Enhanced thermal stability and hardening behavior of a gradient structured Mg-15Gd alloy by pre-aging

This study investigates the influence of pre-aging on the thermal stability of a gradient structured Mg-15Gd (at.%) alloy processed by ultrasonic surface rolling (USRP). USRPed samples and USRPed-pre-aged samples were isothermally annealed at 300 °C and 400 °C, with microstructure characterized by Scanning Electron Microscopy (SEM), Electron Backscatter Diffraction (EBSD), Transmission Electron Microscopy (TEM) and hardness tests. The results demonstrate that pre-aging introduces nanoscale β-Mg 5 Gd precipitates and promotes Gd segregation at grain boundaries, which effectively inhibits grain boundary migration and synergistically enhances both the surface hardening and the thermal stability of the deformed layer in the gradient structure. Pre-aging not only increases the initial surface hardness from 107.1 HV to 126.4 HV via precipitation hardening, but also retards high-temperature grain coarsening through a Zener pinning effect. The severely deformed nanocrystalline surface layer exhibits exceptional microstructural stability after pre-aging, showing a grain growth inhibition of approximately 15.8% after annealing at 400 °C. A multi-mechanism hardening model revealed that the resultant surface hardening stems from the combined contributions of precipitation hardening and grain-boundary segregation hardening. This work provides theoretical insights and a feasible processing strategy for designing gradient-structured magnesium alloys with high thermal stability.