A transmission electron microscopic study was carried out in order to uncover how dislocations and twins accommodated large plastic strains and accumulated in very small nanocrystalline Ni grains during low-temperature deformation. Dislocation patterns that suggested preferential deformation and non-uniform defect storage within the nanocrystalline grains were illustrated. Dislocations were present in individual and dipole configurations. Most dislocations were of 60° type, and piled-up on (111) slip-planes. Various deformation responses, in the forms of dislocations and twinning, could occur simultaneously within a nanocrystalline grain. Evidence for twin boundary migration was obtained. The rearrangement and organization of dislocations, sometimes interacting with the twins, led to the formation of sub-grain boundaries, sub-dividing the nanograin into mosaic domain structures. The observation of strain (deformation)-induced refinement contrasted with the stress-assisted grain growth reported for nanocrystalline metals, and had implications for understanding the stability and deformation behaviour of such highly non-equilibrium materials.

Accommodation of Large Plastic Strains and Defect Accumulation in Nanocrystalline Ni Grains. X.L.Wu, E.Ma: Journal of Materials Research, 2007, 22[8], 2241-53