Formation of gradient microstructure and elimination of tension-compression yield asymmetry of the extruded Mg-4.58Zn-2.6Gd-0.18Zr alloy via free-end torsion

Evident tension-compression yield asymmetry limited the potential use of the extruded Mg-4.58Zn-2.6Gd-0.18Zr alloy as a structural material. Pre-torsion deformation (free-end torsion) was conducted on the extruded alloy and its effects on microstructure and tensile-compressive mechanical properties were investigated in this study. It was found that the as-extruded alloy exhibited homogenous microstructure and <100>−<110 > double-peak fiber texture. Gradient structures along the radial direction were formed after pre-torsion, which contained gradient distribution of dislocations, grain size, {102} extension twin lamellas and texture components. <100>−<110 > double-peak fiber texture was gradually weakened with the torsional angle increasing from 90° to 135° and 180°, and it was related with the re-orientation of extension twinning and dislocation movement. Moreover, extension twinning was more easily to be activated in grains with <110> fiber texture component, and this phenomenon was revealed by calculating the global Schmid factor of extension twinning under pure shear stress state. Tensile and compressive yield strength increased monotonically with the increasing torsional angle. Meanwhile, ductility almost remained unchanged. Tension-compression yield asymmetry was gradually reduced, and it was eliminated after 180° pre-torsion, which was attributed to the texture weakening. The studying provided a promising and cost-effective method to enhance yield strength and reduce tension-compression yield asymmetry while also keep ductility for extruded Mg alloys.