Origin of nucleation and growth of extension twins in grains unsuitably oriented for twinning during deformation of Mg-1%Al

A large number of anomalous extension twins, with low or even negative twinning Schmid factors, were found to nucleate and grow in a strongly textured Mg-1Al alloy during tensile deformation along the extruded direction. The deformation mechanisms responsible for this behaviour were investigated through in-situ electron back-scattered diffraction, grain reference orientation deviation, and slip trace - modified lattice rotation. It was found that anomalous extension twins nucleated mainly at the onset of plastic deformation at or near grain boundary triple junctions. They were associated with the severe strain incompatibility between neighbour grains as a result from the different 〈a〉 basal slip-induced lattice rotations. Moreover, the anomalous twins were able to grow with the applied strain due to the continuous activation of 〈a〉 basal slip in different neighbour grains, which enhanced the strain incompatibility. These results reveal the complexity of the deformation mechanisms in Mg alloys at the local level when deformed along hard orientations.