Gradient Microstructure Evolution and Strength-Ductility Synergy Mechanism in a Mg-7Gd-3Y-1Zn-0.5Zr Alloy Processed by Multi-Directional Forging and Backward Extrusion

In this study, a large-scale conical tube made of Mg–7Gd–3Y–1Zn–0.5Zr alloy was successfully fabricated using a combined process of multi-directional forging and backward extrusion. Systematic characterization revealed that the gradient strain introduced by deformation process led to spatially heterogeneous evolution of the microstructure, including gradients in grain size, morphology and distribution of long-period stacking ordered (LPSO) phases, as well as texture. In the high-strain region, significant grain refinement, fragmentation of LPSO phases, and notable texture weakening were observed. The medium-strain region exhibited dissolution and reprecipitation of solute atoms, resulting in the formation of an alternating α-Mg/14H-LPSO sandwich structure. The low-strain region retained coarse deformed grains and kinked lamellar LPSO phases. This graded microstructure enabled the conical tube to exhibit an excellent combination of high strength (yield strength of 260 MPa) and superior room-temperature ductility (elongation of 21.5%), along with significantly improved mechanical uniformity. Mechanistic analysis indicated that the enhanced performance originated from pronounced grain refinement and coordinated activation of multiple slip systems: basal <a> slip provided fundamental plasticity, while high Schmid factors and favorable intergranular compatibility promoted the activation of pyramidal <c+a> slip, effectively accommodating c-axis strain.