Bragg-edge transmission imaging using a pulsed neutron source is expected to be a new method to investigate the crystallographic and metallographic structure of a material. This method has attracted the attention in the research field of material characterization for materials development and industrial applications because it non-destructively provides the images on the texture and the microstructure inside a material such as a thick steel bulk over the wide area of the material. For deducing such information from the Bragg-edge transmission spectrum, a data analysis code like a Rietveld analysis code for powder diffractometry is indispensable. So far, only the information on the crystallographic anisotropy has been deduced. However, this information is incomplete since both the preferred orientation and the crystallite size affect the Bragg-edge transmission spectrum. Therefore, we have developed a Rietveld-type analysis code, RITS, that allows us to obtain the information on preferred orientation and crystallite size at the same time. To examine the feasibility and the usefulness of the RITS code, we have analyzed the Bragg-edge transmission spectra of rolled and welded -iron plates, and we have successfully obtained the preferred orientation data and the crystallite size data over the wide area of the bulk specimens.
The width of crystal lattice plane spacing (d-spacing) distribution related to microscopic-strain and crystallite size in a martensite phase in a 2 cm thick quenched-ferritic steel sample was quantitatively mapped in real space by a Bragg-edge broadening analysis of spectral data from a pulsed neutron transmission experiment. This analysis was performed under the condition that the instrumental resolution parameters, determined from the data of ferrite in the same sample without microscopic-strain and crystallite size effects, were unchanged over the sample area, and assuming that the d-spacing was distributed according to a Gaussian function in the martensite area. As a result, the full width at half maximum (FWHM) of the Gaussian d-spacing distribution in the martensite was extracted at each position in a sample. Consequently, it was found that the real-space distribution of the FWHM of the d-spacing distribution is closely correlated with a real-space distribution of the Vickers hardness that corresponds to the quantity of martensite. Furthermore, it was indicated that the Vickers hardness was proportional to the FWHM of the d-spacing distribution. The results suggest that it will be possible to measure the Vickers hardness in the martensite non-destructively by using the Bragg-edge neutron transmission method.
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