Architecting multiscale heterostructures in rolled Mg-Zn sheets for synergistic strength-ductility via through-thickness strain gradient

Heterostructured Mg alloys offer a promising pathway to enhance the synergy of strength and ductility, yet the role of multiscale heterostructures induced by strain gradients remains insufficiently understood. Here, a Mg-6Zn-0.5Mn-0.3Ca alloy sheet with multiscale heterostructures has been fabricated via single-pass accumulative roll bonding followed by short-time annealing. The pronounced through-thickness strain gradient simultaneously generated heterogeneous grain structures, textures and precipitate distributions. The final-annealed alloy achieved a superior strength-ductility synergy, with a yield strength (YS) of ∼303 MPa and an elongation of ∼11.1%. Mechanistically, the high YS is attributed to both conventional strengthening mechanisms and an extra Type II back stress (∼28 MPa) contributed by the multiscale heterostructure. More importantly, this work reveals that the precipitate heterogeneity is not merely an accompanying feature of strain-gradient processing, but a key structural parameter that contributes to ductility enhancement in two ways. Precipitate heterogeneity alleviates excessive strength mismatch and local stress concentration while preserving sufficient plastic incompatibility for HDI strengthening. At the same time, precipitates stabilize low-angle boundaries during annealing, enabling them to act as additional dislocation sources during subsequent deformation. Together with grain-structure and texture heterogeneities, this precipitate heterogeneity sustains effective work hardening and improves deformation coordination. This work highlights the precipitate heterogeneity as a critical but underexplored dimension in heterostructure design, providing a robust strategy for developing high-performance Mg alloys.