Structure and stacking faults in layered Mg–Zn–Y alloys: A first-principles study

We use first-principles density functional theory total energy calculations based on pseudo-potentials and plane-wave basis to assess stability of the periodic structures with different stacking sequences in Mg–Zn–Y alloys. For pure Mg, we find that the 6-layer (6 l) structure with the ABACAB stacking is most stable after the lowest energy hcp (2 l) structure with ABAB stacking. Addition of 2 at.% Y leads to stabilization of the structure to 6 l sequence whereas the addition of 2 at.% Zn makes the 6 l energetically comparable to that of the hcp. Stacking fault (SF) on the basal plane of 6 l structure is higher in energy than that of the hcp 2 l Mg, which further increases upon Y doping and decreases significantly with Zn doping. SF energy surface for the prismatic slip indicates activation of non-basal slip in alloys with a 6 l structure. Charge density analysis shows that the 2 l and 6 l structures are electronically similar which might be a cause for better stability of 6 l structure over a 4 l sequence or other periodic structures. Thus, in an Mg–Zn–Y alloy, Y stabilizes the long periodicity, while its mechanical properties are further improved due to Zn doping.