Compound cross-grain boundary extension twin structure and its related twin variant selection in a deformed Mg alloy

Interrupted bi-axial compressions are applied to a hot-rolled Mg alloy with a strong basal texture in order to activate {1 0 - 1 2}-{1 0 - 1 2} double extension twinning. Compound cross-grain boundary (cross-GB) extension twin structures form after the compressions, which consist of cross-GB secondary twin pairs within their host cross-GB primary twin pairs. Shear accommodation of a twin is proven to be proportional to a geometrical m0 factor and the magnitude of its twinning shear. It is found that 60% of cross-GB twin pairs, including both the primary and the secondary ones, have a value of m0> 0.7, indicating twinning shear transmission over GB is a major mechanism. Multi-level twinning shear transmissions over GB contribute prominently to the formation of 40% of the observed compound cross-GB twin structures. About 98.3% of the twins are high Schmid factor (SF) ones, while low SF ones appear due to twinning shear transmission. About 81.7% of secondary twins have a misorientation of < 0 14 -14 1 > 60 degrees with respect to their host grains, indicating a strong confining effect of primary twins. A cross-GB twin pair could form through a mechanism of associated nucleation or isolated nucleation. High m0 value is characteristic of associated nucleation through twin-to-twin accommodation mode. Depending on the process of microstructure evolution, SF, shear accommodation and the confining effect of the matrix if necessary should be synthetically considered to predict twin variant selection related to cross-GB twin pairs. (C) 2017 Elsevier B.V. All rights reserved.