Enhancing the strength–ductility synergy of magnesium alloys through an Mg-5Zn/Mg-3Mn heterostructured composite

Heterostructuring provides an effective strategy for improving the strength–ductility synergy of magnesium alloys, but the deformation accommodation and back-stress strengthening mechanisms in Mg/Mg heterostructured composites remain insufficiently understood. In this study, a Mg-5Zn/Mg-3Mn heterostructured composite was designed to couple a relatively ductile Mg-5Zn region with a stronger Mg-3Mn region while retaining Mg-based interfacial compatibility. Microstructural characterization shows that the designed heterogeneous architecture is well retained after extrusion, with a structurally continuous interface and no obvious continuous reaction layer. Zn exhibits a relatively diffuse distribution near the interface, whereas Mn remains mainly confined within the Mg-3Mn region. The composite exhibits a yield strength of 200 MPa, an ultimate tensile strength of 297 MPa, and an elongation of 17.7%, achieving a higher ultimate tensile strength than the individual Mg-5Zn and Mg-3Mn alloys while retaining considerable ductility. Quasi-in situ EBSD reveals progressive local orientation-gradient and GND accumulation during tensile deformation, without obvious twinning-induced reorientation. Post-deformation TEM and fracture/near-fracture SEM observations further indicate localizeddislocation/strain contrast near the heterointerface and around α-Mn particles, while no large-scale interfacial delamination or catastrophic interface-dominated fracture is detected. Comparative loading–unloading–reloading tests confirm that the composite develops a more pronounced HDI/back-stress response than the monolithic alloys. These results demonstrate that the soft–hard coupled Mg-5Zn/Mg-3Mn heterostructure promotes heterogeneous deformation accommodation, local defect storage and HDI strengthening, thereby improving the strength–ductility synergy of the composite.