Reduced yield asymmetry and excellent strength-ductility synergy in Mg-Y-Sm-Zn-Zr alloy via ultra-grain refinement using simple hot extrusion

An ultrafine-grained (UFG) Mg-RE alloy with superior yield symmetry and strength-ductility synergy was successfully fabricated by simple hot extrusion. The as-extruded specimen was composed of nearly fully recrystallized UFG microstructure with a mean grain size of 0.93 μm and exhibited a nearly random texture (maximum intensity is 1.38). The abundant nano-precipitates and the segregation of alloying elements at grain boundaries played key roles in inhibiting the growth of freshly recrystallized sub-micrometer grains formed during hot extrusion. The mechanical tests revealed that the extruded alloy exhibited an excellent tension/compression yield ratio of 1.03, which was attributed to the suppression of the extension twinning and the extremely weak texture. The suppression of extension twinning mainly resulted from the grain refinement, high concentration of rare earth solute in Mg matrix, precipitates in parts of grains and randomly oriented grains. The main deformation mode of UFG specimen was basal slip in both tension and compression, leading to the similar strain hardening behavior. This work suggests that the high tension-compression symmetry of Mg alloys can be achieved by grain refinement and texture randomization.