Scott Mao1 Yang He1 Chongmin Wang2

1, University of Pittsburgh, Pittsburgh, Pennsylvania, United States
2, Pacific Northwest National Laboratory, Richland, Washington, United States

Twinning is an essential carrier of plastic deformation and critically influences the mechanical behavior of materials. Due to its atomic-scale complexity, the atomic mechanism of twinning nucleation and growth in hexagonal close-packed (HCP) crystals remains theoretical contention to date, which largely impedes the design and processing of high strength alloys. Here, by using in situ transmission electron microscopy, twinning mechanism was directly captured in HCP crystals. In stark contrast to the classical twinning dislocation mechanism, it was found that {1 0 -1 2} twinning nucleation could be dominated by atomic shuffles which were manifested by the direct transformations between prismatic and basal planes; this process established the lattice correspondence of the {1 0 -1 2} twin. During the twin growth, the incoherent B│P-type twin boundaries transformed to coherent twin boundaries which propagated by the movement of classical twinning disconnections. The findings resolve long-standing controversies in twinning of HCP crystals, paved the way for twinning-based design and processing of high strength HCP alloys, and hold broad implication to the deformation in complex crystal structures.