Accommodation and formation of {112̄1} twins in Co single crystals

In situ observation of the atomic shuffles during the { 11 2 ¯ 1 } 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 the11 2 ¯ 1 twinning in hexagonal close packed rhenium nanocrystals. Results show that the11 2 ¯ 1 twinning is dominated by the (b1/2, h1/2) twinning disconnections. In contrast to conventional expectations, the atomic shuffles accompanying the twinning disconnections proceed on alternative basal planes along 1/61 1 ¯ 00 , which may be attributed to the free surface in nanocrystal samples, leading to a lack of mirror symmetry across the11 2 ¯ 1 twin boundary.

Statistical Analysis of Grain-Scale Effects of Twinning Deformation for Magnesium Alloys under Cyclic Strain Loading

To reveal the relationship between grain size and twinning deformation of magnesium alloys under cyclic strain, this study carried out a group of strain-controlled low-cycle fatigue experiments and statistical analysis of microstructures. Experimental results show that the shape of the hysteresis loop exhibits significant asymmetry at different strain amplitudes, and the accumulation of residual twins plays an important role in subsequent cyclic deformation. For the different strain amplitudes, the statistical distribution of the grain size of magnesium alloy approximately follows the Weibull probability function distribution, while the statistical distribution of twin thickness is closer to that of Gaussian probability function. The twin nucleation number (TNN) increases with the increase of grain size, but there is no obvious function relationship between twin thickness and grain size. Twin volume fraction (TVF) increases with the increase of grain size, which is mainly due to the increase of TNN. This work can provide experimental evidence for a more accurate description of the twinning deformation mechanism.

Atomic shuffling dominated mechanism for deformation twinning in magnesium

Deformation twinning is often mediated by partial dislocation activities at the twin boundary. Using molecular dynamics simulations, we have uncovered a new mechanism for the most commonly observed {1012}<101 1> deformation twinning in Mg and other hexagonal close-packed metals. Here the twin growth involves no definable dislocations at the twin boundary, and the twin orientational relationship can be established by local atomic shuffling, directly constructing the twin lattice from the parent lattice.