Investigation of the cryogenic deformation behaviour and subsequent recrystallization in magnesium alloy AZ31

Abstract

Magnesium alloys attract great interest from industry and academia, as they are the lightest structural metals, which can be applied to achieve a weight reduction of car bodies to reduce energy consumption and air pollution. However, their ductility and strength are generally poor at room temperature, mainly because of their coarse grains and strong texture. This study aims to develop a novel method to refine grains and weaken the texture in magnesium (Mg) alloy AZ31, and investigate its underlying deformation and recrystallization mechanisms. This can contribute to improving the applications of Mg alloys. For this purpose, the hot deformation with pre-deformation at cryogenic temperature (CT) and room temperature (RT) was conducted and compared to the direct-hot-deformed Mg alloy. Recrystallization mainly occurred near deformation bands in RT-hot-deformed and direct-hot-deformed samples, while active recrystallization occurred near twin-twin interactions in the CT-hot-deformed sample. This resulted in finer grains and a weaker texture in the CT-hot-deformed sample, and its ductility and strength improved significantly. The pre-deformation behaviour at CT and RT was investigated with a quasi-in-situ Electron Backscatter Diffraction (EBSD). More twin-twin interactions were observed at CT than RT, which contributed to forming a weaker texture and finer twins, and higher hardening rates at CT. To further investigate how twinning and deformation bands evolve, rolling tests at CT and RT were conducted. With increasing strain, abundant deformation bands, evolving from {101 ̅1}-{101 ̅2} double twins, were observed in RT-rolled samples, while abundant twins, starting from {101 ̅2} tension twins to twin-twin interactions and twinning sequence, were observed in CT-rolled samples. During the subsequent annealing process, recrystallization occurred around deformation bands in the annealed RT-rolled sample, resulting in a strong basal texture. However, recrystallization occurred around twin-twin interactions in the annealed CT-rolled sample, forming the weak texture and fine grains.