Regulation Of Entropy And High Temperature Mechanical Properties Of Carbide Ceramics

With the rapid development of the aerospace field,the requirements for the service properties of carbide ceramics are constantly improving.In order to meet the application of carbide ceramics in extreme environments,it is of great significance to study the internal relationship between the entropy regulation of carbide ceramics and the microstructure of materials,so as to improve the high-temperature mechanical properties of materials.At present,the research on entropy regulation of carbide ultra-high temperature ceramics mainly focuses on the regulation of types and the numbers of components of high-entropy ceramics.Due to the effects of entropy increase and lattice distortion,the mechanical properties of materials at room temperature are significantly improved.However,due to the increase of dislocation motion and the decrease of dislocation density,generally,the high temperature mechanical properties of high-entropy ceramics decrease obviously at high temperature（≥1800℃）.In this paper,high-purity single-phase TiC,ZrC,HfC and medium-entropy carbide composite powders were prepared by carbothermal reduction method,respectively.The internal relationship between densification,microstructure evolution and high temperature mechanical properties of W doped low entropy MC carbide ceramics（M=Ta,Zr,Hf）and（Ti,Zr,Hf）C medium-entropy ceramic were studied by hot pressing sintering method.（1）Using commercial TaC powder as raw material,the effects of W doping on the densification,microstructure and mechanical properties of TaC ceramics at room and high temperatures were studied.The results show that the density of TaC ceramics added with5mol%W at 1900℃is up to 97.3%,which is significantly higher than that of TaC ceramics without additives（85.6%）.W additive can improve the sintering property of TaC,reduce the sintering temperature of the material,and enhance the hardness and fracture toughness of the material at room temperature.Since the covalent radius of W（1.38A）and Ta（1.34A）are relatively close,W doping can effectively promote the volume diffusion and grain boundary diffusion in the sintering of TaC ceramics,which leads to the increase of the density of TaC ceramics and also promotes the grain growth of TaC ceramics.（2）In the HfC-W system,due to the large lattice distortion effect,W doping can inhibit the grain growth of HfC to refine the microstructure of the material,reduce the content of closed pores in the grain,increase the strength of grain boundary,and finally significantly improve the mechanical properties of the material at room temperature and high temperature.In HfC-W and TaC-W systems,the increase of configuration entropy caused by W additive is the same,but due to different lattice distortion effect,they have different mechanical properties enhancement mechanisms.（3）In the ZrC-W system,the effect of sintering temperature on the densification of ZrC-10mol%W was further investigated on the basis of the previous research.The results show that the increase of sintering temperature not only improves the density of the material,but also promotes the further growth of grain.As the doping amount of 10mol%W is higher than the solution limit of W in ZrC,a solid solution with core-shell structure is formed.Excess W precipitates on the grain boundary in the form of second phase nanocrystals.W doping significantly enhances the grain boundary strength of the material and improves the high temperature mechanical properties of the material.（4）Using（Ti,Zr,Hf）C and Ti（Zr,Hf）C medium-entropy ceramic composite powders with high purity and low oxygen content as raw materials,the sintering and high temperature mechanical properties of（Ti,Zr,Hf）C medium-entropic carbide ceramics were studied.The results show that compared with ZrC and HfC monocarbide ceramics,the formation of medium-entropy solid solution of（Ti,Zr,Hf）C ceramics can promote the densification of the materials.The high lattice mismatch effect between TiC and ZrC not only inhibits the rapid grain growth during the sintering process of（Ti,Zr,Hf）C,but also increases the grain boundary strength.Compared with(Ti_1/9Zr_5/9Hf_3/9)C and(Ti_2/9Zr_4/9Hf_3/9)C ceramics with non-equal molar ratios,(Ti_1/3Zr_1/3Hf_1/3)C kept good bending strength（580～619 MPa）at25-1800℃.