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When subjected to plastic deformation, grains within ductile face-centred cubic polycrystals fragment into “soft”, low dislocation density cells separated by “hard”, dislocation-rich walls. Using a narrow-bandwidth, sub-micrometre X-ray beam, we have mapped the deformation structure inside a single grain within a deformed Ni polycrystal. Dislocation multiplication and entanglement was found to vary depending on the physical dimensions of the grain. The method we use overcomes current limitations in classical X-ray topography allowing topographic images to be formed from small, highly deformed grains. [ABSTRACT FROM AUTHOR]

Abstract: Deformation in polycrystalline materials does not occur uniformly in the constituent grains, but shows strong inter- and intra-granular variations. The local deformation response depends on lattice orientation, anisotropic elastic-plastic properties, hardening and damage behaviour and the microstructural neighbourhood. Understanding of these complex interactions is vital for the construction and validation of crystal plasticity models, the introduction of mesoscopic deformation effects into polycrystal models and the fundamental understanding of the material’s deformation behaviour. Micro-beam Laue diffraction provides an excellent tool for the study of inter- and intra-granular deformation, allowing, unlike other microscopy methods, measurements within the bulk of the material. A focused “pink” beam is used to probe the interior of individual grains within a polycrystalline sample. The diffracted radiation forms a pattern of Laue spots which is captured by an area detector. From the spot positions, lattice orientation and elastic strains can be deduced. The intensity distribution within individual spots, can be interpreted in terms of the dislocation arrangement within the gauge volume. In this paper we present a novel setup for microbeam Laue diffraction recently developed on the B16 test-beamline at the Diamond Light Source, UK, with the aim of collecting a high quality dataset characterising mesoscopic deformation behaviour within a polycrystal containing a small number of large grains. Such a dataset was collected from a large grained Ni sample in which a number of maps covering several grains were recorded for a sequence of deformation steps. In this paper some first results from this investigation are presented. Ultimately the dataset is intended to allowe a direct comparison between crystal plasticity models and experimental behaviour. [Copyright &y& Elsevier]

Abstract: Extraction of information about residual elastic strain within the bulk is of primary importance in understanding deformation and stress within engineering components. Current techniques for mapping residual strain often require (semi)destructive sample preparation and only provide strain information at discrete points within the sample. The solution proposed here, based on the Bragg edge neutron transmission method, is non-destructive, and in principle allows a three-dimensional reconstruction of the residual strain throughout the bulk of the sample. The fundamental principle underlying the method is the inversion of a two-dimensional data set in order to obtain three-dimensional information, analogous to the familiar tomographic methods used to reconstruct three-dimensional density distributions in conventional imaging. In the present study we reconstruct the strain profiles from a stainless steel water-spray quenched cylinder. The radially symmetric strain distributions present in the sample help to simplify the analysis, however the method could readily be extended to the reconstruction of arbitrarily complex strain distributions. [Copyright &y& Elsevier]

Abstract: Monotonic and cyclic loading of polycrystals causes a complex evolution of the dislocation structure and internal stresses. These phenomena were studied in sheet samples of commercially pure (CP) Ni in heat-treated (large-grained) states. Various microscopy tools were used, namely, scanning electron microscopy (SEM) with electron back-scattered diffraction (EBSD) to image the surface grain structure; Focused Ion Beam (FIB) with channelling contrast to visualise the through-thickness grain arrangement; and synchrotron scanning transmission X-ray microscopy (STXM) to obtain absorption-contrast images. In order to investigate the internal defects and lattice distortion caused by them, synchrotron X-ray diffraction was used in a variety of modes. Reciprocal space mapping (RSM) was used to quantify the amount of lattice re-orientation (rotation) due to plastic deformation. Micro-beam Laue diffraction was used to obtain 2D images containing multiple reflections that undergo “streaking” due to plastic deformation. The combination of reciprocal space mapping and Laue micro-diffraction provided improved insight into the deformation processes within individual grains during plastic deformation. The results are interpreted and discussed in conjunction with dislocation dynamics and finite element modelling of plastic deformation by crystal slip. [Copyright &y& Elsevier]

Interactions between neighboring grains influence the macroscale behavior of polycrystalline materials, particularly their deformation behavior, damage initiation and propagation mechanisms. However, mapping all of the critical material properties normally requires that several independent measurements are performed. Here we report the first grain mapping of a polycrystalline foil using a pixelated energy-dispersive X-ray area detector, simultaneously measuring X-ray fluorescence and diffraction with the Maia detector in order to determine grain orientation and estimate lattice strain. These results demonstrate the potential of the next generation of X-ray area detectors for materials characterization. By scanning the incident X-ray energy we investigate these detectors as a complete solution for simultaneously mapping the crystallographic and chemical properties of the sample. The extension of these techniques to broadband X-ray sources is also discussed. [ABSTRACT FROM AUTHOR]

Abstract: Non-destructive, quantitative and precise determination of internal strain distributions within structural materials and components can be accomplished by only a few experimental techniques, amongst which diffraction of penetrating radiation (X-ray and neutron beams) plays a central role. However, due to the various limitations of these methods, the 2-D and 3-D mapping of internal strains within bulk cross-sections at a spatial resolution of 0.1mm or better has long remained a challenge. The principle of “strain tomography” proposed by the present authors (an instance of broader “rich” tomography methodology) makes use of the reconstruction tomography technique widely employed in imaging applications. In the present paper we report successful practical implementation of this principle to the study of bending strains in sintered polycrystalline zirconia used in the fabrication of multi-unit fixed prosthetic dentures. The results demonstrate the power of the technique, and pave the way to wider application of this approach to cross-sectional strain mapping in complex-shaped components under various loading conditions. [Copyright &y& Elsevier]

Abstract: The overload retardation effect on fatigue crack growth rate (FCGR) in titanium alloy Ti–6Al–4V is studied. Synchrotron X-ray diffraction strain mapping of near-crack tip regions of pre-cracked fatigued samples is used to determine the effective stress intensity factors experienced by the crack tip. The effective stress intensity factor values are computed by finding the best match between the experimental strain maps and linear elastic fracture mechanics (LEFM) predictions. The dependence of the effective stress intensity factor, K, on the applied load is plotted, and an interpretation of the overload retardation effect is proposed. The present approach permits to reconcile the traditional LEFM fatigue crack propagation prediction and the experimental measurement of strain fields. [ABSTRACT FROM AUTHOR]