**4. Effects of Grain Boundary Disorder and Impurities on Dislocation Emission**

Very small amounts of immiscible solutes can dramatically decrease the ability of dislocations to nucleate from GBs. It was recently reported [37] that segregation of Zr to GBs in nanocrystalline (NC) Cu can lead to the development of disorder in the intergranular structure. In this study, the authors employ atomistic computer simulations to investigate how this disorder affects dislocation nucleation from the GBs under applied stress. It was found that a fully disordered GB structure suppresses dislocation emission and significantly increases the yield stress.

The results of the MD simulations [38], which show that, at larger concentrations, the solute effect becomes non-trivial: there are concentration ranges where even a small addition of solutes considerably suppresses the dislocation nucleation from grain boundaries, and there are concentration ranges where the addition of new solutes almost does not change the dislocation nucleation. The authors also provide an atomic mechanism of these effects [38].

Solute additions are commonly used to stabilize NC materials against grain growth and can simultaneously enhance the strength of the material by impeding dislocation emission from the GBs. In this study [38], the authors demonstrate using molecular dynamics (MD) simulations that the effect of solutes on dislocation nucleation depends on the distribution of solutes at the GB. Solutes with a smaller positive size mismatch to the host can be more effective in suppressing dislocation emission from GBs, in comparison to others that have larger mismatch. These findings are relevant to the search for optimal solute additions, which can strengthen NC material by suppressing the nucleation of dislocation slip from GBs, while stabilizing it against grain growth.

Another study [39] reports using atomic simulation on GB dislocation sources in NC copper. The authors provide an insight into dislocation sources in NC copper. Atomistic studies of this type provide details of the emission sequence that enhance understanding of dislocation sources in high angle boundaries.

A three-dimensional model for the generation of split dislocations by GBs in NC Al is proposed in reference [40]. In terms of this model, rectangular glide split-dislocation half-loops nucleate at glide lattice dislocation loops pressed to GBs by an applied stress. The level of the applied stress and the grain size at which the emission of such dislocation half-loops becomes energetically favorable are determined.

A theoretical model is suggested that describes the emission of partial Shockley dislocations from triple junctions of GBs in deformed NC materials [41]. In the framework of the model, triple junctions accumulate dislocations due to GB sliding along adjacent GBs. The dislocation accumulation at triple junctions causes partial Shockley dislocations to be emitted from the dislocated triple junctions and, thus, accommodates GB sliding. Ranges of parameters (applied stress, grain size, etc.) are calculated, in which the emission events are energetically favorable in NC Al, Cu and Ni. The model accounts for the corresponding experimental data reported in the literature.

Finally, it is worth mentioning the work of Swygenhoven et al. [42], which deals with the atomic mechanism responsible for the emission of partial dislocations from GB in NC metals. It is shown that, in a 12 nm grain-size sample, GBs containing grain-boundary dislocations (GBDs) can emit a partial dislocation during deformation by local shuffling and stress assisted free-volume migration.
