Achieving strength-ductility synergy of Mg-Gd-Y-Zn-Zr alloy with dense fine-grained microstructure via continuous alternating differential speed extrusion

Extruded magnesium (Mg) alloys have long been plagued by severe anisotropy, which severely impedes their widespread industrial application despite outstanding lightweight potential. To overcome this persistent challenge, an innovative continuous alternating differential speed extrusion (CADSE) process was proposed in this study, which utilizes asymmetric strain fields to concurrently achieve anisotropy control and mechanical performance enhancement. As a groundbreaking demonstration, Mg-Gd-Y-Zn-Zr alloy sheets with simultaneously enhanced mechanical performance and reduced anisotropy were successfully fabricated for the first time by employing the CADSE process. The effects of the asymmetric cavity structure on microstructure evolution, texture characteristics, and deformation mechanisms of the billet was systematically investigated during the CADSE process. Moreover, the intrinsic mechanisms for the retention of fine-grained structure and strength-ductility enhancement in the extruded sheets were also elucidated. The results indicated that the mechanical properties of the extruded sheets were significantly strengthened, with the E460 (extruded at 460 °C) sheet demonstrating superior stretch forming performance compared to the E430 (extruded at 430 °C) sheet. The superior combination of strength and ductility was obtained in the E460–45° sample, exhibiting tensile yield strength (TYS) of 256 MPa, ultimate tensile strength (UTS) of 323 MPa, and elongation of 22.7 %. The strength improvement originated from microstructural refinement, the retained high-hardness deformed grains, and the precipitation of stacking faults (SFs) within the recrystallized grains. In contrast, the exceptional ductility enhancement stemmed from high proportion of recrystallization and the elimination of intergranular secondary phases. The continuous dynamic recrystallization (CDRX) mechanism served as the primary nucleation mechanism throughout the CADSE process. Conversely, the twinning-induced dynamic recrystallization (TDRX) mechanism played merely a minor role within the first two channels. Moreover, with increasing cumulative strain, the predominant deformation mode in deformed grains shifted from basal <a> slip to prismatic <a> slip. The combined effects of segregation of solute atoms at recrystallized grain boundaries, SFs precipitation within recrystallized grains, and fragmented long-period stacking ordered (LPSO) phases effectively suppressed microstructural coarsening in the extruded sheets. The CADSE process represents a breakthrough in metal forming technology, establishing a new paradigm for anisotropic control and mechanical property enhancement. This innovative methodology establishes a general framework for extruded sheet manufacturing applicable across diverse material systems.