Hydrogen-induced decomposition of Zr-rich cores in an Mg−6Zn−0.6Zr−0.5Cu alloy

This study presents the first experimental evidence of a hydrogen-induced decomposition reaction in an Mg-6Zn-0.6Zr-0.5Cu alloy from combined transmission electron microscopy and atom-probe tomography characterization. The reaction takes place due to the presence of H in the Mg matrix, causes the decomposition of pre-existing, high-temperature Zr-Zn intermetallic rods into Zr-rich hydride and β′ (Zn3Mg2), and forms novel composite precipitates in the Zr-rich cores of the alloy during ageing at 180 °C. The stoichiometry of the Zr-Zn rods was found to be Zn3(Zr1−x, Mgx)2, rather than Zn2Zr3, although both have a similar tetragonal crystal structure. The intrinsic link between the high-temperature Zr-Zn rods and the subsequent elongated composite precipitates, as depicted by the reaction, highlights the importance of engineering the Zr-Zn rod microstructures to control the final precipitate microstructure and effectively strengthen the Zr-rich cores, and hence the advanced Mg alloys.