Microstructure evolution and aging behavior of surface-nanocrystallized WE43 Mg alloy by ultrasonic shot peening at a cryogenic temperature

In this study, the microstructure evolution and aging behavior of surface-nanocrystallized WE43 Mg alloy induced by ultrasonic shot peening at a cryogenic temperature were examined. The results reveal that the original coarse grains with an average grain size of 46.7 mu m were refined to nanocrystalline grains with 31.5 nm in the nanocrystalline region, demonstrating a marked grain refinement effect. After aging treatment, the nanocrystalline region experienced two aging stages. During Stage I, there was a rapid increase in hardness, and in Stage II, it reached a maximum of 154.2 HV, representing a 105.6% improvement over the solution-treated alloy. Further microstructural characterization indicates that solute segregation initially occurred along the grain boundaries, leading to a rapid increase in hardness during Stage I. Subsequently, two types of precipitates were observed both at the grain boundaries and within the grains during Stage II, with the combined effects of solute segregation and precipitation contributing to a further increase in hardness.