Laser shock-induced gradient twinning microstructure in AZ31B alloy

Magnesium alloys are highly promising in biodegradable materials for medical applications. However, their applications are limited by weaknesses in mechanical strength and corrosion resistance. In recent years, advancements in a novel laser method, Laser Shock Peening (LSP), have been explored. Researchers suggest that LSP can induce surface compressive stresses and induce changes in residual stress and grain size gradients, thereby enhancing the material's fatigue resistance, corrosion resistance, and resistance to stress corrosion cracking. However, there is ongoing debate about whether LSP treatment can, in effect, create a gradient microstructure. In this study, we focused on the AZ31B alloy, notable for its dense hexagonal close-packed (HCP) crystal structure. This material is particularly interesting because, due to its low stacking fault energy, it is more prone to twinning deformation than to dislocation slip when it undergoes plastic deformation. To analyze the changes wrought by LSP, we employed optical microscopy (OM), electron backscatter diffraction (EBSD), and X-ray diffraction (XRD) for examining the microstructures both before and after LSP treatment. Our findings revealed that LSP treatment refined the grain size, leading to a gradient distribution. In addition, we observed a direct correlation between the volume fraction of twinning and grain size. In contrast, the geometrically necessary dislocation (GND) density exhibited an inverse relationship. By measuring hardness and residual stress in the region affected by LSP, we determined that LSP could introduce a layer of plastic deformation up to a depth of 800–1000 μm. We also established a link between the gradient microstructure and hardness and provided a thorough discussion on the types of twinning and the mechanisms behind the induction of gradient microstructure on the surface.