Microstructure And Mechanical Properties Of Boride-carbide Dual-phase High Entropy Ceramics

The concept of high entropy alloy aroused great interest in materials science as soon as it was proposed in 2004.Due to the existence of high entropy effect,the material often shows some excellent properties than the single component material.The concept of high entropy was therefore quickly extended to other materials.In the field of high entropy ceramics,borides and carbides high entropy ceramics of transition refractory metals have become the focus of research due to their outstanding performance in mechanics.Therefore,the dual-phase high-entropy ceramics combining the two characteristics has become a new direction for the development of ultra-high temperature ceramics.In this paper,a series of boride and carbide multiphase high entropy ceramics with different compositions were prepared by high energy ball milling assisted hot pressing sintering with transition metal compounds as raw materials by RSSCE.The microstructure and mechanical properties were characterized.The density of sintered ceramics reaches more than 97%,and the materials are basically dense.According to the XRD pattern,the obtained ceramics are all complex phase,namely boride phase of HCP and carbide phase of FCC.For the dual-phase ceramics that sintered by borides with different components and Ti C,the number of components has a significant effect on the microstructure.When the number of cations in the material system reaches five,the grain size is significantly refined.The grains of boride phase change from equiaxed to plate-like.The grain size of THNZ boride phase is 1.55±0.76μm,and that of carbide phase is1.45±0.49μm.The change of microstructure further leads to the change of ceramic properties.As the number of components increases,the amount of distortion in the lattice also increases.Under the joint action of fine grain strengthening,the hardness（～28.83GPa）and bending strength（～1017MPa）of the material are significantly increased.When the number of cations in the material increased to five,the appearance of plate-like grains in the boride phase makes it easy to appear deflection or bridging during the process of crack propagation,which effectively improves the toughness of the material(～6.58MPa·m^1/2).If the number of boride components in the system is kept as four.No matter how the content of Ti C is changed,the grains of boride phase always remain plate-like.The larger the relative content difference between the two phases is,the easier it is to form platy crystal.The presence of platy crystals greatly increases the toughness of the material.The fracture toughness of the ceramics in this material system is all over6.0MPa·m^1/2.When the content of Ti C is 40mol.%,the fracture toughness of（Ti,Nb,Zr,Hf,Ta）B₂-40mol.%（Ta,Hf,Zr,Nb,Ti）C ceramics is up to 7.15±0.76MPa·m^1/2.It can be found that the fracture mode of multiphase high entropy ceramics is transgranular and intergranular mixed fracture.If ten compounds are used to prepare dual-phase high entropy ceramics at the same time,the degree of ion exchange reaction in ceramics is weakened.At this time,although the ceramics are still composed of boride phase and carbide phase,there is no plate-like grains in the microstructure.Toughening mechanisms such as deflection and bridging are difficult to occur during crack growth,so the fracture toughness of the system decreases.（Ti,Nb,Zr,Hf,Ta）B₂-80mol.%（Ta,Hf,Zr,Nb,Ti）C with the highest fracture toughness is only 4.65±0.39MPa·m^1/2.According to the results of energy spectrum,the distribution of five different cations in boron-carbide dual-phase high entropy ceramics is not uniform,but by a certain tendency.The main phase of the boride phase is Ti B₂,while a small amount of other cations are soluted.There is no obvious main phase in the carbide phase and it is a high entropy phase.In general,no matter how the composition changes,Ti elements are always enriched in boride phase,while Ta and Hf elements tend to be enriched in carbide phase.The distribution of Nb and Zr elements depends on the composition.Comparing the mechanical properties of the multiphase high-entropy ceramics prepared by different synthesis pathways,RSSCE is the best way to synthesize the multiphase high-entropy ceramics.（Ti,Nb,Zr,Hf,Ta）B₂-50mol.%（Ta,Hf,Zr,Nb,Ti）C ceramics prepared by hot pressing at 2000℃and 30MPa have the best mechanical properties.Its hardness is up to 28.36GPa,the bending strength is up to 1017MPa,the fracture toughness is up to 6.58MPa·m^1/2.