A controversy concerning the description of {1012} 1011 twinning, whether it is shear-shuffle or pure glide-shuffle or pure shuffle, has developed. There is disagreement about the interpretation of transmission electron microscopic observations, atomistic simulations and theories for twin growth. In this article, we highlight the atomic-level, characteristic, equilibrium and non-equilibrium boundaries and corresponding boundary defects associated with the threedimensional 'normal', 'forward' and 'lateral' propagation of {1012} growth/annealing and deformation twins. Although deformation twin boundaries (TBs) after recovery exhibit some similarity to growth/annealing TBs because of the plastic accommodation of stress fields, there are important distinctions among them. These distinctions distinguish among the mechanisms of twin growth and resolve the controversy. In addition, a new type of disconnection, a glide disclination, is described for twinning. Synchroshear, seldom considered, is shown to be a likely mechanism for {1012} twinning. IMPACT STATEMENTA controversy concerning {1012} 1011 twinning in hcp metals has developed. We present comprehensive understanding of interface structures and twinning mechanisms of {1012} growth/annealing and deformation twins at the atomic level. ARTICLE HISTORY

The three-dimensional nature of twins, especially the atomic structures and motion mechanisms of the boundary lateral to the shear direction of the twin, has never been characterized at the atomic level, because such boundary is, in principle, crystallographically unobservable. We thus refer to it here as the dark side of the twin. Here, using high-resolution transmission electron microscopy and atomistic simulations, we characterize the dark side of deformation twins in magnesium. It is found that the dark side is serrated and comprised of coherent twin boundaries and semi-coherent twist prismatic–prismatic boundaries that control twin growth. The conclusions of this work apply to the same twin mode in other hexagonal close-packed materials, and the conceptual ideas discussed here should hold for all twin modes in crystalline materials.

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.

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