A special physical mode of plastic deformation in nanocrystalline ultra-fine grained and polycrystalline solids was suggested. The mode represented cooperative grain-boundary sliding and nanoscale grain nucleation (occurring via stress-driven splitting and migration of grain boundary) processes. It was revealed theoretically that, for certain ranges of parameters of the defect structure under consideration, the special deformation mode was more energetically favourable than both the so-called pure grain-boundary sliding and the previously examined (Bobylev et al., 2010) cooperative grain-boundary sliding and migration process. In addition, the special deformation mode enhanced the ductility of nanocrystalline and ultrafine-grained solids, and this enhancement effect was more pronounced when compared to that of the cooperative grain boundary sliding and migration process. The component deformation modes, grain-boundary sliding and stress-driven nanograin nucleation, effectively accommodated each other via transformations of grain-boundary disclinations. Under these circumstances, for certain ranges of parameters of the grain-boundary structures, the process was more energetically favourable than both conventional grain-boundary sliding and the previously examined cooperative grain-boundary sliding and migration in nanocrystalline solids. It was theoretically revealed that the intrinsic ductility (specified by the critical plastic strain at which a stable crack with a length at least 3nm was formed) of nanocrystalline Ni was enhanced if the above process predominated; compared to the situations where either pure grain-boundary sliding or cooperative grain-boundary sliding and migration was predominant.

Cooperative Grain Boundary Sliding and Nanograin Nucleation Process in Nanocrystalline, Ultrafine-Grained, and Polycrystalline Solids. S.V.Bobylev, N.F.Morozov, I.A.Ovidko: Physical Review B, 2011, 84[9], 094103