We have demonstrated how atomic-scale simulations can be used to obtain information about the deformation mechanisms in nanocrystalline metals. We find that at these grain sizes the dominating deformation mechanism is sliding in the grain boundaries through a large number of small events in the grain boundary. Each event only involves a few atoms. The result of this large number of small events is a flow in the grain boundaries, permitting the grains to slide past each other with only a minor amount of deformation inside the grains. The deformation mechanism results in a reverse Hall-Petch effect, where the material becomes softer when the grain size is reduced. This is caused by an increase of fraction of the atoms that are in the grain boundaries as the grain size is reduced. We have not yet been able to observe the cross-over between the range of grain sizes where this deformation mechanism dominates, and grain sizes where a conventional deformation mechanism based on dislocation motion dominates. The hardness and yield stress of nanocrystalline metals is expected to reach its maximum in this cross-over region.
In order to simulate larger systems than the ones considered here, the atomic-scale approach must be abandoned. We have provided an overview of simulation techniques at coarser scales, and have indicated how they can be combined with atomic-scale simulations to provide models of the mechanics of metals at multiple length scales. These techniques have not yet become mainstream simulation tools, but it is likely that many of them will gain more widespread usage in the near future.