Search for: All records

Abstract 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$$\left\{11\bar{2}1\right\}$$ $\left(11\overline{2}1\right)$twinning in hexagonal close packed rhenium nanocrystals. Results show that the$$\left\{11\bar{2}1\right\}$$ $\left(11\overline{2}1\right)$twinning is dominated by the (b_1/2, h_1/2) twinning disconnections. In contrast to conventional expectations, the atomic shuffles accompanying the twinning disconnections proceed on alternative basal planes along 1/6$$\left\langle 1\bar{1}00\right\rangle$$ $\left(1\overline{1}00\right)$, which may be attributed to the free surface in nanocrystal samples, leading to a lack of mirror symmetry across the$$\left\{11\bar{2}1\right\}$$ $\left(11\overline{2}1\right)$twin boundary. 

Grain boundary (GB) structural change is commonly observed during and after stress-driven GB migration in nanocrystalline materials, but its exact atomic scale transformation has not been explored experimentally. Here, using in situ high-resolution transmission electron microscopy combined with molecular dynamics simulations, we observed the dynamic GB structural transformation stemming from reversible facet transformation and GB dissociation during the shear-mediated migration of faceted GBs in gold nanocrystals. A reversible transformation was found to occur between (002)/(111) and Σ11(113) GB facets, accomplished by the coalescence and detachment of $\left(\overline{1}\overline{1}1\right)/\left(002\right)$-type GB steps or disconnections that mediated the GB migration. In comparison, the dissociation of (002)/(111) GB into Σ11(113) and Σ3(111) GBs occurred via the reaction of $\left(111\right)/\left(11\overline{1}\right)$-type steps that involved the emission of partial dislocations. Furthermore, these transformations were loading dependent and could be accommodated by GB junctions. This work provides atomistic insights into the dynamic structural transformation during GB migration.