Effects of extrusion speed on the formation of bimodal-grained structure and mechanical properties of a Mg-Gd-based alloy

The effect of ram speed ranging from 1 mm/s to 10 mm/s on the microstructure and mechanical properties of a Mg-11Gd-3Y-0.5Nd-Zr (wt%) alloy was investigated using optical microscopy, scanning electron microscopy, electron backscatter diffraction analysis, transmission electron microscopy, hardness testing and uniaxial tensile testing. The results indicate that a bimodal-grained structure composed of elongated deformed grains and fine recrystallized grains is formed in the 1 mm/s extruded sample. Then, the microstructure completely transformed into equiaxed grains when extruded at a speed higher than 5 mm/s, and the recrystallized grains become coarser as the ram speed increasing. Compared with the 5 mm/s and 10 mm/s extruded samples, the 1 mm/s extruded sample exhibits the best strength and ductility. After T5 treatment, the ultimate tensile strength, tensile yield strength and elongation are 456 MPa, 333 MPa and 8.9%, respectively. The excellent mechanical properties are attributed to grain refinement strengthening, texture weakening, bimodal-grained structure and age hardening. The key to form a bimodal-grained structure is to restrict the temperature rise by controlling the extrusion speed.