First-principles analysis of the Al-rich corner of Al-Li-Cu phase diagram

TL;DR

This study uses first-principles calculations and statistical mechanics to map the Al-rich corner of the Al-Li-Cu phase diagram, revealing stable, metastable phases and their temperature-dependent stability, aiding understanding of alloy aging.

Contribution

It provides the first detailed computational phase diagram of the Al-rich Al-Li-Cu system, including stability and solubility of phases across temperatures.

Findings

Identified stable and metastable phases in Al-rich Al-Li-Cu system.

Mapped temperature-dependent phase stability and transitions.

Aligned computational results with limited experimental data.

Abstract

The phase diagram of Al-Li-Cu system in the Al-rich region was determined by means of first-principles calculations and statistical mechanics. The mixing enthalpies of many configurations for different lattices in the whole Al-Li-Cu system were determined by density functional theory simulations to find the stable phases in the convex hull. They were fitted with a cluster expansion to calculate the free energy of the configurations with different compositions as a function of temperature in the Al-rich region (Al content > 40 at. %) by means of Monte Carlo simulations. It was found that the ground state phases in the Al-rich part of the Al-Li-Cu phase diagram were {\alpha}-Al, {\theta}' (Al2Cu), {\delta}' (Al3Li), {\delta} (AlLi) and T1 (Al6Cu4Li3), while {\theta}'' (Al3Cu), T1' (Al2CuLi) and Al3Cu2Li were found on the lowest mixing enthalpy surfaces of their lattices and were metastable. {\alpha}-Al, {\delta} and T1 are stable phases in the whole temperature range while {\delta}' becomes metastable at very low temperature and {\theta} (Al2Cu) replaces {\theta}' as the stable phase at approximately 550 K due to the vibrational entropic contribution. In addition, the phase diagram in the Al-rich region was built and it was shown in isothermal sections from 100 K to 900 K. They were in good agreement with the limited experimental data in the literature and provided new information regarding the stability, solubility and stoichiometry of the different phases. This information is important to understand the precipitation mechanisms during high temperature aging.