In this work, the {10–12} tensile twins are introduced to improve the drawability of the AZ31 Mg alloy sheet. Concretely, the drawing depth is increased by 32% compared with the as-received sheet at 200 °C. This is because {10–12} tensile twins promote the occurrences of many deformation mechanisms during warm deep drawing, such as slips, detwinning, dynamic recrystallization (DRX) behaviors, etc. Further, based on the different stress states during deep drawing, these mechanisms and their competition relationships, as well as texture evolutions, are systematically studied. Combined with critical resolved shear stress (CRSS) and microstructure evolution, the global Schmid factor (GSF) obtained by quantizing stress states by stress tensor (σ) can accurately predict the activation trend of deformation mechanisms. It is found that the stress states have a reverse influence on the activation trend of the {10–12} twinning and detwinning. The change of stress states affects the competitive relationships between detwinning and DRX, and then affects the process and degree of DRX. The {10–12} tensile twins and large plane strain promote the activation of prismatic slips, and the larger plane strain also deflected the {10–12} twinning lattice. The {10–12} tensile twins and their induced deformation mechanisms can prominently weaken the basal texture and improve the drawability.

Magnesium (Mg) single crystal specimens with three different orientations were prepared and tested from room temperature to 733 K in order to systematically evaluate effects of temperature on the critical resolved shear stress (CRSS) of slips and twinning in Mg single crystals. The duplex non-basal <a> slip took place in the temperature range from 613 to 733 K when the single crystal samples were stretched along the <0110> direction. In contrast, the single basal <a> slip and prismatic <a> slip were mainly activated in the temperature range from RT to 733 K when the tensile directions were inclined at an angle of 45° with the basal and the prismatic plane, respectively. Viscoplastic self-consistent (VPSC) crystal modeling simulations with genetic algorithm code (GA-code) were carried out to obtain the best fitted CRSSs of major deformation modes, such as basal <a> slip, prismatic <a> slip, pyramidal Ⅱ <c+a>, {1012} tensile twinning and {1011} compressive twinning when duplex slips accommodated deformation. Additionally, CRSSs of the basal <a> and the prismatic <a> slip were derived using the Schmid factor (SF) criterion when the single slip mainly accommodated deformation. From the CRSSs of major deformation modes obtained by the VPSC simulations and the SF calculations, the CRSSs for basal <a> slip and {1012} tensile twinning were found to show a weak temperature dependence, whereas those for prismatic <a>, <c+a> slip and {1011} compressive twinning exhibited a strong temperature dependence. From the comparison of previous results, VPSC-GA modeling was proved to be an effective method to obtain the CRSSs of various deformation modes of Mg and its alloys.

To investigate the role of pre-twins in Mg alloy sheets during warm planar deformation, the stretch forming is conducted at 200 ℃. Results suggest the formability of the pre-twinned AZ31 Mg alloy sheet is enhanced to 11.30 mm. The mechanisms for the improved formability and the deformation behaviors during the planar stretch forming are systematically investigated based on the planar stress states. The Schmid factor for deformation mechanisms are calculated, the results reveal that planar stress states extremely affect the Schmid factor for {10-12} twinning. The detwinning is activated and the prismatic slip is enhanced in the pre-twinned sheet, especially under the planar extension stress state in the outer region. Consequently, the thickness-direction strain is accommodated better. The dynamic recrystallization (DRX) type is continuous DRX (CDRX) regardless of the planar stress state. However, the CDRX degree is greater under the planar extension stress state. Some twin lattices deviate from the perfect {10-12} twinning relation due to the planar compression stress state and the CDRX. The basal texture is weakened when the planar stress state tends to change the texture components.