Density Functional Theoretical Study Of Thermodynamic And Mechanical Properties Of (ZrTaNbTi)C At High Temperature

With the continuous development of science and technology,higher requirements have been put forward for the performance of materials.Traditional materials have been unable to meet the increasing needs of social production and life,and the design concept of high entropy alloy material was first proposed under this background.Studies show that the high entropy alloy materials have better comprehensive properties than the traditional alloy material and have broad application prospect.With the advance of research,inspired by the design concept of high entropy alloy,high entropy oxides,high entropy carbides,high entropy nitride compounds,high entropy borides and other high entropy ceramics have gradually developed rapidly.Therein,the high entropy carbides have attracted extensive attention because of their excellent physical and chemical properties.In spite of many reports about the high hardness and Young's modulus of high entropy carbides,studies on the mechanical and thermal properties of high entropy carbides at room and high temperatures are still very rare.Therefore,this paper combines ab initio calculation based on density functional theory with self-consistent quasi harmonic approximation(SC-QHA)to study the thermodynamic properties,mechanical and electronic properties of high entropy(ZrTaNbTi)C at higher temperature.The main research contents are as follows:Firstly,the thermodynamic properties of high entropy ceramic(ZrTaNbTi)C have been first investigated on basis of the special quasi-random structure model.The present results show that high entropy ceramic(ZrTaNbTi)C is thermodynamically stable due to the negative formation enthalpy.Moreover,the phonon dispersion relation computed at equilibrium volume contains no imaginary-frequency,implying the dynamical stability of(ZrTaNbTi)C.Then,the thermodynamic properties of Ti C and(ZrTaNbTi)C have been investigated from ab initio calculation in combination with self-consistent quasi-harmonic approximation(SC-QHA),quasi-harmonic approximation(QHA)and quasi-harmonic Debye-Grüneisen model.The comparison of results for Ti C with these three models exhibits the high accuracy of thermodynamic calculation in this paper.The comparitive study of(ZrTaNbTi)C with three methods suggests that SC-QHA method has not only the high precision characteristic of QHA model,but also the high efficiency of Debye-Grüneisen model,so is an efficient and accurate method to study the thermodynamic properties of the high entropy system.The present results could be of great practical significance for improving the study of lattice dynamics and thermodynamic properties of high entropy ceramics,and for design and development of noval ultra-high temperature ceramics.Secondly,the temperature dependent elastic properties of(ZrTaNbTi)C have been systematically studied from the density functional perturbation theory combined with self-consistent quasi-harmonic approximation.At 0 K,the obtained elastic constants meet the Born stability criteria,implying(ZrTaNbTi)C of mechanical stability.(ZrTaNbTi)C possesses evidently high elastic moduli and hardness.The calculated electronic density of state and Bader charge at 0 K and2000 K show that(ZrTaNbTi)C have covalent characteristics accompanied by ionic feature while the covalency decreases and the ionicity increases as temperature increases.The temperature-dependent elastic properties show that(ZrTaNbTi)C is mechanically stable in temperature range studied and maintains high strength and hardness at high temperature due to the slight softening of strong covalent bonding.Higher Poisson's ratio,Pugh's ratio and Cauchy pressure suggest the apparent brittle-ductile transformation trend as the temperature increases.Moreover,(ZrTaNbTi)C shows less anisotropy at higher temperature from the Zener index A~Z and three-dimensional projections,being beneficial to reduce cracking and improve durability.The present study provides more insight into the high temperature behavior of mechanical properties,would be valuable for understanding and design of high-temperature properties of high entropy carbide ceramics.