Prediction and characterization of an Mg-Al intermetallic compound with potentially improved ductility via orbital-free and Kohn-Sham density functional theory

Magnesium-aluminum (Mg-Al) intermetallic compounds that form as precipitates can significantly influence the mechanical properties of Mg-Al alloys. A computational evaluation of known and unknown Mg-Al intermetallic compounds could help design new Mg-Al alloy microstructures with optimal properties. Here, we employ the cluster-expansion method with energies efficiently calculated with orbital-free density functional theory (OFDFT) and predict a new, metastable intermetallic compound Mg₃Al with a D0₁₉ hexagonal structure that is slightly more stable than an alternative L1₂ cubic structure. We apply Kohn-Sham DFT (KSDFT) to accurately evaluate various metastability criteria for D0₁₉ and L1₂ Mg₃Al, including Born's criterion and phonon dispersion. We show that both Mg₃Al crystalline phases satisfy the metastability criteria and hence should be at least metastable. We further compare ductility metrics for D0₁₉ and L1₂ Mg₃Al to that of hexagonal-close-packed Mg by computing Pugh's ratio and generalized stacking fault energies. The ductility is predicted to follow the order: D0₁₉ Mg₃Al > L1₂ Mg₃Al > Mg, based on the highest Pugh's ratio and the lowest unstable stacking and twinning fault energies of D0₁₉ Mg₃Al compared to that of Mg. We also predict a very low antiphase boundary energy for Mg₃Al and therefore expect D0₁₉ Mg₃Al to be beneficial for improving the ductility of Mg-rich Mg-Al alloys. A computational design of Mg-Al alloy microstructures may become possible by combining the strengths of both OFDFT and KSDFT, i.e., the efficiency of the former and the accuracy of the latter, as demonstrated here.