Surface mechanical attrition-induced hierarchical gradient nanostructuring enables ultrahigh strength and moderate ductility in Mg-Sn-Zn-Zr alloys

The engineering application of lightweight structural magnesium (Mg) alloys is usually impeded by the challenge of achieving high strength together with useful tensile ductility. In this study, rare-earth-free Mg-6Sn-3Zn-0.3Zr alloys with ultimate tensile strengths (UTS) of 385–405 MPa, tensile yield strengths (TYS) of 320–365 MPa and elongations (Els.) of approximately 13% were developed by constructing gradient nanostructures via surface mechanical attrition treatment (SMAT). Microstructural results revealed a hierarchical gradient architecture consisting of a nanograined surface layer, an ultrafine substructured transition layer and a coarse-grained core. The surface nanograins were formed through dislocation accumulation, twinning-assisted grain subdivision, subgrain rotation and in-situ dynamic recrystallization, accompanied by second-phase fragmentation and redistribution. The enhanced strength was mainly attributed to nanocrystalline strengthening, particle-assisted hardening, dislocation/substructure hardening and gradient-structure-induced hetero-deformation-induced (HDI) strengthening, while the retained ductility was enabled by HDI hardening and strain accommodation in the defect-rich transition layer and coarse core. This work demonstrates an effective SMAT-based gradient-design strategy for overcoming the strength-ductility trade-off in rare-earth-free Mg alloys.