A special micromechanism for the formation of elongated nanoscale voids at grain boundaries in deformed nanocrystalline materials was suggested and theoretically described. Within the present description, the formation of nanoscale voids represented a slow (diffusion-controlled) process driven by release of the elastic energy of grain boundary disclination configurations formed due to grain boundary sliding. It was demonstrated that the nucleation of elongated nanoscale voids at grain boundary disclination dipoles occurred as an energetically favourable process in deformed nanocrystalline Ni and Al2O3 (sapphire) in wide ranges of their parameters. The generation of nanovoids at triple junctions of grain boundaries could effectively occur as an energetically favourable process driven by grain-boundary sliding in deformed nanocrystalline materials. Such nanovoids slowly nucleated in the stress fields of grain boundary disclination dipoles formed at triple junctions due to grain boundary sliding. At the same time, the generation of brittle nanoscale cracks was a rapid process that could be kinetically preferred when compared to the slow diffusion-controlled formation of nanovoids in nanocrystalline materials deformed at high strain rates. The theoretical model accounted for the experimental observations of nanovoids at grain boundaries and their triple junctions in deformed nanocrystalline Ni and Au. Within the framework of the model, intense local plastic flow occurred through grain boundary sliding and preceded nanovoid formation. Their formation was a slow diffusion-controlled process, and such nanovoids could serve as nuclei for viscous dimple rupture structures observed experimentally at the fracture surfaces of nanomaterials. Large pile-ups of lattice dislocations, which often induced the generation of voids and brittle cracks in conventional coarse-grained polycrystals, were hardly formed in nanoscale grains of deformed nanocrystalline materials. Therefore, the generation of nanovoids at grain boundaries due to intergrain sliding could play the role of the dominant fracture process at the nanoscale level in nanocrystalline materials showing ductile fracture behaviour.

Elongated Nanoscale Voids at Deformed Special Grain Boundary Structures in Nanocrystalline Materials. I.A.Ovidko, A.G.Sheinerman, N.V.Skiba: Acta Materialia, 2011, 59[2], 678-85