Mechanism of anomalous age hardening response in the multi-directional forged Mg–9Gd–3Y–2Zn–0.5Zr alloy with bimodal microstructure

Bimodal microstructures fabricated via plastic processing improve the strength–ductility synergy in magnesium alloys. However, the mechanism behind their anomalous age-hardening response remains unclear. This study investigates the aging behavior of a Mg–9Gd–3Y–2Zn–0.5Zr alloy processed by multi-directional forging (MDF). Isothermal MDF at 420 °C formed a fully recrystallized structure with fragmented long-period stacking ordered (LPSO) phases, while decreased-temperature MDF, not only was the recrystallized fraction reduced, but the grain size was also refined, resulting in a typical bimodal microstructure. Furthermore, lower forging temperatures led to increasingly pronounced strain-induced precipitation of the β phase during MDF. This process consuming rare earth (RE) solutes and suppressing β′ precipitation during aging, which limited age hardening. Despite the limited hardening, the alloy with the lowest recrystallization degree of ∼26 % and a grain size of ∼1.3 µm attains an ultimate tensile strength of 389 MPa and an elongation of 11.3 % at peak aging. The enhanced strength primarily derives from grain-boundary strengthening, dislocation strengthening, precipitation hardening, and solid-solution strengthening, whereas the improved ductility is attributed to the reduced population of shearable β′ precipitates, which mitigates strain localization.