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

Abstract

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/) structure with the ABACAB stacking is most stable after the lowest energy hcp (2/) structure with ABAB stacking. Addition of 2 at.% Y leads to stabilization of the structure to 6/ 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 2/ 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/ structure. Charge density analysis shows that the 2/ and 6/ structures are electronically similar which might be a cause for better stability of 6/ structure over a 4/ 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.