  Interfacial nucleation is the dominant process of dislocation generation
during the plastic deformation of nano-crystalline materials. Solute additions
intended to stabilize nano-crystalline metals against grain growth, may
segregate to the grain boundaries and triple junctions where they can affect
the process of the dislocation emission. In this Letter we demonstrate that the
effect of solute addition in a nano-crystalline material containing competing
solute segregation sites and dislocation sources can be very complex due to
different rates of segregation at different interfaces. Moreover, at large
concentrations, when the solutes form clusters near the grain boundaries or
triple junctions, the interfaces between these clusters and the matrix can
introduce new dislocation emission sources, which can be activated under lower
applied stress. Thus, the strength maximum can occur at a certain solute
concentration: adding solutes beyond this optimal solute concentration can
reduce the strength of the material.
