First-principles calculations of β″-Mg₅Si₆/α-Al interfaces

The metastable β″-Mg₅Si₆ phase is often the most effective hardening precipitate in Al-rich Al-Mg-Si alloys. Two important factors that control the precipitate morphology are the strain energy and the interfacial energy between the precipitate and the matrix. By means of a first-principles supercell approach and density functional theory calculations, we have studied the interfacial properties between β″-Mg₅Si₆ and α-Al. We carefully construct a large number of interfacial cells in order to elucidate preferred interfacial terminations and orientations, as well as atom alignment and intermixing across the interface. Each of the low-energy interfaces we found possesses two key attributes: a high number of Al-Si bonds across the interface, and a face-centered cubic topological alignment of atoms across those interfaces. Our first-principles results yield quantitative values for the interfacial energies, lattice mismatches and strain energies that can be used in future predictions of precipitate morphologies as a function of size.