It was recalled that nanocrystalline materials contained such a high number density of grain boundaries that orientationally averaged grain-boundary properties could be deduced from measurements of the overall properties by using a rule-of-mixtures approach. Because the excess volume in grain boundaries markedly affected the density and electrical resistivity of nanocrystalline materials, measurements of these 2 properties could be used to determine the excess volume. The observed variation in excess volume, as a function of average crystallite size, suggested that the amount of excess volume which a grain boundary could support was related to the spatial extent of the crystallites that made up the boundary. At the smallest average crystallite sizes, the excess volume appeared to fall significantly below the value that was characteristic of grain boundaries in conventional polycrystalline materials. This behavior was suggested to arise from interactions, between grain boundaries, which were mediated by the boundary-induced strain fields in the crystallites. This effect was expected to have great relevance to any transition between the nanocrystalline and amorphous states.

C.E.Krill, R.Birringer: Materials Science Forum, 1996, 225-227, 263-74