Bimodal Microstructure and Strengthening Mechanisms of a Mg-RE Alloy Processed by Asymmetric Upsetting-Extrusion

This study successfully produced a magnesium alloy bar featuring a bimodal microstructure with high strength via an asymmetric upsetting-extrusion process. The evolution of microstructure, texture, and mechanical properties was systematically investigated using finite element simulation, room-temperature tensile tests, optical microscopy, scanning electron microscopy, and electron backscatter diffraction. Results demonstrate that the bimodal structure forms under the combined effects of shear deformation in the upsetting stage and low-speed, high-ratio deformation in the extrusion stage. This structure consists of coarse deformed grains containing high-density dislocations surrounded by fine dynamically recrystallized grains. A strong <10-10>//ED basal fiber texture also developed, which effectively suppresses basal slip. Continuous dynamic recrystallization was the primary grain refinement mechanism. The 370 °C extruded alloy achieved a high tensile strength of 457.9 MPa, but its elongation was limited to 3.96%. This combination of strength and ductility is attributed to the synergistic influence of the bimodal microstructure, strong basal texture, and high dislocation density.