Abstract
A previously introduced mathematical framework for full-field X-ray orientation microscopy is for the first time applied to experimental near-field diffraction data acquired from a polycrystalline sample. Grain by grain tomographic reconstructions using convex optimization and prior knowledge are carried out in a six-dimensional representation of position-orientation space, used for modelling the inverse problem of X-ray orientation imaging. From the 6D reconstruction output we derive 3D orientation maps, which are then assembled into a common sample volume. The obtained 3D orientation map is compared to an EBSD surface map and local misorientations, as well as remaining discrepancies in grain boundary positions are quantified. The new approach replaces the single orientation reconstruction scheme behind X-ray diffraction contrast tomography and extends the applicability of this diffraction imaging technique to material micro-structures exhibiting sub-grains and/or intra-granular orientation spreads of up to a few degrees. As demonstrated on textured sub-regions of the sample, the new framework can be extended to operate on experimental raw data, thereby bypassing the concept of orientation indexation based on diffraction spot peak positions. This new method enables fast, three-dimensional characterization with isotropic spatial resolution, suitable for time-lapse observations of grain microstructures evolving as a function of applied strain or temperature.
Highlights
A previously introduced mathematical framework for full-field X-ray orientation microscopy is for the first time applied to experimental near-field diffraction data acquired from a polycrystalline sample
As opposed to 3D X-Ray Diffraction (3DXRD) variants based on pencil beam (1D) or line beam (2D) illumination which involve 3D or 2D scanning procedures limiting the ultimate acquisition speed, the full-field variant described in this paper requires only one rotational scan and can be executed on standard X-ray tomography instruments, offering simultaneous X-ray absorption and phase contrast inspection of the illuminated sample volume
Depending on the experiment settings and material characteristics, the reconstruction task related to X-ray orientation microscopy can be sub-divided into one of the following three categories: (1) negligible intra-granular orientation spreads (2) presence of intra-granular orientation distributions which can be described as a 3D vector field and (3) presence of intra-granular orientation distributions which require a description as 3D orientation distributions per sampled volume element[2]
Summary
A previously introduced mathematical framework for full-field X-ray orientation microscopy is for the first time applied to experimental near-field diffraction data acquired from a polycrystalline sample. As demonstrated on textured sub-regions of the sample, the new framework can be extended to operate on experimental raw data, thereby bypassing the concept of orientation indexation based on diffraction spot peak positions. This new method enables fast, three-dimensional characterization with isotropic spatial resolution, suitable for time-lapse observations of grain microstructures evolving as a function of applied strain or temperature. In the first case the task of 3D grain shape reconstruction reduces to the classical problem of image reconstruction from parallel projections and can be solved using algorithms developed in the field of medical imaging[2,3,4,5,6,7] Common to this class of micro-structure reconstruction techniques is a two step process based on poly-crystal indexing followed by grain by grain reconstruction. It has to be noted that the concept of indexing grain orientations from diffraction www.nature.com/scientificreports/
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