Grain boundaries of nanocrystalline materials - their widths, compositions, and internal structures

Nanocrystalline materials contain many atoms at and near grain boundaries. Sufficient numbers of Mossbauer probe atoms can be situated in grain bounda ry environments to make a clear contribution to the measured Mossbauer spec trum. Three types of measurements on nanocrystalline materials are reported here, all using Mossbauer spectrometry in conjunction with X-ray diffracto metry, transmission electron microscopy, or small angle neutron scattering. By measuring the fraction of atoms contributing to the grain boundary comp onent in a Mossbauer spectrum, and by knowing the grain size of the materia l, it is possible to deduce the average width of grain boundaries in metall ic alloys. It is found that these widths are approximately 0.5 nm for fcc a lloys and slightly larger than 1.0 nm for bcc alloys. Chemical segregation to grain boundaries can be measured by Mossbauer spect rometry, especially in conjunction with small angle neutron scattering. Suc h measurements on Fe-Cu and Fe3Si-Nb were used to study how nanocrystalline materials could be stabilized against grain growth by the segregation of C u and Nb to grain boundaries. The segregation of Cu to grain boundaries did not stabilize the Fe-Cu alloys against grain growth, since the grain bound aries were found to widen and accept more Cu atoms during annealing. The Nb additions to Fe3Si did suppress grain growth, perhaps because of the low m obility of Nb atoms, but also perhaps because Nb atoms altered the chemical ordering in the alloy. The internal structure of grain boundaries in nanocrystalline materials pre pared by high-energy ball milling is found to be unstable against internal relaxations at low temperatures. The Mossbauer spectra of the nanocrystalli ne samples showed changes in the hyperfine fields attributable to movements of grain boundary atoms. In conjunction with SANS measurements, the change s in grain boundary structure induced by cryogenic exposure and annealing a t low temperature were found to be somewhat different. Both were consistent with a sharper density gradient between the crystalline region and the gra in boundary region.