Preparation And High Temperature Mechanical Properties Of Medium-Entropy Ceramics (M=Ta,Nb) In (Ti,Hf,M)C

Materials science and engineering is one of the three most rapidly developing scientific and technological fields in the 21st century.The research,development and application of new materials have become an important power point for all countries to compete for new advantages.Especially in recent years,with the rapid development of aerospace industry,the development and research of materials under extreme service conditions will be both an opportunity and a challenge.Therefore,countries around the world have invested a lot of human,material and financial resources in continuous research and development,in order to seize the commanding heights.The appearance of high entropy alloy together with its four core effects(high entropy effect,lattice distortion effect inside the material,hysteresis diffusion effect on atomic migration and diffusion,and cocktail effect on material performance)has pointed out a new direction for the development of ultra-high temperature ceramics,so that they show stronger vitality and vigorous vitality.In this paper,the entropic ceramic powders with low oxygen content(Ti,Hf,Ta)C and(Ti,Hf,Nb)C were prepared by carbothermal reduction method.The sintering densification mechanism of ceramics was explored by studying sintering temperature,sintering method and different component contents,and the related mechanical properties were also explored.(1)(Ti,Hf,Ta)C mesentropic ceramic powders with biphase structure were synthesized by carbothermal reduction method for the first time.The synthesis mechanism of the powders was analyzed by sintering the powders at different temperatures(1400-1700~oC).The grain size of the powders was 0.92±0.4μm after sintering at 2100 ~oC.The(Ti,Hf,Ta)C mesentropic ceramics with a density of 93.9%have better grain size and higher ultrahigh temperature strength than their counterparts,with a high temperature strength of 697±26 MPa at 1800~oC.It is much higher than its corresponding strength of unitary carbide ceramics.(2)The study on non-equimolar ceramics of entropy ceramics in(Ti,Hf,Ta)C shows that changing the content of Hf elements will directly affect the microstructure of ceramics,with the increase of Hf content from 20%to 40%.The relative densities of Th TC-2,THTC and Th TC-4 sintered at 2100~oC are 97.5%,93.3%and 90.4%,respectively.The structure of entropic ceramics in(Ti,Hf,Ta)C will also change,and the smallest grain size(0.92±0.4μm)can be obtained at the same molar ratio.And has the highest flexural strength,The flexural strength(510-697 MPa)of entropy ceramics at 1000-1800~oC is significantly higher than that of(Ti,Hf,Ta)C-20%(144-341 MPa)and(Ti,Hf,Ta)C-40%(308-579 MPa)at equal molar ratio(Ti,Hf,Ta)C.However,the ceramics with 20%Hf content have the worst grain size and mechanical properties,which indicates that Hf element will inhibit the growth of grain due to diffusion during ceramic sintering.In addition,by comparing the(Ti,Hf,Ta)C ceramics sintered by plasma spark,it is found that the ceramic grains are too large and the high temperature strength is too low.(3)(Ti,Hf,Nb)C ceramics were synthesized for the first time.By sintering(Ti,Hf,Nb)C ceramics with self-synthesized powder,the morphologies of the powders and ceramics were analyzed.It was found that the density of(Ti,Hf,Nb)C ceramics sintered at 2100~oC was 94.2%.Moreover,it has a finer grain size(1.75±0.4μm)than the corresponding unitary carbide ceramics,and improves the high temperature mechanical properties of(Ti,Hf,Nb)C ceramics.Compared with the corresponding unitary carbide ceramics,the high temperature strength is much higher than that of the corresponding unitary carbide ceramics.In particular,the flexural strength at 1800~oC reached 658±72MPa.