Theoretical Investigations on Structural Stability and Elastic Properties of MoNbTaW-X (=Ti/V) High Entropy Alloys

A refractory high entropy alloy based on MoNbTaW, with varying amounts of Ti and V, has been studied to elucidate the effect of the alloy composition on the electronic structure, phase stability, thermodynamic properties and the elastic properties. A synergistic approach has been adopted, employing empirical parameters, CALPHAD, and first principles calculations, to verify phase stability and single-phase solid solution formation for this alloy system. First-principles calculations are based on density functional theory, and employ the supercell method for modeling random alloys. The calculated lattice parameter for equiatomic MoNbTaW is in good agreement with available experimental data. The effect of Ti and V in various compositions on the electronic structure of the host alloy has been elucidated via the density of states spectra. Elastic constants of C11, C12, and C44 of 9 alloys based on MoNbTaW with varying amounts of Ti and V, are reported by the stress-strain method and the Voigt-Reuss-Hill approximation. Elastic properties including Young’s modulus, bulk modulus, Poisson’s ratio and Debye temperature of polycrystalline alloys have been reported. All the alloys show mechanical isotropy for bulk modulus and Young's modulus. Addition of Ti and V in increasing amounts shows improved in ductility in the base alloy.