Detector tilt considerations in high-energy Bragg coherent diffraction imaging: a simulation study

This paper addresses three-dimensional signal distortion and image reconstruction issues in x-ray Bragg coherent diffraction imaging (BCDI) in the event of a general non-orthogonal orientation of the area detector with respect to the diffracted beam. Growing interest in novel BCDI adaptations at fourth-generation synchrotron light sources has necessitated improvisations in the experimental configuration and the subsequent data analysis. One such possibly unavoidable improvisation that is envisioned in this paper is a photon-counting area detector whose face is tilted away from the perpendicular to the Bragg-diffracted beam during acquisition of the coherent diffraction signal. We describe a likely circumstance in which one would require such a detector configuration, along with experimental precedent at third generation synchrotrons. Using physically accurate diffraction simulations from synthetic scatterers in the presence of such tilted detectors, we analyze the general nature of the observed signal distortion qualitatively and quantitatively, and provide a prescription to correct for it during image reconstruction. Our simulations and reconstructions are based on an adaptation of the known theory of BCDI sampling geometry as well as recently developed projection-based methods of wavefield propagation. Such configurational modifications and their numerical remedies are potentially valuable in realizing unconventional coherent diffraction measurement geometries and eventually paving the way for the integration of BCDI into new materials characterization experiments at next-generation light sources.Comment: 13 pages, 5 figure

Computational Mining of Meso-Scale Physics From High-Energy X-Ray Data Sets

Migration of grain boundaries in an idealized, well-ordered polycrystalline sample is an often-studied phenomenon in microstructural materials science. It is the subject of many imaging experiments in two dimensions and simulations in two and three dimensions, the collective knowledge of which has given us many insights into behavior of specific materials. This thesis describes the characterization of the grain boundaries in a specially prepared polycrystalline aggregate of high-purity iron with a bodycentered cubic lattice whose microstructure was imaged non-destructively in three dimensions with near-field high-energy diffraction microscopy (nf-HEDM) which uses synchrotron X-rays as a probe. The sample was imaged using nf-HEDM before and after a cycle of annealing in order to activate boundary migration for a short interval of time. Also described are the development of computational techniques for denoising grain boundary images in three dimensions as well as a scheme to solve the inverse problem of computing the dynamical parameters influencing boundary migration from the observed boundary geometries and transport. The two snapshots of the full three-dimensional grain boundary network were used to quantify the geometry and transport of individual grains and track their progress through the annealing. A study of the influence of grain boundary curvature on boundary velocity revealed a weak correlation for a large fraction of the boundaries.</p

Detector tilt considerations in high-energy Bragg coherent diffraction imaging: a simulation study

International audienceThis paper addresses three-dimensional signal distortion and image reconstruction issues in x-ray Bragg coherent diffraction imaging (BCDI) in the event of a general non-orthogonal orientation of the area detector with respect to the diffracted beam. Growing interest in novel BCDI adaptations at fourth-generation synchrotron light sources has necessitated improvisations in the experimental configuration and the subsequent data analysis. One such possibly unavoidable improvisation that is envisioned in this paper is a photon-counting area detector whose face is tilted away from the perpendicular to the Bragg-diffracted beam during acquisition of the coherent diffraction signal. We describe a likely circumstance in which one would require such a detector configuration, along with experimental precedent at third generation synchrotrons. Using physically accurate diffraction simulations from synthetic scatterers in the presence of such tilted detectors, we analyze the general nature of the observed signal distortion qualitatively and quantitatively, and provide a prescription to correct for it during image reconstruction. Our simulations and reconstructions are based on an adaptation of the known theory of BCDI sampling geometry as well as recently developed projection-based methods of wavefield propagation. Such configurational modifications and their numerical remedies are potentially valuable in realizing unconventional coherent diffraction measurement geometries and eventually paving the way for the integration of BCDI into new materials characterization experiments at next-generation light sources

Comparison between diffraction contrast tomography and high-energy diffraction microscopy on a slightly deformed aluminium alloy

International audienceThe grain structure of an Al-0.3 wt% Mn alloy deformed to 1% strain was reconstructed using diffraction contrast tomography (DCT) and high-energy diffraction microscopy (HEDM). 14 equally spaced HEDM layers were acquired and their exact location within the DCT volume was determined using a generic algorithm minimizing a function of the local disorientations between the two data sets. The microstructures were then compared in terms of the mean crystal orientations and shapes of the grains. The comparison shows that DCT can detect subgrain boundaries with disorientations as low as 1 degrees and that HEDM and DCT grain boundaries are on average 4 mm apart from each other. The results are important for studies targeting the determination of grain volume. For the case of a polycrystal with an average grain size of about 100 mm, a relative deviation of about <= 10% was found between the two techniques