Nanocrystalline metals are by definition polycrystalline structures

with a mean grain size below 100 nm. Fig. 1 shows an image taken in a transmission electron microscope of a high-density nanocrystalline (nc)-Cu sample with a mean grain size of 20 nm. The mechanical behavior of a fully-dense nanocrystalline metal is, compared with its coarse-grain counterpart, characterized by a significantly enhanced yield stress and a limited tensile elongation1,2. A simple extrapolation of conventional dislocation behavior to the nanometer regime might lead to the conclusion that plastic deformation is impossible at these small grain sizes and limited ductility is an intrinsic property of such material. Indeed, it is well known that the operation of the usual dislocation sources is grain-size dependent3, in the sense that there is a critical length scale below which sources can no longer operate because the stress to bow out a dislocation approaches the theoretical shear strength. In face-centered cubic (fcc) metals, the critical grain size is believed to lie between 20-40 nm, depending on the nature of the dislocations being considered4. Further, the limited space offered by the nanocrystalline grains strongly limits the operation of the usual intragranular multiplication mechanisms5,6. So long as plasticity is predominantly the result of dislocation activity, the increase in strength with decreasing grain size can be explained on the basis of dislocation pile-ups at grain boundaries. This leads directly to the Hall-Petch relationship where the yield stress is proportional to the inverse square root of the average grain size1,7,8. As grain refinement continues, dislocation activity It is now possible to synthesize polycrystalline metals made up of grains that average less than 100 nm in size. Such nanocrystalline metals contain a significant volume fraction of interfacial regions separated by nearly perfect crystals. The small sizes involved limit the conventional operation of dislocation sources and thus a fundamental question arises: how do these materials deform plastically? We review the current views on deformation mechanisms in nanocrystalline, face-centered cubic metals based on insights gained by atomistic computer simulations. These insights are discussed with reference to recent striking experimental observations that can be compared with predictions made by the simulations.