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

Fultz, B. and Frase, H. N. (2000) Grain boundaries of nanocrystalline materials – their widths, compositions, and internal structures. Hyperfine Interactions, 130 (1-4). pp. 81-108. ISSN 0304-3843. doi:10.1023/a:1011056927880.

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Nanocrystalline materials contain many atoms at and near grain boundaries. Sufficient numbers of Mössbauer probe atoms can be situated in grain boundary environments to make a clear contribution to the measured Mössbauer spectrum. Three types of measurements on nanocrystalline materials are reported here, all using Mössbauer spectrometry in conjunction with X-ray diffractometry, transmission electron microscopy, or small angle neutron scattering. By measuring the fraction of atoms contributing to the grain boundary component in a Mössbauer spectrum, and by knowing the grain size of the material, it is possible to deduce the average width of grain boundaries in metallic alloys. It is found that these widths are approximately 0.5 nm for fcc alloys and slightly larger than 1.0 nm for bcc alloys. Chemical segregation to grain boundaries can be measured by Mössbauer spectrometry, especially in conjunction with small angle neutron scattering. Such measurements on Fe-Cu and Fe₃Si-Nb were used to study how nanocrystalline materials could be stabilized against grain growth by the segregation of Cu 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 boundaries 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 mobility 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 prepared by high-energy ball milling is found to be unstable against internal relaxations at low temperatures. The Mössbauer spectra of the nanocrystalline samples showed changes in the hyperfine fields attributable to movements of grain boundary atoms. In conjunction with SANS measurements, the changes in grain boundary structure induced by cryogenic exposure and annealing at low temperature were found to be somewhat different. Both were consistent with a sharper density gradient between the crystalline region and the grain boundary region.

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Fultz, B.0000-0002-6364-8782
Additional Information:© 2000 Kluwer Academic Publishers. It is a pleasure to acknowledge collaborations on the science of nanocrystalline materials with Prof. H. Ouyang, Dr. Z.-Q. Gao, Dr. L.-B. Hong, Prof. H. Kuwano, Dr. J.L. Robertson, Dr. S. Spooner, Prof. W.L. Johnson, and Dr. T.A. Stephens. This work was supported by the U.S. National Science Foundation under contract DMR-9816617.
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Subject Keywords:57Fe Atom; Isomer Shift; Nanocrystalline Material; Small Angle Neutron Scattering; Boundary Width
Issue or Number:1-4
Record Number:CaltechAUTHORS:20200205-153152101
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Official Citation:Fultz, B. & Frase, H. Hyperfine Interactions (2000) 130: 81.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:101152
Deposited By: Tony Diaz
Deposited On:06 Feb 2020 18:14
Last Modified:16 Nov 2021 18:00

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