Flow asymmetry and nucleation stresses of image twinning and non-basal slip in magnesium

Non-Schmid phenomena leading to yield asymmetry are known to occur in BCC metals and intermetallic compounds due to complex mechanisms such as those related to the three-dimensional core of screw dislocations, and their resulting cross slip activities sensitive to non-Schmid stresses. In this study, we identify and discuss a flow asymmetry due to a possible dependence of image twinning and non-basal slip critical resolved shear stresses. Both crystal plasticity simulations and EBSD serial imaging analyses, where an identical region is analyzed at various strain levels, suggested that these non-Schmid effects correlate with the effect of mantle plasticity, which is sensitive to the stress sign and initial texture. It was deduced from EBSD serial imaging analyses that low misoriented grain boundaries underwent a substantially higher nucleation and growth rates of twinning than highly misoriented grain boundaries. Conventional crystal plasticity based on a pseudo-slip approach for twinning was unable to capture these mantle-induced grain boundary effects related to the magnesium tension–compression asymmetry.