The roles of stress state and pre-straining on Swift effect for an extruded AZ31 Mg alloy

The Swift effect of Mg alloy is sensitive to initial texture. However, dislocation slip is the main deformation mechanism during torsion of Mg alloy. The underlying relation of Swift effect and dislocation slip is still not clarified. The effect of stress state and pre-straining on Swift effect was studied experimentally during free-end torsion for an extruded AZ31 alloy. The free-end torsion was performed with axial tension and compression stress which is lower than yield stress. It is found that the transition of axial deformation from contraction to elongation occurs when the axial stress changes from negative to positive. The pre-dislocations introduced by pre-tension promote axial shortening during torsion. While the pre-twins introduced by pre-compression are inhibition of axial shortening. The change of axial deformation is attributed to competition between twinning and prismatic slip. The axial shortening of extruded Mg alloy is generated by tensile twinning leading to c-axis strain. In contrast, the axial elongation can be generated by the activation of prismatic slip. The magnitude of axial strain generated by twinning is larger than that by prismatic slip. Moreover, the occurrence of detwinning results in axial elongation at low shear strain.