Effects of free-end torsion on the microstructure evolution and fatigue properties in an extruded AZ31 rod

Pre-deformation influenced the microstructure of extruded magnesium alloys such that the mechanical properties and fatigue resistance of these alloys could be enhanced. In this study, free-end torsion was conducted to change the distribution of dislocation, extension twins and texture components in the extruded magnesium alloy AZ31 rod. Results showed that the extension twins at the center were induced not by shear strain but by the Swift effect. The compressive principal stress on the cylindrical surface greatly contributed to the formation of extension twins at the edge. The softening effect of de-twinning, which exceeded the impeditive effect of dislocations, significantly decreased the yield strength of magnesium alloy rods. Strain-controlled low-cycle-fatigue experiments were conducted on specimens subjected to different pre-torsion deformation conditions. The fatigue resistance of magnesium alloy rods was found to be enhanced by pre-torsion deformation, which was mainly related to the reduction of cyclic mean stress and cyclic stress amplitude, as well as to the slow strain hardening during the cyclic tensile-loading stage.