Preparation And Research On Microstructure And Properties Of （TiVNbTaCrW）C₆ High-entropy Ceramics

Since the concept of high entropy has been introduced into the field of ceramic materials,the composition of new ultra-high temperature carbide ceramic materials becomes broader and promising.The existence of high entropy effect makes high-entropy carbide ceramics have better mechanical properties than traditional UHTCs,such as high hardness,high modulus,and low thermal conductivity.According to the law of mixing entropy,the mixing entropy reaches its maximum value with the equal molar ratio constituent elements.Therefore,the equal molar ratio always be the first choice when we design the composition of high-entropy ceramics.However,in actual situations,the metal elements in high-entropy ceramics are not uniformly mixed,and carbon vacancies are easily generated.Therefore,studying the multi-element high-entropy carbides with non-equal molar ratios is also significant for the influence mechanism of the composition element content on the material structure and performance.In this project,the unit cell of（TiVNbTaCrW）C₆high-entropy ceramics with 21components was built by virtual crystal approximation,used to next geometry optimization.After that,the structure and elastic properties of the 21 cells were calculated by first principles calculationl.The calculation results were analyzed and the single-phase forming ability,structural stability and mechanical properties of（TiVNbTaCrW）C₆high-entropy ceramics were predicted.Subsequently,this subject used six transition metal（Me=Ti,V,Nb,Ta,Cr,W）oxides and carbon black as raw materials,and successfully prepared（TiVNbTaCrW）C₆HEC with a two-step method of carbothermic reduction and pressureless sinterings.The carbon content of the composition was adjusted subsequently.The phase composition,microstructure and mechanical properties of（TiVNbTaCrW）C₆high-entropy ceramics with different carbon content were analyzed and tested.And the influence mechanism of carbon content on the structure and properties of（TiVNbTaCrW）C₆high-entropy ceramics was explored.The first-principles calculation results show that the lattice distortion of the 21 components of（TiVNbTaCrW）C₆high-entropy ceramics is between 3.4 and 3.6,which means the feasibility for the formation of a single phase（TiVNbTaCrW）C₆conforms to the lattice distortion criterion.It was found that（TiVNbTaCrW）C₆high-entropy ceramics are thermodynamically unstable by the judgment of the structural stability.This result is due to the numerous carbon vacancies formed in actual HEC,and the metal elements are not mixed uniformly and disorderly,while the virtual crystal approximation ignores the influence of the atomic arrangement,which leads to the situation that the predicted results are inconsistent with the experimental results;Comparing the mechanical properties of different Me-Cr relative content（TiVNbTaCrW）C₆high-entropy ceramics,it can be found that the influence order of the six metal elements on the performance of（TiVNbTaCrW）C₆high-entropy ceramics is:Ti>Ta>MoExperiments have found that the carbothermal reduction reaction is complete under the condition of 1500℃/heating for 1 h,and the high-entropy carbide powder prepared has eliminated the influence of oxygen pollution;pressureless sintering at 2500℃/heating for 1 h can produce a stable single-phase The high-entropy carbide ceramic with face-centered cubic structure has a lattice constant of 4.33?,which is basically consistent with the first-principles calculation result（4.34?）;SEM analysis shows that the high-entropy ceramic of this composition has residual carbon and Cr The element content is lower than the theoretical value,and the content in the crystal is lower than the intergranular phase;the density is 96%,the hardness is 20.3±1.2 GPa,and the elastic modulus is 460.9±11.2 GPa.The thermodynamic analysis of the carbothermic reduction process proves that the reduction reaction is complete at 1600℃;the analysis of the solid solution process shows that the solid solution of（TiVNbTaCrW）C₆high-entropy ceramics is based on Ta C as the main lattice diffusion.The carbon content of the high-entropy system was adjusted,and it was found that the four carbon content components can form a stable single-phase face-centered cubic structure.With the change of the carbon content,the lattice constant has a slight deviation,and 86%is A more appropriate amount of carbon.This has guiding significance for the preparation experiment of high-entropy ceramics.