Grain boundaries of nanocrystalline materials

Nanocrystalline materials contain sufficiently many atoms at and near grain boundaries to make a clear contribution to a Mossbauer spectrum. By measuring the fraction of atoms contributing to the grain boundary component in a Mossbauer spectrum, and by measuring the grain size of the material with x-ray diffractometry and transmission electron microscopy, the average widths of Brain boundaries in metallic alloys are deduced. It is found that these widths are approximately 0.5 nm for fee alloys and slightly larger than 1.0 nm for bcc alloys. The internal structure of grain boundaries in nanocrystalline materials prepared by high energy ball milling is found to be unstable. After cryogenic exposure or annealing at 100 degrees C, the Mossbauer spectra and small angle neutron scattering profiles (SANS) showed changes attributable to movements of grain boundary atoms. The changes in grain boundary structure induced by cryogenic exposure and annealing at low temperature were found to be somewhat different, but both were consistent with a sharper density gradient between the crystalline region and the grain boundary region.