In situ observation of the atomic shuffles during the { } twinning in hexagonal close-packed rhenium.

Twinning, on par with dislocations, is critically required in plastic deformation of hexagonal close-packed crystals at low temperatures. In contrast to that in cubic-structured crystals, twinning in hexagonal close-packed crystals requires atomic shuffles in addition to shear. Though the twinning shear that is carried by twinning dislocations has been captured for decades, direct experimental observation of the atomic shuffles, especially when the shuffling mode is not unique and does not confine to the plane of shear, remains a formidable challenge to date. Here, by using in-situ transmission electron microscopy, we directly capture the atomic mechanism of the twinning in hexagonal close packed rhenium nanocrystals. Results show that the twinning is dominated by the (b, h) twinning disconnections. In contrast to conventional expectations, the atomic shuffles accompanying the twinning disconnections proceed on alternative basal planes along 1/6 , which may be attributed to the free surface in nanocrystal samples, leading to a lack of mirror symmetry across the twin boundary.