Bonding in MgSi and Al-Mg-Si compounds relevant to Al-Mg-Si alloys

The bonding and stability of MgSi and Al-Mg-Si compounds relevant to Al-Mg-Si alloys is investigated with the use of (linearized) augmented plane-wave + (local-orbitals) density-functional theory calculations. We show that the β and β^'' phases found in the precipitation sequence are characterized by the presence of covalent bonds between Si-Si nearest-neighbor pairs and covalent/ionic bonds between Mg-Si nearest-neighbor pairs. We then investigate the stability of two recently discovered precipitate phases, $U1$ and $U2,$ both containing Al in addition to Mg and Si. We show that both phases are characterized by tightly bound Al-Si networks, made possible by a transfer of charge from the Mg atoms.