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 beta and beta(') 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.