Anisotropic yield loci and inverse Swift effect in extruded AZ31 Mg alloy under multi–degree-of-freedom torsional–axial non-proportional loading paths

The deformation mechanisms of Mg alloy under multi–degree-of-freedom axial, torsional, and combined loadings remains critically unclear. This is particularly significant in the case of the anisotropic evolution of yield loci and inverse Swift effect, which are crucial for optimizing the forming technologies for engineering parts. In order to clarify the underlying deformation mechanisms, combined multi–degree-of-freedom axial–torsional non-proportional loading paths are specially designed. The anisotropic evolution of the yield loci and Swift–inverse Swift effects in extruded AZ31 Mg alloys are investigated. The strong loading-path–dependent twinning activities and underlying deformation mechanisms are clarified in detail. The findings reveal that the inverse Swift effect during free rotational tension (FR_Ten) is attributed to the residual shear stress and initial texture heterogeneity, while the spontaneous macroscopic rotation during free rotational compression (FR_Com) is found to originate from the heterogeneous local strain induced by the interactions of the $\left\{10\overline{1}2\right\}$ twins. Free end torsion (FE_Tor) pre-straining induces subsequent yield locus (SYL) rotation towards the positive τ axis and expansion along the negative σ axis. The anisotropic Swift–inverse Swift effects are accurately captured by the plastic strain-components on the yield loci. Tensile twinning and basal slip coordinate the plastic deformation under FR_Com-dominated loading paths, owing to the low twin favourability, and the relative activities of non-basal slips under FR_Ten-dominated loading paths are significantly improved. The evolutions of the Swift–inverse Swift effects are determined by elastic pre-loadings, resulting in loading–path-dependent anisotropic evolutions of mechanical responses.