The role of the finest grains in the strength and ductility of nanocrystalline materials

A theoretical model is suggested which describes the generation and evolution of grain boundary dislocation at triple junctions of grain boundaries in deformed nanocrystalline materials. In the framework of the model, dislocation pileups in nanocrystalline materials are nucleated at triple junctions due to accumulation of the dislocation charge that accompanies grain boundary sliding through triple junctions. The model accounts for experimental observation [Appl. Phys. Lett. 87 (2005) 091904.] of dislocation pileup in deformed nanocrystalline Cu. With results of the model, the effects of the finest grains (grains with sizes approaching the amorphous limit, ranging from 2 to 4 nm) on relieve stress concentration in nanocrystalline materials exhibiting enhanced strength and reasonably good ductility are discussed. A new constitutive model based on the novel properties of the finest grains for nanocrystalline metals was proposed.