With the explosion of research interest in nanocrystalline materials in recent years, one sub-area that has received significant attention is the mechanical behavior of materials with grain sizes less than 100 nm. The great interest in the mechanical behavior of nanocrystalline materials originates from the unique mechanical properties first observed and/or predicted by the pioneers of this field, Gleiter and co-workers, in materials prepared by the gas condensation method (Gleiter, 1989). Among these early observations or predictions were:

• lower elastic moduli than for conventional grain size materials – by as much as 30%–50%;

• very high hardness and strength – hardness values for nanocrystalline pure metals (~10 nm grain size) that are 2–10 or more times higher than those of larger grained (>1 μm) metals;

• increased ductility – perhaps even superplastic behavior – at low homologous temperatures in even normally brittle ceramics or intermetallics with nanoscale grain sizes, believed to be caused by grain boundary, diffusional deformation mechanisms.

While some of these early observations and predictions have been verified by subsequent studies, in particular the high hardness and strength values, some have been found to be caused by high porosity in the early bulk samples (for example, elastic constant behavior) or to other artifacts introduced by the processing procedures.