Segregation-driven exceptional twin-boundary strengthening in lean Mg–Zn–Ca alloys

Twinning-mediated plasticity in hexagonal close-packed crystal structures offers great potential for achieving lean Mg-based alloys with high strength and extended ductility. Micropillars from a lean MgZn1 Ca0.3 (ZX10, in wt%) alloy and pure Mg, each comprising one or more pre-existing {101¯2}<1¯011> extension twin boundaries and a parent orientation with its c-axis perpendicular to the loading direction, were subjected to in situ micropillar compression. By means of correlative characterization using transmission electron backscattered diffraction, transmission electron microscopy and nanoscale energy-dispersive X-ray spectroscopy, we assesed the effects of Zn and Ca solutes on micropillar deformation and on the response of pre-existing twin-boundaries to applied stress. In ZX10, both Zn and Ca solutes periodically segregate at the pre-existing extension twin boundaries, giving rise to a strengthening increment by about 300% compared to pure Mg, along with ductility enhancement. Due to segregation, the twin boundaries remain immobile throughout the deformation process, playing a critical role both in non-basal slip and twin-nucleation events. The former is facilitated by the formation of stable I1 stacking faults, while the latter is stimulated by active slip across the pinned twin interface. In contrast, the pure Mg micropillars respond by massive twin growth followed by higher-order twinning events and profuse basal slip. In the absence of non-basal slip, pure Mg exhibits an earlier onset to failure due to shear incompatibility at the twin interface. The findings provide new insights into the role of solute Zn and Ca in stabilizing twinned microstructures for achieving synergistic strength-ductility enhancement in lean Mg alloys.