Phase stability, mechanical, and thermal properties of high‐entropy transition and rare‐earth metal diborides from first‐principles calculations.

The narrow composition design space of high‐entropy transition metal diborides (HE TMB2) limits their further development. In this study we designed six quaternary and quinary high‐entropy transition metal and rare‐earth diborides (HE TMREB2) and investigated their phase stability using the energy distribution of the local mixing enthalpy of all possible configurations. The results show that both quaternary and quinary HE TMREB2 have higher enthalpic driving forces, which facilitates the formation of single‐phase AlB2‐type structures between TMB2 and REB2. Calculations of elastic constants show that the TMB2 component has the greatest effect on the c44 elastic constant and shear modulus G, while REB2 significantly influences the bulk modulus B. Furthermore, LuB2 and TmB2 substantially affect the elastic modulus anisotropy of HE TMB2. Rare‐earth atoms in HE TMREB2 can enhance the nonharmonic interactions between phonons, which results in a significant hindrance in the thermal transport of low‐frequency phonons as well as an increase in the volume thermal expansion coefficients. Thus, the incorporation of REB2 into HE TMB2 has a significant impact on the phase stability and properties. [ABSTRACT FROM AUTHOR]