Grain boundaries of nanophase materials

Citation

Ouyang, Hao (1993) Grain boundaries of nanophase materials. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/8d25-6574. https://resolver.caltech.edu/CaltechETD:etd-08312007-095729

Abstract

Several iron alloys with nanometer-scale grain sizes were prepared by mechanical attrition in a high energy ball mill. Mossbauer spectrometry was used to measure the internal hyperfine magnetic fields. X-ray diffractometry (XRD) and transmission electron microscopy (TEM) were used to measure the grain size. The proposed structures of nanophase materials consist of two components of comparable volume fractions: a crystalline component and an interfacial component. By correlating features in the Mossbauer spectra to the structural results of XRD and TEM, we identified the hyperfine magnetic fields of 57Fe atoms at grain boundaries. Once identified, the fractions of 57Fe atoms at grain boundaries were determined by the intensities of their magnetic signatures seen in Mossbauer spectra. With data on the fraction of 57Fe atoms at grain boundaries versus grain size, a model of the microstructure can be used to obtain the average width of the grain boundaries. Data from the model were obtained by Monte Carlo simulations. The average grain boundary widths of the fcc alloys Fe-Mn and Ni-Fe are approximately 0.5 nm, but the average widths of grain boundaries in the bcc alloys Cr-Fe and Fe-Ti are larger, approximately 1 nm.

The Debye temperature is a physical parameter of the vibrational spectrum of the solid. The difference in the Debye temperatures between the grain boundaries and the crystalline regions was found. The Debye temperature of the crystallites in nanophase Cr-Fe is 470 K, which is larger than that of grain boundary component, 370 K.

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