Formation of high-strength ^{β ′} precipitates in Mg-RE alloys: The role of the Mg/^{β ″} interfacial instability

Aging reactions in Mg-RE alloys strengthen magnesium, due to the formation of metastable ^β″ and ^β′ precipitates. We use first-principles calculations to critically assess binary Mg-RE (RE = Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm and Y) aging reactions, metastable phases and interfacial energy. We find the following. (i) Our calculations correctly predict the formation of different variants of ^β′ phases for Mg-RE systems across the RE series. (ii) Surprisingly, the Mg/^β″ prismatic interfaces are unstable, with a negative interfacial energy. (iii) This interfacial instability implies the existence of a more energetically stable compound than ^β″, which we show to be the ^β′ precipitate. By exposing the link between Mg/^β″ prismatic interfaces and the ^β′ structure, we propose that ^β′ phase formation is due to an energetic preference for an ordered arrangement of Mg and ^β″. (iv) Our Mg/^β″ interfacial energy results also indicate that atomically thin ^β″ planar Guinier-Preston zones can form as a precursor to ^β′ precipitation.