Elastic and thermodynamic properties of high entropy carbide (HfTaZrTi)C and (HfTaZrNb)C from ab initio investigation

The elastic and thermodynamic properties of novel high entropy carbide (HfTaZrTi)C and (HfTaZrNb)C are studied within density functional theory framework, adopting special quasi-random structure to model the chemical disorder of high entropy compounds. The optimized lattice parameters are in good agreement with the available experimental data. Both high entropy carbides are thermodynamically stable due to the negative formation enthalpies. Then mechanical properties in small elastic range are studied, results show that (HfTaZrTi)C and (HfTaZrNb)C are mechanical stable, and (HfTaZrNb)C has greater strength and stiffness in spite of larger brittleness due to stronger covalent bonds. The derived higher Debye temperature of (HfTaZrNb)C indicates stronger covalent interactions. The calculated electronic structures show covalent characteristics accompanied by ionicity for both (HfTaZrTi)C and (HfTaZrNb)C. The thermodynamic properties are further investigated by further combination with Debye-Gruneisen model. As temperature increases, thermodynamic properties of two materials exhibit essentially similar trend, and (HfTaZrTi)C may be more beneficial for reducing the thermal stress mismatch as a tool coating due to larger thermal expansion coefficient whilst (HfTaZrNb)C has strength advantage of greater bulk modulus in engineering applications. The present research will be valuable for understanding and designing of high entropy carbides.