d because of stress relaxation, and some dislocations crossed back over the grain boundary to the initial right-hand side crystal. During this unloading process, we again observed the grain boundary impediment effect on the lattice dislocation. Finally, one dislocation, indicated by the red arrow in [Fig. 5](#F5){ref-type="fig"}, appeared to be caught by the grain boundary plane.

![Sequential TEM images captured from the movie of the nanoindentation experiment for the low-angle tilt grain boundary.\
Sequential dark-field TEM images captured from the movie recorded during the nanoindentation experiment. The green arrows in each image indicate the grain boundary position. In this experiment, the indenter tip was gradually inserted into the specimen edge from 0 to 85 s and extracted from 86 to 109 s. The introduced lattice dislocations propagated through the crystal and crossed the grain boundary into the adjacent crystal. In the unloading process, the lattice dislocations moved backward, and some dislocations crossed the grain boundary again and back into the initial crystal, where the grain boundary impeded the dislocation propagation. In the end, the dislocation indicated by the red arrow is impeded on the grain boundary plane.](1501926-F5){#F5}

In the present low-angle tilt grain boundary case, the interaction between the lattice dislocations and the grain boundary core is actually the interaction between the lattice screw dislocations and the grain boundary edge dislocations. [Figure 6A](#F6){ref-type="fig"} shows a dark-field image of the grain boundary edge dislocations after the crossing of the lattice screw dislocations. The dislocation lines of the grain boundary edge dislocations are seen to be shifted to the \[$0\overline{1}1$\] direction, which is parallel to the Burgers vector of the lattice screw dislocations, and jogs are formed at the grain boundary dislocations. This indicates that the lattice screw dislocations intersected with the grain boundary edge dislocations during the crossing process ([@