Lattice Distortion In (HfZrTaNbTi)X(X=C,N,NC) And (TiZrHf)Y(Y=C,N) And Their Theoretical Study On Mechanical And Thermodynamic Properties

Inspired by high-entropy alloys,the concept of "high-entropy" has been extended to non-metallic elemental materials,and in particular high-entropy ceramics are developing rapidly with promising applications.Similar to highentropy alloys,high-entropy ceramics are solid solutions consisting of a variety of single compound components in equal or approximately equal atomic proportions.Among the high-entropy systems,high-entropy carbides and highentropy nitrides are the current research hotspots due to their excellent properties,unique crystal structures and great potential for applications.It is accepted that the disordered combination in cationic and anionic sublattices of high-entropy ceramics can yield more unique physicochemical properties,which is expected to promote and expand the potential applications of high-entropy ceramics in functional,structural and other fields.However,little research has been conducted on polyanionic high-entropy ceramics,therefore the development and exploration of new high-entropy ceramics are urgent.Moreover,the excellent properties of high-entropy materials are due to their "four effects",of which the lattice distortion effect is closely related to the structural and mechanical properties of high-entropy ceramics,and a proper understanding of lattice distortion in highentropy ceramics is important to reveal their unique toughening behavior.Although severe lattice distortion is one of the core effects of high-entropy materials,the quantitative description of distortion remains a challenge.Therefore,in this paper,lattice distortion is quantified using both mean atomic displacement and bond length distribution.The lattice distortions of five high-entropy ceramics and their effects on structural stability,mechanical properties,electronic structure and thermodynamic properties are then systematically investigated based on density general function theory combined with a special quasi-random structure model.Details are as follows:Firstly,lattice distortions and their effects on key properties are investigated using first-principles calculations based on density flooding theory for(HfZrTaNbTi)C,(HfZrTaNbTi)N and(HfZrTaNbTi)(NC).The results show that the lattice distortion reduces the enthalpy of formation and enthalpy of mixing of the material,thereby enhancing the structural stability of the high-entropy ceramics.The distortion is further quantified and is in the order of(Hf Zr Ta Nb Ti)N >(Hf Zr Ta Nb Ti)(NC)>(Hf Zr Ta Nb Ti)C.Calculations of the elastic properties show that the distortion improves the ductility of the material and also exhibits a softening effect.The effect of lattice distortion on the thermodynamic properties of this system is further investigated in conjunction with the Debye-Grüneisen model.The present results show that lattice distortion improves the thermal expansion and thermal storage capacity of the material.At the same time,the distortion also increases the entropy of the system,thus improving the stability of the ceramic in high-temperature environments.Secondly,the lattice distortion in(TiZrHf)C and(TiZrHf)N and its effects on structural,mechanical,electrical and thermodynamic properties are investigated using first-principles calculations based on density functional theory.The results show that the distortion in(Ti Zr Hf)N is more severe and the lattice distortion can significantly improve the stability of the material,and increase the plasticity at the slight expense of strength and hardness.Studies of the thermodynamic properties show that lattice distortion exacerbates the hightemperature softening of the material,and can increase the vibrational and total entropy,especially the increase in entropy values implying an increase in thermodynamic stability.Thus,lattice distortion can improve the thermodynamic stability of high-entropy ceramics.This study is important for the understanding of the lattice distortion effect,regulating and optimizing the overall performance of high-entropy ceramics.