First-principles equation of state and phase stability for the Ni-Al system under high pressures

TL;DR

This paper develops a generalized first-principles equation of state for Ni-Al alloys under high pressures, incorporating magnetic effects and phase stability, and investigates the high-pressure order-disorder transition behavior.

Contribution

It introduces a cluster expansion-based EOS model for Ni-Al alloys, including magnetic effects and high-pressure phase stability, validated by experiments and ab initio calculations.

Findings

Good agreement with experimental EOS data

Magnetic energy influences phase stability

Order-disorder transition follows Simon melting equation

Abstract

The equation of state (EOS) of alloys at high pressures is generalized with the cluster expansion method. It is shown that this provides a more accurate description. The low temperature EOSs of Ni-Al alloys on FCC and BCC lattices are obtained with density functional calculations, and the results are in good agreement with experiments. The merits of the generalized EOS model are confirmed by comparison with the mixing model. In addition, the FCC phase diagram of the Ni-Al system is calculated by cluster variation method (CVM) with both spin-polarized and non-spin-polarized effective cluster interactions (ECI). The influence of magnetic energy on the phase stability is analyzed. A long-standing discrepancy between ab initio formation enthalpies and experimental data is addressed by defining a better reference state. This aids both evaluation of an ab initio phase diagram and understanding the thermodynamic behaviors of alloys and compounds. For the first time the high-pressure behavior of order-disorder transition is investigated by ab initio calculations. It is found that order-disorder temperatures follow the Simon melting equation. This may be instructive for experimental and theoretical research on the effect of an order-disorder transition on shock Hugoniots.