Enhanced non-linearity during unloading by LPSO phase in as-cast Mg-Zn-Y alloys and slip-dominated non-linear unloading mechanism

The non-linear unloading behavior and the underlying deformation mechanism of as-cast Mg-Zn-Y alloys with the long period stacking ordered (LPSO) phase were investigated. To evaluate the stress-strain behavior, compression-unloading tests under stress control were performed for as-cast alloys with six different volume fractions of the LPSO phase, which were produced using the high-frequency induction melt casting technique in an Ar atmosphere. The experimental results showed that the non-linearity during unloading was more significant for alloys with a higher volume fraction of the LPSO phase. The LPSO single-phase alloy, in which deformation twinning was hardly activated, exhibited the most significant non-linear unloading behavior, which originated from the small linear elastic region and the steep decrease in the instantaneous hardening modulus during unloading. Crystal plasticity finite-element analysis for the compression-unloading of the LPSO single-phase alloy indicated that the activity of the basal slip system during unloading led to non-linear unloading behavior. The heterogeneous stress distribution that developed during compressive deformation led to the activity of the basal slip system during unloading owing to the local stress tensor, whose normal stress component in the axial direction had opposite sign to that of the macroscopically imposed stress. Comparisons between numerical results with different implemented deformation modes, i.e., the basal slip system and the second-order pyramidal slip system, indicated that strong plastic anisotropy contributes to the development of a pronounced heterogeneous stress field and an insignificant increase in the critical resolved shear stress before unloading, which leads to a higher non-linearity during unloading.