Three-dimensional character of the deformation twin in magnesium

Deformation twins are three-dimensional domains, traditionally viewed as ellipsoids because of their two-dimensional lenticular sections. In this work, we performed statistical analysis of twin shapes viewing along three orthogonal directions: the ‘dark side’ (DS) view along the twin shear direction ( η 1 ), the twinning plane normal (TPN) view ( k 1 ) and the ‘bright side’ (BS) view along the direction λ (= k 1 × η 1 ). Our electron back-scatter diffraction results show that twins in the DS and BS views normally exhibit a lenticular shape, whereas they show an irregular shape in the TPN view. Moreover, the findings in the TPN view revealed that twins grow faster along λ the lateral direction than along η 1 the forward propagation direction at the initial stages of twin growth. These twin sections are irregular, indicating that growth is locally controlled and the overall shape is not perfectly ellipsoidal. We explain these findings using atomistic models, and ascribe them to differences in the mobility of the edge and screw components of the twinning dislocations. Imaging deformation twins in three dimensions is difficult and they are usually viewed as two-dimensional ellipsoids. Here, the authors statistically analyze more than two hundred deformation twins in magnesium observed in three different views and show lateral twin expansion is faster than forward propagation.