Stress-driven migration of grain boundaries and fracture processes in nanocrystalline ceramics and metals

Theoretical models are suggested that describe the effects of stress-driven migration of grain boundaries (GBs) on both the formation of nanoscale cracks (nanocracks) and the growth of comparatively large cracks in deformed nanocrystalline ceramics and metals. The GB migration under consideration is 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. In this paper, the conditions at which the formation of nanocracks is energetically favorable are 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 ðAl2O3Þ are estimated. In addition, we estimated the effect of the stress-driven migration of GBs on the growth of pre-existing, comparatively large cracks in nanocrystalline Ni with the finest grains.