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 elongation 1,2 .
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 dependent 3 , 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 considered 4 . Further, the limited space offered by the nanocrystalline grains strongly limits the operation of the usual intragranular multiplication mechanisms 5,6 . 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 size 1,7,8 . As grain refinement continues, dislocation activity