Deformation of electrodeposited nanocrystalline nickel

The mechanisms of deformation and damage evolution in electrodeposited, fully dense, nanocrystalline Ni with an average grain size of ~30 nm and a narrow grain size distribution were investigated by recourse to (i) tensile tests performed in situ in the transmission electron microscope and (ii) microscopic observations made at high resolution following ex situ deformation induced by compression, rolling and nanoindentation. Particular attention was also devoted to the characterization of the structure in grain interiors and in the vicinity of grain boundaries at Angstromlevel resolution in the as-deposited material and following compression, and to the real-time video-imaging of the evolution of dislocation activity and damage during deformation; these images are presented in this paper and in the web sites provided as supplementary material to this paper. These observations clearly reveal that dislocation-mediated plasticity plays a dominant role in the deformation of nanocrystalline Ni examined in this study. Fracture surface examination confirms dimpled rupture with the scale of the dimples being several times larger than the grain size. Dislocation emission at grain boundaries together with intragranular slip and unaccommodated grain boundary sliding facilitate the nucleation of voids at boundaries and triple junctions. Individual monocrystal ligaments, formed by the growth/linking of these voids, undergo extensive local plasticity to the extent that many of them neck down to a chisel point. These voids as well as those that may have existed prior to deformation can act as nucleation sites for dimples leading to fracture that does not occur preferentially along grain boundaries. The transmission electron microscopy observations of in situ and ex situ deformed specimens are synthesized to formulate a mechanistic framework that provides new insights into the mechanisms of flow and fracture in nanostructured metals.  2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.