Diffraction imaging of polycrystalline materials
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A new diffraction imaging technique for the characterization of polycrystalline materials is proposed and applied to obtain direct information about individual grains and their size and shape distributions and, in turn, strains in these materials. Unlike traditional powder diffractometry, where divergent and focusing x-ray optics are essential to collect information from an ensemble of grains, the nearly parallel and monochromatic beam available from a synchrotron x-ray source is employed to observe and measure diffraction images from individual grains and component particles in consolidated materials prepared by various processes. Images can be recorded by traditional methods, such as film and pulse counting detectors, but modern image detectors, such as charge coupled device (CCD) detectors and image analyzers, make the proposed imaging technique more practical. Unlike traditional diffractometry, this new technique provides the ability to measure shape, size, and strain without model based analyses. The spatial distribution of strain within individual grains, displayed as a diffraction image (topograph), indicates the presence of defects, such as dislocations, subgrain boundaries, and precipitates, and sheds new light on the origins of residual strains (stresses) in industrial materials. The resolution of the imaging system used is limited to grains ~10 /xm or larger due to diffraction broadening (~20" from the size effect) and the resolution of the recording medium.
I. INTRODUCTION Consolidation of component materials as industrial products often requires information about the size, shape, and crystallographic orientation of individual crystal grains before and after processing. Even in the preprocess stage (green-state), shaping of the powder may alter the state of the powder, although changes in size and shape most likely take place during processing. Normally, particle size, shape, orientation, strain, and the distributions thereof, are determined by traditional diffraction profile analysis using a commercial 9-26 scanning diffractometer.1 However, this analysis method is not only cumbersome, but can be inaccurate, depending on the investigator's chosen mathematical models of particle shape and the assumption of a statistically uniform distribution of particles. The advent of a nearly parallel (less than a few arcsec divergence) x-ray beam prepared from synchrotron radiation eliminates dependence on these models. The purpose of this paper is to present a diffraction imaging technique that directly determines these particle quantities without any models. In conventional diffractometry, a large sample volume is bathed by a divergent x-ray beam and a pulsecounting detector is placed on an x-ray focusing circle (Rowland circle). This x-ray optical arrangement relies on the assumption that grains of different sizes and strains are uniformly and abundantly distributed over the dimensions of the experimental probe (that is, the beam
size). The analysis of these diffraction profiles normally requires a similar shap
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