We report on a comprehensive first-principles study of phase stability in the Ni-Al binary, both at zero Kelvin and at finite temperature. First-principles density functional theory calculations of the energies of enumerated orderings on fcc and the sublattices of B2 not only predict the stability of known phases, but also reveal the stability of a family of ordered phases that combine features of $\mathrm{L}{1}_2$ and $\mathrm{L}{1}_0$ in different ratios to adjust their overall composition. The calculations also confirm the stability of vacancy ordered B2 derivatives that are stable in the Al-rich half of the phase diagram. We introduce strain order parameters to systematically analyze instabilities with respect to the Bain path connecting the fcc and bcc lattices. Many unstable orderings on both fcc and bcc are predicted around compositions of $x_{\text{Ni}}=0.625$, where a martensitic phase transformation is known to occur. Cluster expansion techniques together with Monte Carlo simulations were used to calculate a finite-temperature-composition phase diagram of the Ni-Al binary. The calculated phase diagram together with an analysis of Bain instabilities reveals the importance of anharmonicity in determining the phase bounds between the B2 based $\beta$ phase and the $\mathrm{L}{1}_2$ based ${\gamma }^{\prime }$ phase, as well as properties related to martensitic transformations that are observed upon quenching Ni-rich $\beta$.

• Received 20 June 2016

DOI:https://doi.org/10.1103/PhysRevB.94.094111