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.

镁合金的斯威夫特效应对初始织构很敏感。然而，位错滑移是镁合金扭转过程中的主要变形机制。斯威夫特效应与位错滑移的潜在关系仍不清楚。通过实验研究了挤压 AZ31 合金在自由端扭转过程中应力状态和预应变对 Swift 效应的影响。自由端扭转是在低于屈服应力的轴向拉伸和压缩应力下进行的。研究发现，当轴向应力由负向正变化时，轴向变形从收缩向伸长转变。预张力引入的预位错促进扭转过程中的轴向缩短。而预压缩引入的预孪晶则抑制轴向缩短。轴向变形的变化归因于孪晶和棱柱滑移之间的竞争。挤压镁合金的轴向缩短是通过拉伸孪晶产生的，从而导致c轴应变。相反，轴向伸长可以通过棱柱滑移的激活来产生。孪晶产生的轴向应变大小比棱柱滑移大。此外，解孪的发生导致低剪切应变下的轴向伸长。