Ab initio study of the likely orientation relationships of interphase and homophase interfaces in a two-phase HCP + BCC Mg-Li alloy

Abstract

Density functional theory is employed to examine the properties of interfaces generated between the hexagonal close packed (HCP) α and body centred cubic (BCC) β phases in a Mg-8wt.%Li binary alloy. While there are four experimentally-observed and geometrically-predicted orientation relationships (OR) associated with these adjoining HCP and BCC structures, interfacial energy considerations govern the most likely choice of OR. Here, the Burgers OR was found to have the lowest interfacial energy and the largest work of separation. It was also shown that homophase β/β interfacial energies are much higher than the interfacial energy of the α/β phases exhibiting the Burgers OR as the preferred OR. The findings are consistent with the hypothesis that the energy of a preferred interface corresponds to a local minimum in the five-dimensional boundary geometrical phase space. In addition, it was revealed that occupied s bands of the local electronic density of states of atomic layers in the vicinity of the α/β interface exhibiting the Burgers OR shift towards lower electronic energies, thereby increasing the interface stability. The crystal orbital Hamiltonian population provided further quantitative evidence that such an interphase interface of Burgers OR represents the strongest covalent bonding, thereby indicating that chemical bonding constitutes an essential contribution to the interfacial energy. Hence, the simple association between geometrical parameters of an interface and interfacial energy is incomplete.