A new mechanism for fracture toughness enhancement in nanocrystalline metals and ceramics was suggested. The mechanism represented the cooperative grain boundary sliding and stress-driven grain boundary migration process near the tips of growing cracks. It was shown that this mechanism could increase the critical stress intensity factor for crack growth in nanocrystalline materials by a factor of three or more and thus considerably enhance the fracture toughness of such materials. The cooperative grain-boundary sliding and migration deformation mechanism was shown to increase the critical stress intensity factors in nanocrystalline metals and ceramics by several times and, as a result, it might lead to a significant enhancement of fracture toughness of these materials. It was shown that among the two constituents of the examined deformation mechanism (grain-boundary sliding and grain-boundary migration), sliding played the main role in the enhancement of fracture toughness in nanocrystalline solids. Migration was essential to the accommodation of sliding associated with the transfer of high-angle grain boundaries (with a misorientation angle exceeding 21°). Therefore the enhancing effect of the cooperative sliding and migration process on the fracture toughness increased with increasing fraction of high-angle grain boundaries. In the case of sliding associated with the transfer of low-angle grain boundaries near to crack tips, sliding could be accommodated by lattice dislocation emission from triple-junctions or diffusion. The effect of each mechanism upon the fracture toughness of a nanocrystalline specimen depended upon the structure of the specimen and its loading conditions. It was the effective combined action of various deformation mechanisms at certain conditions that could provide the experimentally observed high fracture toughness of nanocrystalline metals and ceramics. The fracture toughness of nanocrystalline materials could also be analyzed using a mechanism-independent approach which was based upon a theory of nano-elasticity. That is, linear elasticity enhanced by the Laplacian of strain or stress accounted for the higher-order deformation gradients induced by the small-volume nanoscale constraints. In particular, enhancement of fracture toughness of nanocrystalline materials could be deduced from the fact that the theory of nano-elasticity produced non-singular stress and strain distributions at the crack tip and predicted a maximum stress ahead of it whose value depended upon the gradient coefficient of nanoscale internal length. Therefore a size-dependent critical stress intensity factor could be determined, leading to an enhanced fracture toughness that depended upon the value of the relevant internal length parameter.

Effect of Cooperative Grain Boundary Sliding and Migration on Crack Growth in Nanocrystalline Solids. I.A.Ovidko, A.G.Sheinerman, E.C.Aifantis: Acta Materialia, 2011, 59[12], 5023-31