Thermodynamic stability and electronic structure of η-Ni6Nb(Al,Ti) from first principles

First principles calculations in combination with special quasirandom structures are used to investigate the thermodynamic stability and electronic structure of a partially ordered hexagonal phase with chemistry Ni6Nb(Al,Ti) observed in Allvac 718Plus. The results agree with the experimental observations by confirming the structural stability of the alloy over a wide range of compositions. At finite temperature, vibrational and configurational contributions to the free energy stabilize a competing orthorhombic phase, which is shown to become energetically favourable for high Ti concentrations.