First principles prediction of the Al-Li phase diagram

TL;DR

This study uses first principles calculations combined with statistical mechanics to accurately predict the Al-Li phase diagram, providing new insights into phase stability, microstructure optimization, and thermodynamic properties relevant for alloy processing.

Contribution

It introduces a comprehensive first principles approach to predict the Al-Li phase diagram and microstructure, including phase stability and transformation pathways, with polynomial thermodynamic expressions.

Findings

Al3Li is a stable phase with low energy barriers to transformation.

The structure of GP zones was identified as Al0.5Li0.5 monolayers.

Al-Li phase diagram predictions agree with experimental data.

Abstract

The phase diagram of the Al-Li system was determined by means of first principles calculations in combination with the cluster expansion formalism and statistical mechanics. The ground state phases were determined from first principles calculations of fcc and bcc configurations in the whole compositional range while the phase transitions as a function of temperature were ascertained from the thermodynamic grand potential and the Gibbs free energies of the phases. Overall, the calculated phase diagram was in good agreement with the currently accepted experimental phase diagram but the simulations provided new insights that are important to optimize microstructure of these alloys by means of heat treatments. In particular, the structure of the potential GP zones, made up of Al0.5Li0.5 (001) monolayers embedded in Al matrix, was identified. It was found that Al3Li is a stable phase although the energy barrier for the transformation of Al3Li into AlLi is very small (a few meV) and can be overcome by thermal vibrations. Moreover, bcc AlLi was found to be formed by martensitic transformation of fcc configurations and Al3Li precipitates stand for favorable sites for the nucleation of AlLi because they contain the basic blocks of such fcc ordering. Finally, polynomial expressions of the Gibbs free energies of the different phases as a function of temperature and composition were given, so they can be used in mesoscale simulations of precipitation in Al-Li alloys.