Theoretical models were suggested that describe the effects of stress-driven migration of grain boundaries on both the formation of nanoscale cracks (nanocracks) and the growth of comparatively large cracks in deformed nanocrystalline ceramics and metals. The grain-boundary migration under consideration was driven by the applied stress, carries plastic flow and produces quadrupoles of disclination defects in nanocrystalline materials. The disclinations create high local stresses capable of initiating the formation of nanocracks. Here, the conditions under which the formation of nanocracks was energetically favorable were theoretically described. The external stress values needed to initiate nanocrack formation near the disclinations in nanocrystalline metals (Al and Ni) with the finest grains and nanoceramics (Al2O3) were 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