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Plastic deformation in coarse-grained metals is governed by dislocation-mediated processes. These processes lead to the accumulation of a residual dislocation network, producing inhomogeneous strain and an inreversible broadening of the Bragg peaks in x-ray diffraction. We show that during plastic deformation of electrodeposited nanocrystalline nickel, the peak broadening is reversible upon unloading; hence, the deformation process does not build up a residual dislocation network. The results were obtained during in situ peak profile analysis using the Swiss Light Source. This in situ technique, based on well-known peak profile analysis methods, can be used to address the relationship between microstructure and mechanical properties in nanostructured materials.
In coarse-grained polycrystalline metals, plasticity is carried by dislocations generated by sources within the grains. Such dislocations propagate and interact with preexisting structures and also with each other, and can partly annihilate each other. At a given deformation level, all the dislocation segments that have not annihilated make up the final microstructure of the deformed state and are stored as extrinsic grain boundary (GB) dislocations, dislocation walls and subgrain boundaries, or individual dislocation segments (1).
There is a critical length scale below which a dislocation source can no longer operate (2) because the stress to bow out a dislocation approaches the theoretical shear stress. For face-centered cubic (fcc) metals the critical grain size is believed to lie between 20 and 40 nm, depending on the nature of the dislocations considered. Whether plasticity in nanocrystalline metals with mean grain sizes below 100 nm and an intrinsic grain size distribution is still carried by dislocations, and whether these dislocations are of the same nature as those observed in coarse-grained metals, are questions that remain unresolved (3). Postdeformation analysis in compressed or indented nanocrystalline Ni (nc-Ni) material does not indicate major dislocation debris (4), whereas deformation twinning is observed in ground nc-Al (5). In situ deformation in a transmission electron microscope (TEM) demonstrates some dislocation activity, often at the crack tip; however, the samples investigated are thinned for electron transparency, and the observed dislocation activity might therefore be a result of other dislocation sources such as surface defects (4, 6-8). On the other hand, atomistic simulations suggest that GBs in the nanocrystalline regime promote sliding and act as both a source and...