Understanding the twinning-detwinning mechanism in the Swift effect for Mg–3Al–1Zn bar during free-end torsion

The Swift effect, namely the axial response accompanying torsion, is strongly affected by twinning mechanisms in magnesium alloys, yet detwinning occurs during torsion, and its quantitative connection to the axial response remains insufficiently clarified. In this work, an extruded AZ31 Mg alloy bar is studied under a pre-compression-free-end torsion loading path to tailor various initial {10–12} extension twin fractions. A continuous transition of the Swift effect from axial contraction to axial elongation is observed with increasing pre-strain, indicating the evolutionary change in the dominant twinning-related contribution to the axial response. Moreover, the elastic visco-plastic self-consistent model with twinning and detwinning scheme, together with torsion-specific finite-element approach (TFE-EVPSC-TDT) reproduces the first-order shear response and captures the overall evolution trend of the second-order axial strain. A novel criterion based on the modified global Schmid factor (GSF) for twinning and detwinning under torsion is proposed: the nucleation and growth of {10–12} extension twin occur in grains or twin structures with a positive GSF, while detwinning is favored in prefabricated twins with a negative GSF, and the orientations of pre-twins may promote either re-twinning or detwinning depending on the orientations of the parent grains. In addition, an analytical “twinning-only” upper-bound model is established to quantify the axial contribution of extension twinning under torsion. The analysis indicates that the maximum twinning-related axial contribution reaches ~6.66%, and the remaining deviation of the measured axial strain can be attributed to the additional slip-assisted axial extension that becomes increasingly important as shear straining. The findings in the present work provide a new and significant understanding of the twinning and detwinning mechanism in the Swift effect of Mg alloys.