A computational study of the configurational and vibrational contributions to the thermodynamics of substitutional alloys: the Ni3Al case

TL;DR

This paper introduces a combined computational approach to analyze the thermodynamics of order-disorder transitions in substitutional alloys, specifically applied to Ni3Al, highlighting vibrational and configurational entropy contributions.

Contribution

It develops a methodology integrating nonequilibrium methods and Monte Carlo simulations to evaluate configurational and vibrational entropy effects in alloys.

Findings

Vibrational entropy decreases at the transition due to bond proportion effects.

Volume increase at transition raises vibrational entropy by 0.08 kB/atom.

Including vibrations lowers the predicted transition temperature by about 30%.

Abstract

We have developed a methodology to study the thermodynamics of order-disorder transformations in n-component substitutional alloys that combines nonequilibrium methods, which can efficiently compute free energies, with Monte Carlo simulations, in which configurational and vibrational degrees of freedom are simultaneously considered on an equal footing basis. Furthermore, by appropriately constraining the system, we were able to compute the contributions to the vibrational entropy due to bond proportion, atomic size mismatch, and bulk volume effects. We have applied this methodology to calculate configurational and vibrational contributions to the entropy of the Ni3Al alloy as functions of temperature. We found that the bond proportion effect reduces the vibrational entropy at the order-disorder transition, while the size mismatch and the bond proportion effects combined do not change the vibrational entropy at the transition. We also found that the volume increase at the order-disorder transition causes a vibrational entropy increase of 0.08 kB/atom, which is significant when compared to the configurational entropy increase of 0.27 kB/atom. Our calculations indicate that the inclusion of vibrations reduces in about 30 percent the order-disorder transition temperature determined solely considering the configurational degrees of freedom.