Enhanced fracture toughness in a high-strength extruded Mg-Gd-Y-Zn-Mn alloy via synergistic intrinsic-extrinsic toughening

Magnesium (Mg) alloys are promising lightweight structural materials. However, they usually exhibit a strength-toughness trade-off, which limits their engineering applications. In this study, a bimodal microstructure incorporating lamellar long-period stacking-ordered (LPSO) phases was engineered in an extruded Mg-10Gd-3Y-1Zn-0.5Mn (GWZ1031M, wt.%) alloy. A superior strength-toughness synergy was achieved, with an ultimate tensile strength of 450 MPa, yield strength of 370 MPa, elongation of 12.7% and fracture toughness of 26.5 MPa·m ½ . At the crack initiation stage, the bimodal microstructure generates pronounced strain incompatibility between textured coarse grains and randomly oriented fine grains, resulting in GND accumulation and HDI hardening. Meanwhile, the associated local strain gradients facilitate the activation of non-basal 〈 c + a 〉 slip and deformation twinning, which enhance plastic accommodation near the crack tip and delay crack nucleation. During crack propagation, under triaxial tensile stress state, weak α-Mg/LPSO interfaces facilitate crack deflection and interfacial delamination, which effectively dissipates fracture energy and alleviates crack-tip stress concentration. The synergy between HDI-induced intrinsic toughening and delamination-induced extrinsic toughening enables the simultaneous enhancement of strength and fracture toughness, while the high yield strength is primarily attributed to HDI hardening arising from strain partitioning. This study provides insights into the microstructural tailoring of advanced high-performance Mg-based structural materials.