Fig. 1. Nucleation of deformation twins in bicrystal W nanowires. (a) Pristine structure of a [$\bar{1}10$] -oriented bicrystal W nanowire. (b, c) Compression-induced deformation twins in the nanowire. Two deformation twins nucleated from the GB upon deformation. The size of twin 2 was 0.75 nm after nucleation (b) which grew into a thick twin rapidly (c). (d) The FFT pattern of twin 2. (e-h) Atomistic process of GB-assisted nucleation of twin 2. The deformation induced the migration of some GB segments (e-f), and the emission of a twin from the GB with a thickness of 6 atomic layers (g-h). (i) The distribution of thickness of twin nuclei in 22 W nanowires right after the nucleation.

Fig. 2. Discrete growth dynamics of deformation twins in W nanowires. (a-e) The nature of growth of a GB-nucleated deformation twin. The twin thickened repeatedly with an increment of 0.41 nm, corresponding to three-layers of {112} planes. (f) Atomic level observation of the discrete advancement of a TB with steps of approximately 0.41 nm thick. Note that due to the resolution limit, the atomic configuration in front of the TB step cannot be clearly identified. (g) A plot of twin thickness for the growth of a deformation twin, demonstrating that most growth events are approximately 3 atomic layers thick. (h) A deformation twin in bulk Nb deformed at 77 K exhibiting the stepwise TB structure, with step heights of about three atomic layers.

Fig. 3. DFT simulations of the twinning GSF curve in W. (a) Schematic showing the deformation process used to create the twinning GSF curve. (b) The GSF twinning curve computed using DFT for BCC W. The inset highlights just the valleys and peaks of the curve. (c) The amplitude of the Fourier transform, i.e. the spectrum, of the twinning GSF curve as a function of frequency. Two primary frequencies occur in the curve, with the main frequency at 1 cycles/bp and a secondary frequency at 0.276 cycles/bp. (d) The GSF twinning curves by the simultaneous propagation of a single, double, triple and quadruple twinning dislocation.

Fig. 4. Size-dependent deformation twinning in [$\bar{1}10$] -oriented W bicrystal nanowires. (a-c) Dislocation-dominated plasticity in a [$\bar{1}10$]-oriented W nanowire with a diameter of 65 nm. The inset in (a) is the corresponding diffraction pattern. Before deformation, no lattice defects existed in the nanowire (a); straining caused the nucleation of dislocations from multiple sources in the nanowire (b); a deformation band formed by extensive localized slip (c). The red arrows in (b, c) point out the dislocation lines, while the pink arrow in (c) indicates the deformation band. (d) The observed dominant deformation mechanism plotted as a function of diameter in [$\bar{1}10$] -oriented W bicrystal nanowires. From this plot, there is a mechanism transition size of -40 nm. (e-g) Twinning-controlled plasticity in W nanowires with the diameter of 11 nm. (h-i) An enlarged image and corresponding FFT pattern confirm the deformation twin.

Fig. 5. Competition between deformation twin and dislocation slip in a 39 nm W nanowire. (a) Pristine bicrystal W nanowire with a diameter of 39 nm. (b, c) The W nanowire initially deformed by deformation twin (from a surface) and subsequent growth. (d) The combination of deformation twinning and dislocation slip during further straining, due to the dislocation emissions from the free surface, twin front, and GB.