Three-dimensional plastic response in polycrystalline coppervianear-field high-energy X-ray diffraction microscopy

The evolution of the crystallographic orientation field in a polycrystalline sample of copper is mapped in three dimensions as tensile strain is applied. Using forward-modeling analysis of high-energy X-ray diffraction microscopy data collected at the Advanced Photon Source, the ability to track intragranular orientation variations is demonstrated on an ∼2 µm length scale with ∼0.1° orientation precision. Lattice rotations within grains are tracked between states with ∼1° precision. Detailed analysis is presented for a sample cross section before and after ∼6% strain. The voxel-based (0.625 µm triangular mesh) reconstructed structure is used to calculate kernel-averaged misorientation maps, which exhibit complex patterns. Simulated scattering from the reconstructed orientation field is shown to reproduce complex scattering patterns generated by the defected microstructure. Spatial variation of a goodness-of-fit or confidence metric associated with the optimized orientation field indicates regions of relatively high or low orientational disorder. An alignment procedure is used to match sample cross sections in the different strain states. The data and analysis methods point toward the ability to perform detailed comparisons between polycrystal plasticity computational model predictions and experimental observations of macroscopic volumes of material.

Plastic Deformation and Recrystallization Studied by the 3-D x-ray Microscope

A newly developed synchrotron instrument – the so-called 3D X-ray microscope – is presented. The instrument is placed at the Materials Science beamline at ESRF and dedicated to local μm scale structural characterization within bulk materials. In this paper, emphasis is on in situ studies of thermomechanical processing. The potential of the instrument for characterization of single nuclei and grains is described and discussed based on both first results and planned experiments.

High Spatial Resolution Strain Measurements Within Bulk Materials by Slit-Imaging

High energy synchrotron radiation is employed for residual strain measurements from local gauge volumes within the bulk of polycrystalline materials. The longitudinal spatial resolution is defined by placing a narrow imaging slit behind the sample and recording the intensity distribution on a position sensitive detector. It is shown that the sample to slit distance can be increased without sacrificing longitudinal resolution by applying a reconstruction technique. Hence, space is provided for large samples and sample environments. The reconstruction technique is described and validated by measuring the residual strain profile of a shot-peened Al sample. A longitudinal gauge length of 95 üm is achieved at 52 keV with a sample to slit distance of 10 cm.

Residual stress distribution in steel butt welds measured using neutron and synchrotron diffraction

70 keV synchrotron radiation and thermal neutrons have been employed to investigate the residual stress characteristics in a fully restrained, steel, butt weld. The focus is on the values of the subsurface and through-thickness strain/stress variation in the middle of the weld. The advantages and limitations of the techniques have been addressed, in relation to the gauge volume, the stress-free reference sample and positioning. The measurement of residual stress around the weld achieved in this work significantly improves the resolution at which residual stress in welded components has been determined.

Separating the recrystallization and deformation texture components by high-energy X-rays

High-energy synchrotron diffraction offers great potential for experimental study of recrystallization kinetics. Measurements on partially recrystallized samples using a monochromatic high-energy synchrotron beam show that recrystallized grains generate sharp diffraction spots, whereas the intensity from the deformed matrix varies smoothly along the Debye–Scherrer rings. Based on these observations, a method has been developed to separate the recrystallization texture components from those originating from the deformation matrix. The validity of this method is demonstrated with partially recrystallized interstitial-free steel.

Three-dimensional maps of grain boundaries and the stress state of individual grains in polycrystals and powders

A fast and non-destructive method for generating three-dimensional maps of the grain boundaries in undeformed polycrystals is presented. The method relies on tracking of micro-focused high-energy X-rays. It is verified by comparing an electron microscopy map of the orientations on the 2.5 × 2.5 mm surface of an aluminium polycrystal with tracking data produced at the 3DXRD microscope at the European Synchrotron Radiation Facility. The average difference in grain boundary position between the two techniques is 26 µm, comparable with the spatial resolution of the 3DXRD microscope. As another extension of the tracking concept, algorithms for determining the stress state of the individual grains are derived. As a case study, 3DXRD results are presented for the tensile deformation of a copper specimen. The strain tensor for one embedded grain is determined as a function of load. The accuracy on the strain is Δ∊ ≃ 10−4.

A conical slit for three-dimensional XRD mapping

Traditionally, depth resolution in diffraction experiments is obtained by inserting pinholes in both the incoming and diffracted beam. For materials science investigations of local strain and texture properties this leads to very slow data-acquisition rates, especially when characterization is performed on the level of the individual grains. To circumvent this problem a conical slit has been manufactured by wire-electrodischarge machining. The conical slit has six 25 microm-thick conically shaped openings matching six of the Debye-Scherrer cones from a face-centred-cubic powder. By combining the slit with a microfocused incoming beam of hard X-rays, an embedded gauge volume is defined. Using a two-dimensional detector, fast and complete information can be obtained regarding the texture and strain properties of the material within this particular gauge volume. The average machining and assemblage errors of the conical slit are found both to be of the order of 5 microm. An algorithm for alignment of the slit is established, and the potential of the technique is illustrated with an example of grain mapping in a 4.5 mm-thick Cu sample.

A scattering filter for energy-dispersive optics

A filtering technique to remove parasitic scattering from X-ray absorption spectra that are acquired in energy-dispersive mode has been developed and tested at the European Synchrotron Radiation Facility. The improved set-up removes small-angle scattering of the sample or the windows of sample cells which may spoil the energy resolution or reduce the intensity of prominent features in the absorption spectrum, such as the white line at the Pt L(III) edge. The sample is placed behind the curved monochromator and between two plane perfect crystals in the Bonse-Hart configuration. The dispersion of the Bonse-Hart double-crystal camera is matched to the dispersion of the curved monochromator by inclining the scattering planes of the two optical elements against each other.

Focusing Optics for High-Energy X-ray Diffraction

Novel focusing optical devices have been developed for synchrotron radiation in the energy range 40-100 keV. Firstly, a narrow-band-pass focusing energy-tuneable fixed-exit monochromator was constructed by combining meridionally bent Laue and Bragg crystals. Dispersion compensation was applied to retain the high momentum resolution despite the beam divergence caused by the focusing. Next, microfocusing was achieved by a bent multilayer arranged behind the crystal monochromator and alternatively by a bent Laue crystal. A 1.2 micro m-high line focus was obtained at 90 keV. The properties of the different set-ups are described and potential applications are discussed. First experiments were performed, investigating with high spatial resolution the residual strain gradients in layered polycrystalline materials. The results underline that focused high-energy synchrotron radiation can provide unique information on the mesoscopic scale to the materials scientist, complementary to existing techniques based on conventional X-ray sources, neutron scattering or electron microscopy.

Microfocusing of hard X-rays with cylindrically bent crystal monochromators

High-energy X-ray focusing with bent-crystal monochromators is known to be hampered by so-called depth or crystal-thickness aberrations. A theoretical model of focus broadening based on the geometrical theory of X-ray diffraction in slightly deformed crystals is presented and compared with experimental data. First, it is shown that depth broadening can be avoided in the Laue geometry by an appropriate choice of asymmetry angle. Based on this finding, a monochromator for high-pressure diffraction experiments has been designed and a source-size-limited focal spot below 10 microns is observed. As a consequence of the box-shaped rocking curve of bent Laue crystals, the focus is free of long-ranging tails. Diffraction patterns of standard powder samples were recorded on imaging plates and a theoretical description of the energy-dispersion-related peak broadening is given. Finally, diffraction patterns of N2 at 180 kbar demonstrate the excellent data quality achievable with this monochromator.