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- ave 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 $(6l)$ structure with the ABACAB stacking is most stable after the lowest energy hcp $(2l)$ structure with ABAB stacking. Addition of 2 at.% Y leads to stabilization of the structure to $6l$ sequence whereas the addition of 2 at.% Zn makes the $6l$ energetically comparable to that of the hcp.Stacking fault (SF) on the basal plane of $6l$ structure is higher in energy than that of the hcp $2l$ 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 $6l$ structure. Charge density analysis shows that the $2l$ and $6l$ structures are electronically similar which might be a cause for better stability of $6l$ structure over a $4l$ 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.