An evolution of subsequent yield loci under proportional and non-proportional loading path of ‘as-received’ extruded AZ31 magnesium alloy: Experiments and CPFEM modeling

This work presents an experimental investigation of as-received extruded AZ31 magnesium alloy subjected to proportional and non-proportional loading paths at various finite levels of pre-strain. The measured subsequent yield loci evolutions in the σ11−3σ12 stress space, using a 10µε offset linearity definition of yield, with increasing plastic deformation are reported in this paper. Initial yield locus deviates substantially from a von-Mises yield loci. Subsequent yield loci under proportional loading show an usual nose in the loading direction and almost a flat shape in reverse loading direction. In addition, yield loci in all proportional loading paths (tension, free end torsion and combined tension-torsion) have significant kinematic and some isotropic hardening. Subsequent yield loci under non-proportional loading (tension after torsion and torsion after tension) show distortion in addition to significant kinematic and isotropic hardening, as well as, demonstrate a strong path-dependency. During these experiments with the increasing plastic strain, elastic moduli within each subsequent yield loci are also measured. These experimentally determined yield loci of as received AZ31 alloy are significantly different from the observed ones for aluminum alloys by Khan et al. (2009) and Iftikhar et al. (2021). However, these results are comparable to the ones reported recently by Iftikhar and Khan (2021) for annealed extruded AZ31 alloy. Numerical modeling using crystal plasticity models was carried out to study the origin of the distortion of the yield locus. The CP model is calibrated using the tensile and shear flow curves. Using the CP framework, initial and subsequent yield loci are predicted. CP predictions show good predictions. The CP simulations can capture the observed distortion and asymmetry in the yield loci. In addition, the CP simulations can give insight on the origin of the kinematic hardening, distortion and asymmetry. They show that these are correlated with activity of various slip systems and the amount of twinning occurring during the loading along different loading paths.