Three-dimensional investigation of grain boundary-twin interactions in a Mg AZ31 alloy by electron backscatter diffraction and continuum modeling

The effect of grain boundary (GB) misorientation (theta) on twinning in a Mg AZ31 alloy is investigated using a three-dimensional (3-D) experimental and modeling approach, in which 3-D electron backscattered diffraction is performed in a volume consisting of a central grain, favorably oriented for twinning, and surrounded by three boundaries, with theta ranging from 15 degrees to 64 degrees. This study corroborates previous observations that twin nucleation and propagation are favored at low theta. Furthermore, it reveals that non-Schmid effects, such as the activation of low Schmid factor (SF) variants or of double tensile twins, are absent in the vicinity of low misorientation boundaries and that they become more abundant as theta increases. The 3-D morphology of individual twin variants is found to be related to their SF. High SF variants have well-established plate morphology, while low SF variants adopt irregular shapes. A crystal plasticity continuum model recently proposed by the authors is used in a very high intragrain resolution and large-scale finite element polycrystalline aggregate model of the experimental specimen. This model is shown to successfully capture the influence of theta on twin propagation and variant selection. It ultimately predicts (i) a rise in local non-basal slip with increasing theta, (ii) that low theta GB favor twin nucleation by non-Schmid stress concentrations, but that propagation is immediately accommodated by the macroscopic stress, and (iii) that high theta GB are not favorable twin nucleation sites, despite having high von Mises stress concentrations. (C) 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.