Grain refinement and improved mechanical properties of Mg-4Zn-0.5Ca-0.5RE magnesium alloy by thermomechanical processing

Abstract

The high strength Mg-4Zn-0.5Ca-0.5RE magnesium alloy was subjected to different hot deformation processes of extrusion, forging, and their combination for grain refinement and improvement of mechanical performance. A fine dynamically recrystallized (DRX) microstructure was obtained by the extrusion process accompanied with the fragmentation and dispersion of the eutectics toward eliminating their deleterious effects, which led to a major improvement in the mechanical properties compared with the as-cast counterpart. However, the forging process led to the development of a partially recrystallized microstructure, indicating the necessity for the application of high strains. On the other hand, detailed electron backscatter diffraction (EBSD) analysis revealed that the combination of extrusion and forging is effective for more intense grain refinement (compared to the extrusion process), leading to the improved yield and ultimate tensile strength while maintaining high ductility (total elongation). A threshold value of grain orientation spread (GOS) was proposed for separating the DRX grains from the deformed ones (below 2° for the DRX grains). Moreover, the DRX grain size was correlated with the Zener-Hollomon parameter. These findings can shed light on the industrial application of this wrought Mg alloy.