Characterizing the boundary lateral to the shear direction of deformation twins in magnesium

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 [Image: see text] deformation twins in magnesium. It is found that the dark side is serrated and comprised of [Image: see text] coherent twin boundaries and semi-coherent twist prismatic–prismatic [Image: see text] 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.