The role of double twinning on transgranular fracture in magnesium AZ61 in a localized stress field

The current work combines electron backscattered diffraction and Schmid's law numerical analysis to study the micro-mechanisms responsible for transgranular fracture in magnesium AZ61 subjected to a triaxial stress field operating at the vicinity of a notch. It is argued that the susceptibility of an individual grain to transgranular cracking depends strongly on its orientation relative to the stress condition. Grains in which the stress field creates high shear stresses on their {1 0 1¯ 1} twinning systems are more susceptible to transgranular fracture. The primary {1 0 1¯ 1} twinning mode was determined to follow a Schmid-type behavior in most of the studied grains. An inspection of the misorientation boundaries between the parent and twinned segments reveals the role of {1 0 1¯ 1}–{1 0 1¯ 2} double twinning, which essentially involves the rotation of 12¯ 1 0/37.5° relative to the parent, on the formation of transgranular micro-cracks. This twinning mode intensifies the shear stresses resolved on the basal slip systems, leading to early shear localization and transgranular void/crack initiation.