New nanostructured phases with reversible hydrogen storage capability in immiscible magnesium-zirconium system produced by high-pressure torsion

Mg and Zr are immiscible in the solid and liquid states and do not form any binary phases. In this study, Mg and Zr were significantly dissolved in each other by severe plastic deformation (SPD) through the high-pressure torsion (HPT) method and several new metastable phases were formed: Nanostructured hcp, nano-twinned fcc, bcc or ordered bcc-based phases. These supersaturated Mg-Zr phases, which did not decompose up to 773 K, exhibited reversible hydrogen storage capability at room temperature. They absorbed ∼1 wt.% of hydrogen under 9 MPa in ∼20 s and fully desorbed the hydrogen in the air. First-principles phonon calculations revealed that the disordered hcp and fcc solid solutions were dynamically stable in the whole composition range of the Mg-Zr system. The bcc or bcc-based ordered phases, which were formed only as intermediate phases during the phase transformation to the hcp solid solution alloy, were energetically higher and were dynamically stable only under limited conditions in the Mg-rich compositions.