Enhancement of Mechanical Properties of Rolled AZ31 Alloy by Utilizing Ultrasonic Nano-crystalline Surface Modification (UNSM) Treatment and Heat Treatment

Mg alloys have great potential as lightweight structural materials due to their low density, but their application is limited due to lower mechanical properties compared to other lightweight alloys. The Ultrasonic Nano-crystalline Surface Modification (UNSM) technique, which induces severe plastic deformation on the material surface using ultrasonic impacts, can result in a gradient microstructure with a refined surface consisting of nano-sized grains, and an intermediate microstructure with residual strain, ultimately enhancing mechanical properties. While the nano-crystalline surface contributes to improved strength and ductility, the residual strain in the intermediate microstructure brings about a significant reduction in ductility. However, the residual strain can be beneficial for both strength and ductility through recovery and recrystallization, achieved through an appropriate annealing treatment. In this study, we applied the UNSM-treatment, followed by subsequent annealing at 200 °C, to a homogenized rolled AZ31 alloy to simultaneously improve strength and ductility by generating a gradient microstructure.