Theoretical models were suggested which described the effects of the stress-driven migration of grain boundaries upon the formation of nanoscale cracks (nanocracks) and the growth of comparatively large cracks in deformed nanocrystalline ceramics and metals. The grain boundary migration was driven by the applied stress, involved plastic flow and produced quadrupoles of disclination defects in nanocrystalline materials. The disclinations created high local stresses which were capable of initiating the formation of nanocracks. The conditions under which the formation of nanocracks was energetically favorable were described theoretically. The external stress values needed to initiate nanocrack formation near to disclinations in nanocrystalline metals (Al, Ni), having the finest grains, and nanoceramics (Al2O3) were estimated. In addition, the effect of the stress-driven migration of grain boundaries upon the growth of comparatively large pre-existing cracks in nanocrystalline Ni having the finest grains, was estimated.

Stress-Driven Migration of Grain Boundaries and Fracture Processes in Nanocrystalline Ceramics and Metals. I.A.Ovidko, A.G.Sheinerman, E.C.Aifantis: Acta Materialia, 2008, 56[12], 2718-27