Microstructural characteristics of AZ31 alloys rolled at room and cryogenic temperatures and their variation during annealing

This study investigates the microstructural characteristics of AZ31 Mg alloys rolled at room temperature (RT) and cryogenic temperature (CT) and the variation in their microstructure and hardness during subsequent annealing. Cryorolling induces the formation of more side cracks than does RT rolling, because of the reduction in the ability of the material to accommodate deformation at CT. Numerous {10–11} contraction and {10–11}-{10–12} double twins are formed in both the material rolled at RT and that rolled at CT, because the grains of the initial material are favorably oriented for {10–11} twinning under rolling. The RT-rolled material has a higher dislocation density than the cryorolled material, and more twins are uniformly distributed throughout the former material. As a result, static recrystallization during subsequent annealing is more pronounced in the RT-rolled material, which results in the formation of a highly recrystallized homogeneous microstructure after annealing. In contrast, the formed twins are predominantly present along the shear bands in the cryorolled material, as a result of which this material has an inhomogeneous bimodal structure containing a large amount of coarse unrecrystallized grains after annealing. The hardness of the annealed RT-rolled material is higher than that of the annealed cryorolled material owing to the finer grain structure of the former.