Modeling the temperature dependent effect of twinning on the behavior of magnesium alloy AZ31B sheet

Uniaxial compression test data were obtained from magnesium alloy AZ31B sheet material tested along three sample directions (rolling, transverse and normal direction) over the temperature range T = 22–250 °C. The yield point during in-plane compression is insensitive to temperature, up to 200 °C, suggesting that athermal mechanisms are responsible for yielding. The in-plane compression samples exhibit very low r-values, which provides another signature of significant twinning activity in magnesium sheet, in addition to the characteristic sigmoidal strain hardening curve. By varying the critical resolved shear strengths (CRSS) and hardening behaviors of the deformation mechanisms, it is possible to model the changes in the flow stress profile, the strain anisotropy, and texture evolution using a viscoplastic self-consistent polycrystal model. Notably, the CRSS values for basal slip were observed to be constant, while that of twinning increased slightly, and the CRSS values of thermally activated slip modes, i.e., prismatic and pyramidal 〈c + a〉 slip, decrease over the temperature range investigated. Because deformation twinning is observed to be significantly active over the entire temperature range, and the ductility improves markedly as the temperature is increased, it is concluded that twinning is not intrinsically detrimental to the ductility. However, the poor ductility during in-plane compression at the lower temperatures appears to be connected with the twinning reorientation since there is a very limited ability to accommodate c-axis compression.