Revealing the precursor-mediated atomic-scale pathway to multi-modal precipitation in a Mg-Zn alloy

The atomic-scale evolution from Guinier-Preston (G.P.) zones to multi-modal MgZn precipitates, i.e. rod-like β1′-MgZn2 (P63/mmc and Cmcm), rod-like β1′-Mg4Zn7 (C2/m), lath-like β1′-MgZn2 (P63/mmc) and disc-like β2′-MgZn2 (P63/mmc) during ageing in a Mg-8Zn-1Al-0.5Mn alloy has been systematically investigated. Rod-like and lath-like β1′-MgZn variants originate from spherical and newly discovered strip-like G.P. zones governed by Zn-vacancy complexes, respectively. Furthermore, Mg6Zn7 substructural units mediate multidirectional irregular stacking with C14/C15-MgZn2 substructural units to generate rod-like β1′-MgZn, while single C14-MgZn2 substructural units enforces ordered stacking, yielding lath/disc-like MgZn2. These insights into precursor-mediated MgZn precipitation provide a foundation for designing high-performance Mg-Zn alloys.