B. Fultz et Hn. Frase, Grain boundaries of nanocrystalline materials - their widths, compositions, and internal structures, HYPER INTER, 130(1-4), 2000, pp. 81-108
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.