Abstract: The evolution mechanism of the second-order mechanical behaviors under the complex pre-torsion path has not been clarified, which limits its potential application for optimizing precision forming processes of Mg alloy structural components. In this work, the combined loading of reciprocating free-end torsion (FET) and free-rotational tension (FRT) was performed on the extruded AZ31 solid rod with basal fiber texture. The corresponding mechanical response, microstructure evolution, and twinning behaviors were investigated, and the physical mechanism of the second-order behaviors (both the Swift and inverse Swift effects) was discussed. The results indicate that the two-stage deformation of reciprocating torsion is capable of activating tensile twins, effectively refining the surface grains, and thus improving the yield asymmetry. Due to the reverse load, detwinning dominates the deformation of reverse FET (RFET), while detwinning in FRT deformation is caused by the spontaneous rotation of the inverse Swift effect. The twin variants with c-axis tending to extrusion direction (ED) are predisposed to detwinning, and the bimodal texture introduced by FET is degraded. The plastic deformation of RFET is primarily coordinated by basal and prismatic slips. The large strain FRT is dominated by basal slip due to its low $\overline{S F}$ /CRSS. The interaction between twinning and detwinning dominates the multi-directionality of the Swift effect during reciprocating torsion, and the cumulative effect of strain hardening further enhances the dependence of RFET axial strain on basal slip. The dγ/dε index successfully captures the evolution of the inverse Swift effect during FRT. The forward rotation of the solid rod is caused by the inversion of the radial residual shear stress field, and the internal stress introduced by the torsional inhomogeneity is compensated. The subsequent reverse rotation is the result of balancing the orientation inhomogeneity and local strain heterogeneity introduced by the shear deformation. The increase in the reverse rotation rate is caused by detwinning.

Key words: Mg alloy, Reciprocating torsion, Twin activities, Swift effect, Inverse Swift effect