Twin nucleation at grain boundaries in Mg analyzed through in situ electron back-scatter diffraction and high-resolution digital image correlation

Abstract

Nucleation of extension twins at grain boundaries in weakly textured pure Mg was investigated through in situ mechanical tests in combination with electron backscatter diffraction and high-resolution digital image correlation. The Mg alloy exhibited a weak basal texture in which the Schmid factor for twinning under tensile deformation was low (or even negative) in most grains. Over 150 twin nucleation events at grain boundaries were recorded and classified into different groups. The most important ones were single twin nucleation and twin co-nucleation at grain boundaries and triple junctions while the number of twin transfer events was very small. It was found that twin nucleation/co-nucleation began at large grains suitably oriented for basal slip and gradually occurred in smaller grains with lower basal SF as the applied deformation increased while the twinning SF was not able to predict extension twinning. Thus, basal slip leads to the creation of stress concentrations at the grain boundaries that trigger twin nucleation. The microstructural features that dictate each type of twin nucleation event were analyzed and provided novel information about the influence of the microstructure of twin variant selection in Mg. In particular, it was found that twin variant selection was dictated in many cases by the orientation of twin shear strain with respect to the grain boundaries and was independent of the twinning Schmid factor.