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Understanding the evolution of chemical composition and morphology of battery materials during electrochemical cycling is fundamental to extending battery cycle life and ensuring safety. This is particularly true for the much debated high energy density (high voltage) lithium-manganese rich cathode material of composition Li(1 + x)M(1 - x)O2 (M = Mn, Co, Ni). In this study we combine full-field transmission X-ray microscopy (TXM) with X-ray absorption near edge structure (XANES) to spatially resolve changes in chemical phase, oxidation state, and morphology within a high voltage cathode having nominal composition Li1.2Mn0.525Ni0.175Co0.1O2. Nanoscale microscopy with chemical/elemental sensitivity provides direct quantitative visualization of the cathode, and insights into failure. Single-pixel (∼ 30 nm) TXM XANES revealed changes in Mn chemistry with cycling, possibly to a spinel conformation and likely including some Mn(II), starting at the particle surface and proceeding inward. Morphological analysis of the particles revealed, with high resolution and statistical sampling, that the majority of particles adopted nonspherical shapes after 200 cycles. Multiple-energy tomography showed a more homogeneous association of transition metals in the pristine particle, which segregate significantly with cycling. Depletion of transition metals at the cathode surface occurs after just one cycle, likely driven by electrochemical reactions at the surface.

The development of ultra-brilliant synchrotron X-ray sources enables characterization methods that are particularly important for nuclear materials. Here we discuss emerging synchrotron methods with unprecedented signal-to-noise, spatial and time resolution. Microprobe methods are discussed that extend virtually any X-ray characterization measurement to ultra-small sample volumes. This ability is critical to resolve heterogeneities in nuclear materials and for studies on volumes with vastly lower activity than are needed for traditional X-ray characterization. Specific methods discussed include microdiffraction for the characterization of local crystal structure and micro-spectroscopy techniques that allow for characterization of elemental distributions with sensitivity for daughter products, oxidation states and diffusion through buffer layers. Opportunities are also discussed that exploit the high brilliance and pulsed nature of synchrotron radiation to reduce backgrounds from sample radiation and to study materials dynamics.

This paper describes fabrication methods used to demonstrate the advantages of nested or Montel optics for micro/nanofocusing of synchrotron X-ray beams. A standard Kirkpatrick-Baez (KB) mirror system uses two separated elliptical mirrors at glancing angles to the X-ray beam and sequentially arranged at 90° to each other to focus X-rays successively in the vertical and horizontal directions. A nested KB mirror system has the two mirrors positioned perpendicular and side-by-side to each other. Compared to a standard KB mirror system, Montel optics can focus a larger divergence and the mirrors can have a shorter focal length. As a result, nested mirrors can be fabricated with improved demagnification factor and ultimately smaller focal spot, than with a standard KB arrangement. The nested system is also more compact with an increased working distance, and is more stable, with reduced complexity of mirror stages. However, although Montel optics is commercially available for laboratory X-ray sources, due to technical difficulties they have not been used to microfocus synchrotron radiation X-rays, where ultra-precise mirror surfaces are essential. The main challenge in adapting nested optics for synchrotron microfocusing is to fabricate mirrors with a precise elliptical surface profile at the very edge where the two mirrors meet and where X-rays scatter. For example, in our application to achieve a sub-micron focus with high efficiency, a surface figure root-mean-square (rms) error on the order of 1 nm is required in the useable area along the X-ray footprint with a ∼0.1 mm-diameter cross section. In this paper we describe promising ways to fabricate precise nested KB mirrors using our profile coating technique and inexpensive flat Si substrates.

X-ray microbeams are an emerging characterization tool with broad implications for science, ranging from materials structure and dynamics, to geophysics and environmental science, to biophysics and protein crystallography. We describe how submicrometer hard x-ray beams with the ability to penetrate tens to hundreds of micrometers into most materials and with the ability to determine local composition, chemistry, and (crystal) structure can characterize buried sample volumes and small samples in their natural or extreme environments. Beams less than 10 nanometers have already been demonstrated, and the practical limit for hard x-ray beam size, the limit to trace-element sensitivity, and the ultimate limitations associated with near-atomic structure determinations are the subject of ongoing research.

Diffuse scattering caused by L12 type Ni3 (Al, Si) precipitates after stress annealing of Ni-Al-Si alloys is studied. Peculiarities of diffuse scattering in the asymptotic region as compared to the Huang scattering region are discussed. Coupling between the stress annealing direction and the precipitate shape is demonstrated. Experimental reciprocal space maps (RSMs) are compared to theoretical ones. Oscillations of diffuse scattering due to Ni3 (Al, Sc) precipitates are observed. The strengths of the precipitates are estimated from the analysis of the diffuse scattering oscillations.

In this paper, designs of a novel rotary weak-link stage for a vertical rotation axis and a two-dimensional tip-tilting system are presented. Applications of these new stage systems include: an advanced X-ray stereo imaging instrument for particle tracking velocimetry, an alignment stage system for hard X-ray nano-focusing Montel mirror optics, and an ultra-precision crystal manipulator for cryo-cooling optical cavities of an X-ray free-electron-laser oscillator (XFELO).

A nested Kirkpatrick–Baez mirror pair has been designed, fabricated and tested for achromatic nanofocusing synchrotron hard X-rays. The prototype system achieved a FWHM focal spot of about 150 nm in both horizontal and vertical directions., The first test of nanoscale-focusing Kirkpatrick–Baez (KB) mirrors in the nested (or Montel) configuration used at a hard X-ray synchrotron beamline is reported. The two mirrors are both 40 mm long and coated with Pt to produce a focal length of 60 mm at 3 mrad incident angle, and collect up to a 120 µm by 120 µm incident X-ray beam with maximum angular acceptance of 2 mrad and a broad bandwidth of energies up to 30 keV. In an initial test a focal spot of about 150 nm in both horizontal and vertical directions was achieved with either polychromatic or monochromatic beam. The nested mirror geometry, with two mirrors mounted side-by-side and perpendicular to each other, is significantly more compact and provides higher demagnification than the traditional sequential KB mirror arrangement. Ultimately, nested mirrors can focus larger divergence to improve the diffraction limit of achromatic optics. A major challenge with the fabrication of the required mirrors is the need for near-perfect mirror surfaces near the edge of at least one of the mirrors. Special polishing procedures and surface profile coating were used to preserve the mirror surface quality at the reflecting edge. Further developments aimed at achieving diffraction-limited focusing below 50 nm are underway.

The depth-dependent strain gradients near buried interfaces in a model system of NiAl–Mo composite were nondestructively probed with 3-D X-ray microdiffraction. Coupled with micromechanical analysis, our study shows that the relaxation of the residual thermal strains in the NiAl–Mo composites results in the formation of a near-surface “slip zone” with large strain gradients in both the reinforcing Mo fibers and NiAl matrix. Based on these results an approach to calculate the fiber–matrix interface strength for composite materials is suggested.

Renewed interest in composite materials is driven by the fact that their mechanical properties can be superior to those of individual constituent phases. Interfaces between the phases and the properties of individual phases are the two key elements responsible for the unique micro-mechanisms of plastic deformation in composites. In this research summary we show how the depth-dependent residual strain distributed in the two phases and partitioned across the composite interfaces can be directly measured at submicron length-scales using x-ray microdiffraction and compared to a detailed simulation within the framework of micromechanical stress analysis. Interface strength is determined from the analysis of the so-called “slip zone” caused by the near-surface stress relaxation. Two examples are discussed: Mo-NiAl and AG-15 composites.

Three-dimensional spatially resolved strains were mapped in a model NiAl/Mo composite after nanoindentation. The depth-dependent strain distributed in the two phases and partitioned across the composite interfaces is directly measured at submicron length scale using X-ray microdiffraction and compared with a detailed micromechanical stress analysis. It is shown that indentation-induced deformation in the composite material is distinct from deformation expected in a single-phase material. This difference arises in part from residual thermal strains in both phases of the composite in the as-grown state. Interplay between residual thermal strains and external mechanical strain results in a complex distribution of dilatational strain in the Mo fibers and NiAl matrix and is distinct in different locations within the indented area. Reversal of the strain sign (e.g., alternating tensile/compressive/tensile strain distribution) is observed in the NiAl matrix. Bending of the Mo fibers during indentation creates relatively large ∼1.5° misorientations between the different fibers and NiAl matrix. Compressive strain along the 〈0 0 1〉 direction reached −0.017 in the Mo fibers and −0.007 in the NiAl matrix.

The indentation-induced elastic-plastic zone in an IN 740 Ni-based superalloy was studied by three-dimensional (3-D) x-ray microdiffraction and electron back scattering diffraction (EBSD). Large lattice reorientations and the formation of geometrically necessary dislocations are observed in the area with a radius of ∼75 μm. A residual compression zone is found close to the indent edge. An elastic-plastic transition is observed at ∼20 μm from the indent edge. Depth dependent dislocation densities are determined at different distances from the indent edge.

We report on novel 3D spatially resolved X-ray diffraction microscopy studies of self-affine behavior in deformed single crystals. This study extends surface profile measurements of self-affined morphology changes in single crystals during deformation to include local lattice rotations and sub-surface behavior. Investigations were made on the spatial correlation of the local lattice rotations in 8% tensile deformed Cu single crystals oriented with [1 2 3], [1 1 1] and [0 0 1] axes parallel to the tensile axis. The nondestructive depth-resolved measurements were made over a length scale of one to hundreds of micrometers. Self-affined correlation was found both at the surface and below the surface of the samples. A universal exponent for the power-law similar to that observed with surface profile methods is found at the surface of all samples but crystallographically sensitive changes are observed as a function of depth. Correlation lengths of the self-affine behavior vary with the [1 2 3] crystal exhibiting the longest self-affine length scale of 70 μ m with only 18 μ m for the [1 1 1] and [0 0 1] crystals. These measurements illuminate the transition from surface-like to bulk-like deformation behavior and provide new quantitative information to guide emerging models of self-organized structures in plasticity.

Neutron and synchrotron X-ray diffractions were used to study the texture development in the face-centered-cubic nickel-based superalloy over gauge volumes ranging from hundreds of cubic millimeters down to sub-cubic micrometers. The bulk averaged results find a uniform texture development from collective slip. However, X-ray microbeam studies at microscale find that the plastic deformation within a single grain is mediated by limited slip, as evidenced by the local strain distribution. Polychromatic microdiffraction shows the formation of several distinct structural zones even in the same grain. A hierarchical heterogeneous geometrically-necessary dislocations arrangement and distinct slip bands are observed within each grain. A depth-dependent change of the stereographic projection within the slip band in the grain is demonstrated. Correlated oscillations of the lattice orientation at the submicron scale evolve into an overall texture at the macroscale. Possible reasons for this observed structural evolution are discussed.

This research presents a combined experimental-modeling study of lattice rotations and deviatoric strain gradients induced by focused-ion beam (FIB) milling in nitride heterostructures. 3D X-ray polychromatic microdiffraction (PXM) is used to map the local lattice orientation distribution in FIB-structured areas. Results are discussed in connection with microphotoluminescence (μ-PL), fluorescent analysis, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) data. It is demonstrated that FIB-milling causes both direct and indirect damage to the InGaN/GaN layers. In films subjected to direct ion beam impact, a narrow amorphidized top layer is formed. Near the milling area, FIB-induced stress relaxation and formation of complicated 3D strain fields are observed. The resulting lattice orientation changes are found to correlate with a decrease and/or loss of PL intensity, and agree well with finite element simulations of the three-dimensional strain fields near the relaxed trenches. Experimentally, it is found that the lattice surface normal has an in-plane rotation, which only appears in simulations when the GaN-substrate lattice mismatch annihilates the InGaN-substrate mismatch. This behavior further supports the notion that the film/substrate interface is incoherent.

The high source intensity of the Spallation Neutron Source (SNS), together with efficient detectors and large detector solid angles, now makes possible neutron experiments with much smaller sample volumes than previously were practical. Nested Kirkpatrick-Baez supermirror optics provide a promising and efficient way to further decrease the useable neutron sample size by focusing polychromatic neutrons into microbeams. Because the optics are nondispersive, they are ideal for spallation sources and for polychromatic and wide bandpass experiments on reactor sources. Theoretical calculations indicate that nested mirrors can preserve source brilliance at the sample for small beams and for modest divergences that are appropriate for diffraction experiments. Although the flux intercepted by a sample can be similar with standard beam-guided approaches, the signal-to-background is much improved with small beams on small samples. Here we describe the design, calibration and performance of a nested neutron mirror pair for the Spallation Neutrons At Pressure (SNAP) beamline at the SNS. High-pressure neutron diffraction is but one example of a large class of neutron experiments that will benefit from spatially-resolved microdiffraction.

3D spatially-resolved polychromatic microdiffraction was used to nondestructively obtain depth-dependent elastic strain gradients and dislocation densities in the constituent phases of a directionally solidified NiAl–Mo eutectic composite consisting of ∼500–800 nm Mo fibers in a NiAl matrix. Measurements were made before and after the composite was compressed by 5% and 11%. The Mo fibers were analyzed both in their embedded state and after the matrix was etched to expose them as pillars. In the as-grown composite, due to differential thermal contraction during cooldown, the Mo phase is under compression and the NiAl phase is in tension. After the prestrains, the situation is reversed with the Mo phase in tension and NiAl matrix in compression. This result can be explained by taking into account the mismatch in yield strains of the constituent phases and the elastic constraints during unloading. The dislocation density in both the Mo and NiAl phases is found to increase after prestraining. Within experimental uncertainty there is little discernible difference in the total dislocation densities in the Mo phase of the 5% and 11% prestrained specimens. However, the density of the geometrically necessary dislocations and the deviatoric strain gradients increase with increasing prestrain in both the Mo and NiAl phases.

In this study, the fatigue-induced microstructure produced in a nickel-based polycrystalline superalloy that was subjected to cyclic loading was characterized by polychromatic x-ray microdiffraction (PXM) together with in-situ neutron diffraction and transmission-electron microscopy (TEM). In-situ neutron-diffraction measurements reveal two distinct stages of the fatigue damage: cyclic hardening followed by cyclic softening. Three-dimensional spatially resolved PXM micro-Laue measurements find an increase in the density of geometrically necessary dislocations near the grain boundaries, which is accompanied by lattice rotations and grain subdivisions. The PXM results are in agreement with the in-situ neutron-diffraction and TEM results.

A spatially resolved X-ray diffraction method – with a submicron 3D resolution together with SEM and OIM analysis are applied to understand the arrangements of voids, geometrically necessary dislocations and strain gradient distributions in samples of Al (1 2 3) and Cu (0 0 1) single crystals shocked to incipient spallation fracture. We describe how geometrically necessary dislocations and the effective strain gradient alter white beam Laue patterns of the shocked materials. Several distinct structural zones are observed at different depths under the impact surface. The density of geometrically necessary dislocations (GNDs) is extremely high near the impact and back surface of the shock recovered crystals. The spall region is characterized by a large density of mesoscale voids and GNDs. The spall region is separated from the impact and back surfaces by compressed regions with high total dislocation density but lower GNDs density. Self-organization of shear bands is observed in the shock recovered Cu single crystal.

MicroLaue diffraction combines the oldest x-ray diffraction method–Laue diffraction–with the most modern x-ray sources, optics and detectors. The combination can resolve complex materials into single-crystal-like submicron volumes. This unique ability to nondestructively map crystal structure at and below a sample surface, with high spatial and strain resolution can address long-standing fundamental issues in materials science. For example, the three-dimensional evolution of mesoscale structure and the self organization of defects can be observed nondestructively to understand the origins of inhomogeneous grain growth, deformation and fracture.

Kirkpatrick–Baez (KB) neutron supermirrors can efficiently focus polychromatic neutron beams to micrometre dimensions. The ultimate size is determined mainly by the perfection of the mirrors and by the size of the beam needed to have sufficient experimental signal. Nested or Montel KB mirrors can collect ∼2.6 times more beam than standard sequential KB optics, but require good figure perfection at the edge of one mirror. This paper describes the characterization of the figure errors over the important reflective portions of the two mirrors needed for a Montel focusing pair. The measurements are placed in context with theoretical predictions and are used to predict mirror focusing performance. Strategies to improve on the focusing of this class of optics are suggested and early results from these mirrors installed on the Spallation Neutrons at Pressure (SNAP) Beamline 3 at the Spallation Neutron Source (SNS) at Oak Ridge are presented.

Severe plastic deformation during Friction Stir Processing (FSP) of an IN738 Ni-based superalloy was studied by means of X-ray polychromatic microdiffraction, EBSD, scanning electron and optical microscopies. Modeling of the physical properties and phase composition was also performed. Several distinct zones are formed during FSP including a stir zone (SZ), a thermal-mechanical affected zone (TMAZ) and a heat affected zone (HAZ). Each zone has distinct microstructure after FSP. The initial dendrite structure is preserved in the HAZ, while strengthening γ′-phase particles partially dissolve and coagulate. Plastic deformation of the base material dendrites takes place in the TMAZ and a large number of geometrically necessary dislocations are formed. The extent of deformation increases toward the SZ and the dendrite structure is completely destroyed in the SZ and replaced by a fine submicrocrystallinne microstructure.

With a high-energy 3rd generation source like the Advanced Photon Source (APS), it is possible to push the performance of polychromatic microdiffraction far beyond current levels and to approach the intrinsic limit of the technique based on sample damage and the diffraction limit of X-rays. We describe ongoing efforts to improve the spatial, temporal and momentum transfer resolution of polychromatic microdiffraction on beamline 34-ID-E at the APS. The goal of this effort is to provide high-resolution images of 3D crystal structures over sufficient volumes and with sufficient detail to clarify the underlying physics of inhomogeneous structure and evolution on mesoscopic length scales. The performance of a high-speed amorphous Si area detector system and the ongoing development of advanced focusing optics will be described and discussed in light of the ultimate limits set by the physics of X-rays and materials, and in light of opportunities to field specialized insertion devices and optics for polychromatic microdiffraction.

We compare experimental measurements of inhomogeneous plastic deformation in a Ni bicrystal with crystal plasticity simulations. Polychromatic X-ray microdiffraction, orientation imaging microscopy and scanning electron microscopy, were used to characterize the geometrically necessary dislocation distribution of the bicrystal after uniaxial tensile deformation. Changes in the local crystallographic orientations within the sample reflect its plastic response during the tensile test. Elastic strain in both grains increases near the grain boundary. Finite element simulations were used to understand the influence of initial grain orientation and structural inhomogeneities on the geometrically necessary dislocations arrangement and distribution and to understand the underlying materials physics.

Timely development of materials for the demanding fusion energy environment requires a broad range of advanced scientific tools, including advanced structural characterization methods. The current state-of-the-art and emerging capabilities in electron microscopy, atom probe tomography, neutron scattering and X-ray scattering are reviewed with respect to potential applications in fusion materials research and development. Recent dramatic advances in capabilities in all four of these characterization tools are transforming the spatial precision and quantitative information that can be extracted during structural characterization. Examples include spectroscopic identification of single atoms in bulk materials, three-dimensional mapping of millimeter-scale volumes of materials with nanometer resolution, and high-resolution in situ measurements of internal stress and strain during mechanical testing.

This paper analyzes local lattice rotations introduced in severely deformed polycrystalline titanium by friction stir welding. Nondestructive three-dimensional (3D) spatially resolved polychromatic X-ray microdiffraction, is used to resolve the local crystal structure of the restructured surface from neighboring local structures in the sample material. The measurements reveal strong gradients of strain and geometrically necessary dislocations near the surface and illustrate the potential of polychromatic microdiffraction for the study of deformation in complex materials systems.

A combined experimental/computational approach is employed to study slip-system-related dislocation-substructure formation during uniaxial tension of a single-phase, face-centered-cubic (fcc), nickel-based alloy. In-situ neutron-diffraction measurements were conducted to monitor the peak-intensity, peak-position, and peak-broadening evolution during a displacement-controlled, monotonic-tension experiment at room temperature. The measured lattice-strain evolution and the macrostress/macrostrain curves were used to obtain the material parameters required for simulating the texture development by a visco-plastic self-consistent (VPSC) model. The simulated texture compared favorably with experimentally-determined texture results over a range of 0 to 30 pct engineering strain. The grain-orientation-dependent input into the Debye-intensity ring was considered. Grains favorably oriented relative to the two detector banks in the geometry of the neutron experiment were indicated. For the favorably oriented grains, the simulated slip-system activity was used to calculate the slip-system-dependent, dislocation-contrast factor. The combination of the calculated contrast factor with the experimentally-measured peak broadening allows the assessment of the parameters of the dislocation arrangement within the specifically oriented grains, which has a quantitative agreement with the transmission-electron-microscopy results.

The availability of intense, focused synchrotron X-ray microbeams has enabled new techniques for materials investigations with sub-micron spatial resolution. The scanning microbeam setup we have developed at the XOR-UNI beamline at the Advanced Photon Source (APS) is versatile in that it provides 1D, 2D or 3D scans (including depth resolution), and can alternate between polychromatic- and monochromatic-beam modes. Focusing in both modes uses elliptical Kirkpatrick-Baez (K-B) reflecting mirrors. Beam diameters of {approx} 500 nm FWHM are now routine, and 90 nm focus has been demonstrated. In white-beam mode, a CCD detector records a complete Laue diffraction pattern, which is analyzed with an automated indexing program. These X-ray diffraction patterns provide real-space maps of the local lattice structure, crystal orientation, grain morphology, and strain tensor. Spatially-resolved X-ray microdiffraction studies are now providing previously unavailable measurements of local microstructures. These measurements, in turn, yield new insights in several classic fields of materials science. This paper will illustrate the application of polychromatic scanning X-ray microscopy with examples from 1D, 2D and 3D materials systems. In 1D systems, we have mapped the structure and orientation of an individual ZnO nanorod along with the associated Ge catalyst particle used to control the nanocrystal growth. Inmore » 2D systems, X-ray microdiffraction studies have revealed the mechanisms for local crystallographic tilting in epitaxial oxide films grown on textured Ni substrates for superconducting applications. In 3D systems, X-ray microscopy investigations have included in-situ studies of microstructural evolution during thermal grain growth in polycrystalline aluminum. In general, these spatially-resolved measurements provide important new insights and are valuable as input for theoretical and computer modeling studies of a wide range of material processes.« less

In a given work, the detailed systematic analysis of the discovered phenomenon is carried out. This phenomenon of unique informativity of the dynamical-scattering pattern in crystals with many types of defects is described, its physical nature and possibilities of practical application are determined. The phenomenon nature lies in the fact that, for each reflex of the scattering pattern, while transition from kinematic case to dynamical one takes place, the dependences on conditions of dynamical diffraction are radically different from each other for Bragg and diffuse components, which always remain the same in the kinematic theory. This essential difference is caused by the substantially different nature and magnitudes of the fundamental characteristics of materials, namely, scattering factors, for Bragg and diffuse components. For the Bragg component, it is the scattering potential averaged over all defects’ configurations (periodical), and for the diffuse one, these ones are the fluctuating deviations of scattering potential from its average value (nonperiodical). As a result, the dynamical extinction factors for the coherent and diffuse-scattered waves differ from each other as well as other regularities of the multiple-scattering effects for Bragg and diffuse components. The ascertainment of a nature of the mentioned effects has provided the unique possibility of the operated change of diffuse-component contribution for the fixed defect structure just due to the purposive variation of the dynamical-diffraction conditions. The influences of defects on the Bragg and diffuse components of the scattering pattern have opposite nature, i.e. they always decrease the Bragg component, but increase the diffuse one. Due to this fact, the abovementioned phenomenon provides the possibility of the controllable variations of the total defects’ influence on the scattering pattern as a whole, which is used for its characterization. In case of the defects’ diagnostics in the dynamical diffraction conditions, this one governed the essential revision of principles of the quantitative and qualitative classification of defects by their influence on the scattering pattern, which has been performed within the scope of the kinematic theory. The possibility to control the diffuse-component contribution is the most important advantage of the dynamical diffraction, which has increased radically the informativity and has provided the fundamentally new functional possibilities of the dynamical diffractometry. The dynamical diffraction only gives the possibility of realization and combined (joint) processing of full set of the independent diffraction experiments due to the changing diffraction conditions for a sample of fixed defect structure with the goal to solve an inverse problem of the multiparametric diagnostics of crystalline materials and products of nanotechnology with several types of defects. Thus, the new generation of crystallography, i.e. the diffuse dynamical combined diffractometry of complex defect structures in single crystals and heterogeneous systems, has been developed.

An in-situ neutron diffraction technique was used to investigate the lattice-strain distributions and plastic deformation around a crack tip after overload. The lattice-strain profiles around a crack tip were measured as a function of the applied load during the tensile loading cycles after overload. Dislocation densities calculated from the diffraction peak broadening were presented as a function of the distance from the crack tip. Furthermore, the crystallographic orientation variations were examined near a crack tip using polychromatic X-ray microdiffraction combined with differential aperture microscopy. Crystallographic tilts are considerably observed beneath the surface around a crack tip, and these are consistent with the high dislocation densities near the crack tip measured by neutron peak broadening.

The plasticity behavior of the annealed HASTELLOY C-22HSTM alloy, a face-centered cubic (FCC), nickel-based superalloy, was examined by the in-situ neutron-diffraction experiments at room temperature. Monotonic-tension and low-cycle-fatigue experiments were conducted to observe the plastic behavior of the alloy. The tension straining and cyclic-loading deformation were studied as a function of the stress. The plastic behaviors during the deformation are discussed in the light of the relationship between the stress and dislocation-density evolutions. The calculated dislocation-density evolutions within the alloys reflect the strain hardening and cyclic hardening/softening. Experimental lattice strains are compared to verify the hardening mechanism at the selected stress levels for tension and cyclic loadings. Combining with the calculations of the dislocation densities, the neutron-diffraction experiments give an evidence of the strain and cyclic hardening of the alloy.

The crack-tip deformation behavior during a single overload, fatigue test of ferritic stainless steel, and Ni-based HAYNES 230 superalloy is studied at different structural levels using (1) neutron-diffraction, from which both the elastic-lattice strain and volume-averaged total dislocation densities are obtained, (2) polychromatic X-ray microdiffraction to probe the geometrically necessary dislocations and boundaries distribution, and (3) an irreversible and hysteretic cohesive interface model which has been implemented into a finite element framework to simulate the stress/strain evolution near the fatigue crack tip. Neutron strain measurements and finite element simulations are in qualitative agreement on the macroscopic length scale. Large plastic deformation induced by the overload and the resulting compressive residual strains are observed in front of the crack tip after the overload, and are the principal reason for the fatigue-crack-growth retardation. Strong strain gradients surrounding the crack...

Polychromatic microdiffraction is an emerging materials-characterization tool made practical by powerful X-ray and neutron sources, and by advanced optics and software. With polychromatic techniques, local crystalline properties including phase, texture (orientation), elastic strain, and defect density can be mapped with submicron spatial resolution in three dimensions. Here, we describe the evolving ability to nondestructively map local crystal structure in three dimensions and discuss how future advances will help address long-standing issues of inhomogeneous grain growth, deformation, fracture, and elastic strain. Current and future applications impact virtually all materials including electronic, solar, and light-emitting-diode (LED) materials, nanomaterials, structural materials, and joining materials. In addition, the ability to focus small beams on small samples dramatically increases signal-to-noise and greatly reduces the cost for extreme environmental chambers required for high-pressure, high-temperature, high-magnetic field or corrosive environments. Polychromatic techniques efficiently use source brilliance and minimize the required sample volume, which is essential for hard-to-make materials, irreplaceable materials, and for radioactive, toxic, or otherwise dangerous materials. New polychromatic neutron capabilities will significantly extend the range of samples that can be studied with neutrons and presents important new scientific opportunities for studies of magnetic materials, low Z elements, fragile crystal structures, and small samples in extreme environments.

A novel white-beam microdiffraction analysis of defects, strains and tilts in a free standing m -plane GaN film grown via hydride vapor phase epitaxy is presented. It is shown that misfit dislocations are grouped within cell boundaries creating local lattice rotations (tilts) between the growing cells. Distribution of lattice rotations in the film is not homogeneous. Regions of large rotations are separated by low rotations regions. The dominating rotation axis is parallel [110] direction. High in plane shear stress component is observed along [0001]. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The nature of residual damage from atomic displacement cascades in iron has been investigated by diffuse X-ray scattering. A single crystal iron sample was irradiated at ∼60 °C in the High Flux Isotope Reactor at the Oak Ridge National Laboratory to a fluence of 1 × 10 23 n/m 2 ( E > 0.1 MeV), or ∼0.01 dpa. Diffuse X-ray scattering measurements were carried out at the Advanced Photon Source at the Argonne National Laboratory on three specimens: unirradiated, as-irradiated, and irradiated and annealed for one hour at 450 °C (IA). The specimens were cooled to ∼40 K to minimize thermal diffuse scattering. Subsequent measurements of the specimen lattice parameter were completed at ORNL. The diffuse scattering in the as-irradiated specimen was dominated by interstitial defect clusters exhibiting a tetragonal distortion consistent with a 〈1 0 0〉 type defect. Substantial recovery of this defect component was observed following the anneal at 450 °C. A lattice parameter increase was observed following irradiation, which is also consistent with a substantial population of interstitial type defects. A net lattice parameter decrease was observed in the IA specimen, indicating loss of the interstitial defects with a residual population of vacancy-type defects.

The spatially resolved distribution of strain, misfit and threading dislocations, and crystallographic orientation in uncoalesced GaN layers grown on Si(111) by maskless cantilever epitaxy or by pendeo epitaxy on SiC was studied by white beam Laue X-ray microdiffraction, scanning electron and orientation imaging microscopy. Tilt boundaries formed at the column/wing interface depending on the growth conditions. A depth dependent deviatoric strain gradient is found in the GaN. The density of misfit dislocations as well as their arrangement within different dislocation arrays was quantified. Two different kinds of tilt (parallel and perpendicular to the stripe direction) manifested themselves by mutually orthogonal displacements of the (0006) GaN Laue spot relative to the Si(444) Laue spot. The origin of the tilts is discussed with respect to the miscut of the Si(111) surface and misfit dislocations formed at the interface. Regular oscillations of the conventional wing tilt were observed. Irregular crystallographic tilts fluctuations were found in the direction parallel to stripes. The amplitude of fluctuations is an order of magnitude smaller for layers with a lower defects density.

The dislocation density tensor has been an important variable in the theoretical characterization of dislocations in deformed crystals since its introduction over 5 decades ago. However, the non-destructive, three-dimensional (3D) measurements of lattice rotations and elastic strain needed to determine dislocation density tensors with micron spatial resolution over mesoscopic length scales have until now not been available. We have used 3D X-ray microscopy with sub-micron point-to-point spatial resolution to demonstrate 3D, spatially resolved measurements of the dislocation density tensor in elastically and plastically deformed silicon single crystal plates. Measurements were made of the dislocation density tensor along a line in a ∼35 µm thick silicon plate that was bent (elastically) to a 5.42 mm radius of curvature at room temperature, and in a similar sample deformed plastically by annealing to 700°C under bending stress. We discuss the theoretical background for the dislocation density tensor with re...

Crystalline precipitates in a bulk-metallic-glass (BMG) braze were investigated with an intense x-ray microbeam. The precipitates were found in the Pd40Cu30P20Ni10 BMG braze matrix after joining crystalline Ti-6Al-7Nb. However, the role (if any) played by the precipitates in improving the mechanical bond of the BMG/crystalline joint is unknown. X-ray microdiffraction and microfluorescence measurements from small sample volumes were made with an ~ 0.5 x 0.5 μm2 beam. Spatially-resolved Laue diffraction and x-ray fluorescence measurements were made on several second-phase crystals within the BMG matrix. Although precipitate crystals with the observed compositions were anticipated to be predominantly hexagonal, one of the crystals was found to be cubic or tetragonal. The instrumentation includes capabilities for 3D depth-resolved measurements of crystal structure and for fluorescence analysis of elemental composition. Depth profiling gave information about the grain distribution and morphology in the BMG matrix.

Atomic structure is perhaps the information most critical to the understanding of materials behaviour, hence the great importance of x-rays and neutrons as probes. Although scattering is sensitive to pair and higher-order correlations, in most applications only the pair correlation is recovered. However, pair correlation is inadequate for a complete description of homogeneous systems in thermodynamic equilibrium; all correlations are required. It is often assumed that the pair correlations extracted from scattering experiments either uniquely determine or greatly restrict higher-order correlations. Here we argue on the basis of simulations and classical density functional theory that when the Hamiltonian is of pair-potential form the pair correlations do uniquely determine all higher-order correlations. However, we also demonstrate by simulation and prove algebraically that for specific many-body Hamiltonians additional information beyond pair correlations is needed to determine higher-order correlations. The derivations are underpinned by the close connection between fluctuations, applied fields, and correlations and identify approaches that hold promise for extracting higher-order correlations.

Polychromatic microdiffraction uses small X-ray beams to characterize the local crystallographic structure of materials. When combined with a depth resolving technique called differential aperture microscopy, the phase and local orientation of femto-liter volumes (0.5 × 0.5 × 0.7 μm 3 ) can be resolved beneath the surface of a sample. In addition, the local elastic strain and dislocation tensors can also be determined and the local dislocation type can be modeled. Here we present recent technical developments, including example applications and emerging research directions.

The spatial distribution of strain, dislocations, and crystallographic orientation in uncoalesced and coalesced GaN layers grown on striped Si or SiC substrates was studied by polychromatic X-ray microdiffraction and high resolution monochromatic X-ray diffraction. Tilt boundaries formed at the column/wing interface depending on the growth conditions and geometry of the striped substrate. The measurements revealed that the free-hanging wings are tilted upward at room temperature. A misorientation between the GaN(0001) and the Si(111) or SiC(0001) surface normal is observed. Distinct structural properties of the pendeo and cantilever epitaxially grown samples are discussed.

The paper relates aspects of interfaces, residual stresses and dislocations with joint behaviour in the Ni-based single crystals. It is focused on the understanding the fundamental driving forces and response of the materials which leads to the formation of dislocation structure during local melting and solidification. Oscillations in the dislocation structure are observed at both macro and micro scales. The underlying mechanism of these processes is essential to real world applications.

Intrinsic stresses due to lattice mismatch, high densities of threading dislocations, and extrinsic stresses resulting from the mismatch in the coefficients of thermal expansion, are present in almost all III-Nitride heterostructures. Stress relaxation in the GaN layers occurs in conventional, cantilever (CE) and in pendeo-epitaxial (PE) films via the formation of additional misfit dislocations, domain boundaries, elastic strain and wing tilt. Polychromatic X-ray microdiffraction, high resolution monochromatic X-ray diffraction and SEM analysis have been used to determine the crystallographic properties, misfit dislocations distribution and crystallographic tilts in uncoalesced GaN layers grown by PE and CE. The crystallographic tilt between the GaN(0001) and Si(111) planes was detected in the CE grown samples on Si(111). In contrast there was no tilt between GaN(0001) and SiC(0001) planes in PE grown samples. The wings are tilted upward for both the PE and CE grown uncoalesced GaN layers.

The effects of the as-cast microstructure on the oxidation characteristics of two Ni-Al-Cr alloys with either γ or γ′ primary solidification were investigated with an in-situ, time-resolved X-ray diffraction (TRXRD) technique using synchrotron radiation. The measurements, carried out during rapid heating and cooling, showed that a segregated microstructure in these cast alloys leads to the preferential formation of zirconium oxide before the formation of aluminum oxides is detected. The oxidation leads to a change in the phase stability and to the modification of surface microstructures. Computational thermodynamic models were used to explain the preferential formation of oxides in the as-cast microstructure.

The emergence of intense synchrotron X-ray sources, efficient focusing optics and high-performance X-ray sensitive area detectors allows for measurements of diffuse scattering from cubic micron-scale sample vol umes. Here we present an experiment that illustrates methods for studying the local structure and defect content of tiny sample volumes. In the experiment, an X-ray microbeam illuminating about ∼5 μm3 of a Ni-based superalloy single crystal, is used to collect Laue patterns and reciprocal space volume maps around fundamental and a superstructure reflections. This measurement illustrates how diffuse reciprocal-space distributions can be collected with good spatial and momentum-transfer resolution from a tiny real-space sample volume. This example demonstrates that emerging diffuse scattering techniques can provide fundamentally new information about crystallographic organization and defect content over many length scales.

Polychromatic single-crystal diffraction (pSXD) offers important advantages compared with monochromatic diffraction, such as ultrafast data collection and the ability to collect diffraction data without sample rotation. Despite the relevance of these advantages for applications in high-pressure experiments, pSXD has never been successfully applied for full structure determination at high pressure. Here the theory of polychromatic microdiffraction, strategies developed to adapt it to high-pressure applications, and results of pilot experiments are described. Special emphasis is placed on experiments with microcrystals and multigrain aggregates, and on the use of monochromator scans to extend the pressure range for single-crystal structure determination to the megabar region. Closely related applications to materials research problems are also discussed.

Variations in the local crystallographic orientation due to the presence of geometrically necessary dislocations and dislocation boundaries smear the distribution of intensity near Laue reflections. Here, some simple model distributions of geometrically necessary dislocations, GNDs, are used to estimate the dislocation tensor field from the intensity distribution of Laue peaks. Streaking of the Laue spots is found to be quantitatively and qualitatively distinct depending on the ratio between the absorption coefficient and the GND density gradient. In addition, different slip systems cause distinctly different Laue-pattern streaking. Experimental Laue patterns are therefore sensitive to stored dislocations and GNDs. As an example, white beam microdiffraction was applied to characterize the dislocation arrangement in a deformed polycrystalline Ni grain during in situ uniaxial tension.

Three-dimensional (3D) measurements of strain tensor distributions with subgrain resolution are now possible due to an emerging class of instrumentation: the 3D X-ray crystal microscopes. These instruments combine ultra-intense synchrotron X-ray sources and advanced X-ray optics to probe polycrystalline materials with submicron spatial resolution. By employing polychromatic X-ray beams and a virtual pinhole camera method, called differential aperture microscopy, 3D distributions of the local crystalline phase, orientation (local texture) and elastic and plastic strain tensor distributions can be measured with resolution below the grain size of most materials. The elastic strain tensor elements can typically be determined with uncertainties less than 100 ppm in brittle materials. Orientations can be quantified to ∼0.01° and the local unpaired dislocation density can be measured in ductile materials. Efforts are underway to improve the spatial resolution and increase the depth probed. Because the 3D X-ray crystal microscope is a penetrating nondestructive tool, it is ideal for studies of mesoscale structural evolution in materials. The unprecedented information that can be obtained with this new class of instrumentation is certain to advance our understanding of the materials physics underpinning the behavior of polycrystalline materials.

We describe X-ray and finite element analysis of dislocations, strain and crystallographic tilt in uncoalesced GaN layers grown by maskless pendeoepitaxy. The experimental results reveal that the GaN wings are tilted upward at room temperature. The distribution of strain and tilt depends on the width-to-height ratio of the GaN column. As the width-to-height ratio increases the wing tilt increases and changes more abruptly at the column/wing interface. Broadening of the GaN Laue reflections is more pronounced in the column region then in the wing region; this is in line with the inhomogeneity of both the strain and the defect density. A split in the Laue reflections from GaN films at the column/wing interface indicates the development of a low-angle tilt boundary for samples with a large width-to-height ratio of the column. Spacing between the dislocations in the tilt boundary at the column/wing interface for GaN grown with maskless pendeoepitaxy is an order of magnitude larger than the dislocation spacing observed for masked lateral epitaxial growth.