Theoretical Study Of Thermodynamic And Mechanical Properties Of High Entropy (Tanbhftizr)C And (tanbhftizr)N

With the development and progress of society,science and technology,the demand for alloys is more and more increasing.So,the traditional alloy has been unable to meet the needs of human society,and researchers break through the traditional idea.Then a new design concept of multi-principal component alloys,namely high entropy alloy,is developed,which has been concerned by numerous researchers at home and abroad.High entropy alloys have excellent properties and wide application prospects.Based on the concept and characteristics of high-entropy alloys,high entropy compounds,such as high entropy carbides,high entropy nitrides,high entropy oxides,high entropy borides,are also developed rapidly.Moreover,high entropy carbides and high entropy nitrides have excellent properties.However,studies on the thermodynamic properties and mechanical properties of high entropy materials are very rare.Therefore,thermodynamic properties of(TaNbHfTiZr)C and(TaNbHfTiZr)N are comparatively studied by combining the quasi-harmonic Debye-Griineisen model with density functional theory,and the mechanical and electronic properties of high entropy(TaNbHfTiZr)C and(TaNbHfTiZr)N are also further studied by using density functional theory.The main contents of this research are as follows:Based on density functional theory calculation in conjunction with quasi-harmonic Debye-Griineisen model,thermodynamic properties of(TaNbHfTiZr)C and(TaNbHfTiZr)N are studied,in which the chemical disorder is modeled with the special quasi-random structure.Thermodynamic entropies for both materials rise significantly due predominantly to vibrational contribution,and(TaNbHfTiZr)N is more stable at high temperatures because its entropy increment is more dramatic with increasing temperature.Although the high temperature softening effect is very similar,(TaNbHfTiZr)N is more beneficial for higher temperature applications because of the much larger bulk modulus.Thermal expansion coefficients for both materials increase first rapidly then slowly,and(TaNbHfTiZr)N is more slightly susceptible to temperature.For(TaNbHfTiZr)C and(TaNbHfTiZr)N,the heat capacities at constant volume and pressure demonstrate similar behavior,being larger than metal high entropy HfNbTaTiZr.The present investigations of thermodynamic behaviors for both materials provide valuable evidents for the practical applications.Using density functional theory in conjunction with special quasi-random structures,mechanical and electronic properties of(TaNbHfTiZr)C and(TaNbHfTiZr)N are studied.Based on the empirical criterion for the formation of high entropy compounds,lattice constant difference and enthalpy of mixing indicate the formation of high entropy solid solution phases(TaNbHfTiZr)C and(TaNbHfTiZr)N.The derived elastic stiffness constants also indicate the mechanical stability of high entropy(TaNbHfTiZr)C and(TaNbHfTiZr)N.At zero pressure,the calculated elastic mechanics for both materials obeys the rule of mixture,whereas Vickers hardness is slightly larger than the average value of constituent binary carbides and constituent binary nitrides.The computed elastic parameters show that(TaNbHfTiZr)C and(TaNbHfTiZr)N are brittle.Under high pressure,for both materials the lattice constants decrease slightly,and mechanical properties are improved,even the brittleness-ductility transition takes place.The calculated electronic structures show that(TaNbHfTiZr)C and(TaNbHfTiZr)N have covalent bond with ionicity.With increasing pressure,covalence in(TaNbHfTTiZr)C and(TaNbHfTiZr)N decreases while ionicity increases.The present research will be valuable for understanding and designing of high-entropy materials.