₂C–yNi,TiC–xTiN–15WC–yNi,TiC–xTiN–15Mo–yNi and TiC–xTiN–15W–yNi（wt.%,0≤≤15,y=20,40）cermets have been systematically investigated,in order to reveal mechanisms of magnetic transition,strengthening and hardening.The main conclusions are as follows:1.Regardless of TiN content,Mo₂C-containing,WC-containing,Mo-containing and W-containing cermets consist of Ti-based carbide（or carbonitride）ceramic grains and Ni-based binder phase.Ceramic grains have the typical black-core/white-inner-rim/grey-outer-rim structure,except that part of small ones have the white-core/grey-rim structure.However,with increasing TiN content,ceramic grains become fine,and the total solid solubility of Ti and Mo（or W）increases in binder phase.At 15 wt.%TiN,microstructure of Mo-containing and W-containing cermets is poorly homogenous.Ceramic grains in Mo₂C-containing and Mo-containing cermets are finer and their white inner rims are thinner,and ones with white core are less in number,compared with those of WC-containing and W-containing cermets.At the same TiN and Ni contents,the total solid solubility of Ti and Mo（or W）in binder phase decreases in the following order:Mo-containing cermets,Mo₂C-containing ones,W-containing ones and WC-containing ones.2.Room-temperature（RT）magnetism of Mo₂C-containing,WC-containing,Mo-containing and W-containing cermets became weak with increasing TiN content,which is attributed to the total solid solubility of non-ferromagnetic Ti and Mo（or W）in Ni-based binder phase increases with increasing TiN content.Valence electrons of Ti and Mo（or W）solute atoms are partially transferred to the spin-down subband in 3d band of Ni atoms,thus weakening the magnetic exchange interaction among electrons of Ni atoms.At the same TiN and Ni contents,room-temperature magnetism of cermets become strong in the following order:Mo-containing,Mo₂C-containing,W-containing and WC-containing ones.Regardless of TiN content,Mo₂C-containing and Mo-containing cermets are non-magnetic at RT,and the lowest Curie temperature can be reduced to below–260℃.However,only when TiN content is high enough,WC-containing and W-containing cermets become non-magnetic at room temperature.Cermets become non-magnetic at room temperature,due to that the total solid solubility of Ti and Mo（or W）in binder phase excesses the critical value which makes binder phase nonmagnetic at RT.3.Transverse rupture strength（TRS）of Mo₂C-containing（except for 40 wt.%Ni）,WC-containing,Mo-containing and W-containing cermets first increases and then decreases with increasing TiN content,and appropriate TiN content is helpful to improve their TRS,mainly due to refining strengthening of ceramic grains and solid solution strengthening of binder phase.Hardness of Mo₂C-containing（except for 40 wt.%Ni）and WC-containing cermets does not significantly change with increasing TiN content,mainly due to the synthetic effect of refining hardening of ceramic grains,solid solution hardening of binder phase and the decrease in hardness of ceramic grains.Hardness of Mo-containing and W-containing cermets first has no obvious change and then decreases rapidly,mainly due to the synthetic effect of refining hardening of ceramic grains,solid solution hardening of binder phase,and the decrease in hardness of ceramic grains,and poor homogeneity of microstructure.For Mo₂C-containing cermets containing 40 wt.%Ni,both TRS and hardness increase with increasing TiN content,mainly due to refining strengthening and hardening of ceramic grains and solid solution strengthening and hardening of binder phase increase with increasing TiN content.4.On the whole,there is a significant synergistic effect of TiN and Mo₂C on room-temperature non-magnetization and strengthening of Ti（C,N）–Ni-based cermets.For example,TRS and hardness of non-magnetic TiC–6TiN–15Mo₂C–20Ni and TiC–15TiN–15Mo₂C–40Ni（wt.%）cermets are 2205 MPa and 91.0 HRA,2735 MPa and 85.8 HRA,respectively.