Anisotropic yielding and spontaneous shear plastic deformation of extruded AZ31 alloy in σ-τ space: Inverse Swift effect

• The yield point is detected by the physical meaning offset strain for the first time. • The combined non-proportional loading paths are proposed to obtain the yield surface. • The rotation and distortion of SYS are dominated by pre-torsion and anisotropy. • Recoverable shear plasticity is dominated by residual shear stress. • The inverse Swift effect is governed by pre-twinning and multi-axial loading. The physical mechanism underlying the generation and evolution of the inverse Swift effect in hexagonal close-packed (HCP) Mg alloy has not been clarified. The traditional yield surface is orthogonal to the normal stress direction during free rotational tension (FRT) due to the uni-directional pre-loading paths, which contradicts the inverse Swift effect experimentally observed. Therefore, multi-step loading experiments including free end torsion (FET) and FRT have been performed. Based on specially designed non-proportional loading paths changing in elastic domain, the evolution mechanism of the yield surfaces and anisotropic hardening behaviors in σ-τ space were explored. It is found that the {10_12} tensile twinning introduced by FET leads to the rotation and distortion of the initial yield surface (IYS). Both Swift- and inverse Swift effects can be detected simultaneously by IYS and subsequent yield surface (SYS) by current designed loading paths. The recoverable shear plastic deformation is driven by residual shear stress after the complete nonlinear unloading. Reverse rotation is accelerated by detwinning until the initial twins are exhausted. A new-type inverse Swift effect, i.e., spontaneous positive rotation during FRT, is found to be dominated by local heterogeneous strain and orientation inhomogeneity.