Stability and strain-driven evolution of β′ precipitate in Mg-Y alloys

Alloying Mg with rare-earth elements, such as Nd, Gd, or Y, enables hardening from the precipitation of metastable coherent phases during aging at low temperatures. While the aging potential of binary Mg-Nd alloys is relatively limited due to the nucleation of coarse β1 and β precipitates at the expense of the strengthening β‴ precipitates, binary Mg-Y alloys exhibit exceptional stability of the strengthening coherent βS′ phase when aged at 200 °C. Through combination of high-resolution characterization, density functional theory calculations, and finite-element elasticity studies, we demonstrate that the strengthening βS’ phase is thermodynamically stable at low temperatures (as opposed to metastable as in other Mg-RE binaries) and that misfit strains play a key role in not only controlling precipitate structure and morphology but also their unusual evolution into an interconnected network.