FCC phase formation in immiscible Mg-Hf (magnesium-hafnium) system by high-pressure torsion

Magnesium and hafnium, two hydride-forming and biocompatible metals with hexagonal close-packed crystal structures, are thermodynamically immiscible even in the liquid form. In this study, these two elements were mechanically mixed by high-pressure torsion straining, and a new FCC (face-centered cubic) phase was formed although these two elements do not form the FCC phase even under high pressure or at high temperature. Microstructural examination by scanning-transmission electron microscopy combined with an ASTAR automatic crystal orientation and phase mapping technique confirmed that the FCC phase was stabilized mainly in the Hf-rich nanograins with localized supersaturation. Attempts to control the phase transformations under a hydrogen atmosphere to produce ternary magnesium-hafnium hydrides for hydrogen storage applications were unsuccessful; however, the material exhibited enhanced hardness to an acceptable level for some biomedical applications.