Search

A novel regression method for estimating intragranular strain in polycrystalline materials from three-dimensional X-ray diffraction data is presented and evaluated. The method incorporates an equilibrium constraint to the reconstructed strain field by using a Gaussian process., A new method for estimation of intragranular strain fields in polycrystalline materials based on scanning three-dimensional X-ray diffraction (scanning 3DXRD) data is presented and evaluated. Given an a priori known anisotropic compliance, the regression method enforces the balance of linear and angular momentum in the linear elastic strain field reconstruction. By using a Gaussian process (GP), the presented method can yield a spatial estimate of the uncertainty of the reconstructed strain field. Furthermore, constraints on spatial smoothness can be optimized with respect to measurements through hyperparameter estimation. These three features address weaknesses discussed for previously existing scanning 3DXRD reconstruction methods and, thus, offer a more robust strain field estimation. The method is twofold validated: firstly by reconstruction from synthetic diffraction data, and secondly by reconstruction of a previously studied tin (Sn) grain embedded in a polycrystalline specimen. Comparison against reconstructions achieved by a recently proposed algebraic inversion technique is also presented. It is found that the GP regression consistently produces reconstructions with lower root-mean-square errors, mean absolute errors and maximum absolute errors across all six components of strain.

Journal of applied crystallography 53(2), 314 - 325 (2020). doi:10.1107/S1600576720001016, Two methods for reconstructing intragranular strain fields are developed for scanning three-dimensional X-ray diffraction (3DXRD). The methods are compared with a third approach where voxels are reconstructed independently of their neighbours [Hayashi, Setoyama & Seno (2017). Mater. Sci. Forum, 905, 157–164]. The 3D strain field of a tin grain, located within a sample of approximately 70 grains, is analysed and compared across reconstruction methods. Implicit assumptions of sub-problem independence, made in the independent voxel reconstruction method, are demonstrated to introduce bias and reduce reconstruction accuracy. It is verified that the two proposed methods remedy these problems by taking the spatial properties of the inverse problem into account. Improvements in reconstruction quality achieved by the two proposed methods are further supported by reconstructions using synthetic diffraction data., Published by Wiley-Blackwell, [S.l.]

This work aims to improve the understanding of grain sub-structuration mechanisms and dynamics during deformation of polycrystals. For this purpose, in situ synchrotron experiments and EBSD acquisitions were coupled to study the response of single grains of an Al-0.1%wt.Mn polycrystal during tensile deformation. The specimen deformed in situ at the synchrotron was analyzed by 3DXRD. A new method provided a grain-by-grain analysis of the intragranular misorientation axes and their orientation distribution, through the investigation of the azimuthal broadening of diffraction spots. The 3DXRD results were cross-checked by classical EBSD analysis. Three acquisitions were carried out over the same region of interest at the undeformed state and after the application of 1% and 5% strain. Thanks to the available spatial resolution, the EBSD results allow for a better comprehension of the creation and dynamics of intragranular orientation gradients, and are in good agreement with pre-existing theoretical models. In addition, the characterization of grain sub-structuration and intragranular strain was performed through a novel X-Ray scanning technique, the K-map. The strain was found to be very heterogeneous with high compressive and tensile values in the grain interior and near two grain boundaries, respectively. Dislocations were found to be at the origin of deformation., Ce travail vise à améliorer la compréhension des mécanismes et de la dynamique de sous-structuration des grains pendant la deformation des matériaux polycristallins. Pour cela, des experience in situ en synchrotron et des acquisitions EBSD ont été menées afin d’étudier les comportements des grains individuels d’un polycrystal d’Aluminium déformé plastiquement. Une éprouvette d’un alliage Al-0.1%Mn a été déformée en traction in situ et analysée par microscopie 3D par diffraction des Rayons-X (3DXRD). Une nouvelle méthode de dépouillement a été développée pour determiner les axes de désorientation intragranulaires et les distributions d’orientation, grâce à l’analyse de l’élargissement azimutal des taches de diffraction. La technique EBSD a été appliquée pour obtenir des cartographies de désorientation des grains individuels d’une éprouvette déformée en traction. Trois acquisitions ont été réalisées sur la même région d’intérêt à l’état non déformé et après l’application des deformations 1% et 5%. Ces résultats permettent une meilleure comprehension de la formation et de l’évolution des gradients d’orientations intragranulaires, et son ten bon accord avec les modèles théoriques pré-existants. Aussi, la caractérisation de la sous-structuration des grains et de la deformation intragranulaire a été réalisée grâce à la technique K-map. La deformation était très hétérogène avec des valeurs élevées de compression et de traction à l’intérieur des grains et en proximité de deux joints de grains, respectivement. La distribution de la norme des vecteurs de diffraction a montré que les dislocations sont à l’origine de la deformation.

International audience; Quick scanning X-ray microscopy combined with three-dimensional reciprocal space mapping was applied to characterize intragranular orientation and strain in a single grain of uniaxially deformed Al polycrystal. The strain component perpendicular to the direction of the applied tensile load was found to be very heterogeneous with high compressive and tensile values in the grain interior and near two grain boundaries, respectively. The distribution of the magnitude of diffraction vectors indicates that dislocations are the origin of the strain. The work opens new possibilities for analysing dislocation structures and intragranular residual stress/strain in single grains of polycrystalline materials.

In polycrystals implanted by light ions, a thin layer close to the surface is deformed. X-ray microdiffraction in Laue mode is used to measure the induced strain. In the resulting Laue patterns, the diffraction spots are observed to split, forming double spots, one corresponding to the nondeformed substrate and the other to the deformed layer. A specific image analysis, using bi-Gaussian shape functions, has been developed to improve diffraction spot detection. This is used in association with several numerical tools (conditioning, goodness-of-fit, hat matrix etc.), based on least-squares techniques and statistics, for detecting incorrect data and estimating the accuracy of the result. The use of these tools is not restricted to the study of ion-implanted polycrystals but should find a systematic application for strain analysis from Laue patterns.