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.