A fast methodology to determine the characteristics of thousands of grains using three-dimensional X-ray diffraction. I. Overlapping diffraction peaks and parameters of the experimental setup

Abstract

A fast methodology to determine the characteristics of thousands of grains using three-dimensional X-ray diffraction. I. Overlapping diffraction peaks and parameters of the experimental setup A data-analysis methodology is presented for the characterization of threedimensional microstructures of polycrystalline materials from data acquired using three-dimensional X-ray diffraction (3DXRD). The method is developed for 3DXRD microscopy using a far-field detector and yields information about the centre-of-mass position, crystallographic orientation, volume and strain state for thousands of grains. This first part deals with pre-processing of the diffraction data for input into the algorithms presented in the second part . J. Appl. Cryst. 45, 705-718] for determination of the grain characteristics. An algorithm is presented for accurate identification of overlapping diffraction peaks from X-ray diffraction images, which has been an issue limiting the accuracy of experiments of this type. The algorithm works in two stages, namely the identification of overlapping peaks using a seeded watershed algorithm, and then the fitting of the peaks with a pseudo-Voigt shape function to yield an accurate centre-of-mass position and integrated intensity for the peaks. Regions consisting of up to six overlapping peaks can be successfully fitted. Two simulations and an experiment are used to verify the results of the algorithms. An example of the processing of diffraction images acquired in a 3DXRD experiment with a sample consisting of more than 1600 grains is shown. Furthermore, a procedure for the determination of the parameters of the experimental setup (global parameters) without the need for a calibration sample is presented and validated using simulations. This is immensely beneficial for simplifying experiments and the subsequent data analysis. research papers J. Appl. Cryst. (2012). 45, 693-704 Hemant Sharma et al. Grain characterization I 695

Advances in X-ray diffraction contrast tomography: flexibility in the setup geometry and application to multiphase materials

“…Friedel pairs can be exploited in far-field data too and have been used for indexing (Moscicki et al, 2009), strain measurements (Aydiner et al, 2009) and setup calibration (Sharma et al, 2012a).…”

On the calibration of high-energy X-ray diffraction setups. II. Assessing the rotation axis and residual strains

“…It is important to emphasize that, because of correlations between the lattice parameter and the sample-to-detector distance D, at high energies any method requires the value of the stress-free lattice parameter in order to determine D. a 0 was therefore obtained from a different experiment. Inspired by the Bond (1960) method, an independent determination of a 0 was recently suggested by Sharma et al (2012a). The procedure was tested only on simulated data; however, to obtain a good accuracy for beam centre coordinates, sample-detector distance and detector tilt the authors suggested the use of a large number of reflections corresponding to many grains.…”

“…Papers dealing with strain determination (Oddershede et al, 2010;Sharma et al, 2012a) usually consider the initial undeformed state of a polycrystal as reference, which consequently allows calibration of the setup. However, in order to work properly the method requires diffraction information from a large number of grains and is additionally coupled to the assumption that single grains in the reference specimen have identical cell parameters (Oddershede et al, 2010;Sharma et al, 2012a). This assumption is difficult to check in practice and very probably depends on the chemical composition of the analysed specimen (e.g.…”

Characterization of neutron-irradiated HT-UPS steel by high-energy X-ray diffraction microscopy

“…Those spots were first identified using intensity threshold and their positions were corrected for systematic distortions using the instrument parameters obtained from calibration data. The spots are then fitted and indexed [29,30] to obtain the COMs, crystallographic orientations, and strains of the constituent grains in the illuminated volume.…”