A flexible and standalone forward simulation model for laboratory X-ray diffraction contrast tomography

Abstract

Laboratory X-ray diffraction contrast tomography (LabDCT) has recently been developed as a powerful technique for non-destructive mapping of grain microstructures in bulk materials. As the grain reconstruction relies on segmentation of diffraction spots, it is essential to understand the physics of the diffraction process and resolve all the spot features in detail. To this aim, a flexible and standalone forward simulation model has been developed to compute the diffraction projections from polycrystalline samples with any crystal structure. The accuracy of the forward simulation model is demonstrated by good agreements in grain orientations, boundary positions and shapes between a virtual input structure and that reconstructed based on the forward simulated diffraction projections of the input structure. Further experimental verification is made by comparisons of diffraction spots between simulations and experiments for a partially recrystallized Al sample, where a satisfactory agreement is found for the spot positions, sizes and intensities. Finally, applications of this model to analyze specific spot features are presented.

Optimizing laboratory X-ray diffraction contrast tomography for grain structure characterization of pure iron

“…A polyhedron-meshing-based forward simulation model as described by Fang et al (2020) was used to analyze the details about individual diffraction spots, including from which grains and which (hkl) planes they originate as well as the sizes, intensities and energies of the X-rays producing the spots. For the simulations, the experimentally determined 3D grain structures were used as input to calculate the X-ray energies for each diffraction spot, and to relate that to the experimentally measured intensity of the same diffraction spot.…”

“…In the model, an X-ray energy spectrum from a W target was used to calculate the intensity of the diffraction spots, taking both sample absorption and detective quantum efficiency into account. More details about the analysis procedure are given by Fang et al (2020). To evaluate the reconstructed results, diffraction spots from grains with sizes <25 mm were generated by the forward simulation and used to find the corresponding experimental spots.…”

“…efficiencies for low-energy photons while the efficiency gets significantly decreased for photon energies higher than 60-80 keV (Fang et al, 2020), the energy range for the low {hkl} families should be prioritized by tuning the X-ray spectrum from the source, especially for samples with small grains and low atomic number. For samples with high atomic number, a comparatively higher energy range and a sample with smaller diameter is preferred.…”

“…X-rays with different wavelengths incoming from different angles are focused by crystal lattice planes (Guinier & Tennevin, 1949;McDonald et al, 2015). This is different from standard Bragg or Laue diffraction and typically leads to elliptical diffraction spots -the short axis of the spot is a result of the focusing effect and is roughly parallel to the diffraction vector, while the long axis is due to the geometrical magnification from the conical shape of the beam and is along the direction perpendicular to the short axis (Fang et al, 2020). Laue focusing requires good crystal perfection of the grains, and only small imperfections can be tolerated.…”

“…The aim of the present work is to study the impacts of different experimental parameters, including accelerating voltage (maximum energy of the incoming X-rays), exposure time and number of projections, on the quality of the 3D reconstruction by conducting a series of LabDCT measurements on the same sample volume. The X-ray energies and intensities of individual diffraction spots from the same grain are determined with the assistance of a forward simulation model (Fang et al, 2020), as the basis for comparing different experimental conditions. The results obtained are important for understanding the influence of each parameter on the diffraction spots from grains with different sizes, and therefore important for optimizing LabDCT measurements.…”