Fig. 1. Grain boundary structure of an embedded nanograin in a NC Au sample. (a) As-prepared NC Au sample consisting of a nanograin G1 surrounded by three other grains G2-G4. (b-d) Magnified images revealing the atomic structures of GBs of G1, corresponding to b-d labeled in (a), respectively. Dislocations are marked with upside-down “T”, where different colors represent dislocations with different Burgers vectors. The high-angle GB1-4 is characterized with structural units of “C” and “D”. The yellow lines in the triangles represent (002) planes, and the white lines represent closely-packed {111} planes.

Fig. 2. Sequential images showing the shrinkage of G1 dominated by coordinated GB migration. The white dashed lines indicate the current positions of the GBs. The yellow and red arrows in (b) represent the directions of shear loading and GB migration, respectively.

Fig. 3. Atomistic mechanism of correlated GB migration. (a-c) Limited motion of GB1-2, where GB dislocations at the vertical and horizontal GB segments migrate and climb with a similar distance. The yellow dashed lines and circles in (c) indicate the previous positions of GB1-2 and GB dislocations in (b), respectively. (d-f) Migration of GB1-3 mediated by dislocation glide and localized dislocation climb. (g-i) Migration of dissociated high-angle GB1-4. The yellow dashed lines in (g-i) serve as a reference for GB migration. The directions of dislocation climb and GB migration are indicated by the white and red arrows, respectively.

Fig. 4. GB dynamics during the grain shrinkage revealed by MD simulations. The size of the simulated nanocrystal and the structure of the surrounding GB network are identical to those in the real nanocrystal in Fig. 3. Insets in (a-c) present a Burgers vector analysis of the GB dislocations, where the blue lines indicate 1/2 < 110> full dislocations and green lines indicate 1/6 < 112> Shockley dislocation partials. The directions of Burgers vectors and GB migration are denoted by the blue and yellow arrows, respectively.

Fig. 5. Grain annihilation in shear cycling. (a-b) Continuous migration of GB1-2 and the dissociated GB1-4 in reversed shear. The yellow dashed lines and circles in (b) denote the initial GB positions and GB dislocations in (a), respectively. (c-d) Successive migration of GB1-2 in a mode of collective climb and localized slip of GB dislocations, which induces a clockwise rotation of G1 measured about 5°. In the meantime, the SFs in dissociated GB1-4 continued to migrate upward, with its lattice planes rotating clockwise continuously. (d-e) The glide of dislocations from the left (e.g., A and B) further react with the dislocations on GB1-2 and SFs in the dissociated GB1-4, inducing the grain rotation and annihilation of G1. (e-f) Emission and annihilation of SFs at neighboring GBs.

Fig. 6. Misorientation evolutions of G1/G2 (the blue curve) and G1/GB1-4 (the red curve) in shear cycling. The signs of misorientation changes are defined by the inset in (a), where the embedded G1 is selected as a reference. (b) shows the misorientation changes during the fast annihilation under reversed loading, where the labels 5a-5e on the curve correspond to the deformation snapshots in Fig. 5(a-e).

Fig. 7. Temporal evolution of the area covered by G1 and its surrounding GBs' migration distances in shear cycling.

Fig. 8. The atomistic process of grain annihilation within a shear loading cycle in MD simulations. (a-c) Grain shrinkage dominated by GB migration under the rightward shear loading. (e-g) Grain annihilation upon the reversed shear. The corresponding shear stress distribution is analyzed for each snapshot. (d, h) Temporal changes of the total potential energy in the processes of rightward shear (d) and reversed leftward shear (h), respectively. The labels on the curves correspond to the deformation snapshots in (a-c) and (e-g), while “i” in (h) indicates an energy decrease after G1 rotates into G2's lattice and thereby causing its eventual annihilation.

Fig. 9. Coordinated deformation between different GBs within the GB network in shear cycling. (a-b) and (d-e) Deformation snapshots and the corresponding geometrical phase analysis (GPA) of stress state around the G1 under the rightward and reversed shear loading, respectively. (c, f) Schematics illustrating the coordinated deformation between different GBs. The length of the arrows indicates the magnitude of shear strain around G1 and migration of the GBs.

Fig. 10. Generality of shear-induced grain shrinkage in various GB networks. (a-d) Sequential snapshots showing the shrinkage processes of nanograins with different GB structures. Corresponding shear stress analysis is presented for each case. The yellow arrows indicate the shear directions.

Fig. 11. Temporal evolution of the shrinkage ratio of G1 embedded in various GB networks presented in Fig. 8(a-c) and 10. The shrinking rate of G1 within each GB network was attained by linearly fitting the data before t = 1.12 ns (marked by a dashed line).