Twinning-like lattice reorientation without a crystallographic twinning plane

Twinning on the {10 (1) over bar2} plane is a common mode of plastic deformation for hexagonal-closepacked metals. Here we report, by monitoring the deformation of submicron-sized single-crystal magnesium compressed normal to its prismatic plane with transmission electron microscopy, the reorientation of the parent lattice to a 'twin' lattice, producing an orientational relationship akin to that of the conventional {10 (1) over bar2} winning, but without a crystallographic mirror plane, and giving plastic strain that is not simple shear. Aberrationcorrected transmission electron microscopy observations reveal that the boundary between the parent lattice and the ` twin' lattice is composed predominantly of semicoherent basal/prismatic interfaces instead of the {10 (1) over bar2} winning plane. The migration of this boundary is dominated by the movement of these interfaces undergoing basal/prismatic transformation via local rearrangements of atoms. This newly discovered deformation mode by boundary motion mimics conventional deformation twinning but is distinct from the latter and, as such, broadens the known mechanisms of plasticity.