Showing papers on "Electron backscatter diffraction published in 2006"

Three-dimensional mapping of a deformation field inside a nanocrystal

Abstract: Synchrotron X-ray radiation, produced by electron accelerators at central facilities, can now be produced in extremely narrow coherent beams. When these X-rays illuminate a crystal of nanometre dimensions a diffraction pattern emerges that is highly resolved. This provides a powerful new tool for structural analysis, as the fine features of the diffraction pattern can be interpreted in terms of sub-atomic distortions within the crystal attributable to its contact with an external support. Coherent X-ray diffraction patterns derived from third-generation synchrotron radiation sources can lead to quantitative three-dimensional imaging of lattice strain on the nanometre scale. Coherent X-ray diffraction imaging is a rapidly advancing form of microscopy: diffraction patterns, measured using the latest third-generation synchrotron radiation sources, can be inverted to obtain full three-dimensional images of the interior density within nanocrystals1,2,3. Diffraction from an ideal crystal lattice results in an identical copy of this continuous diffraction pattern at every Bragg peak. This symmetry is broken by the presence of strain fields, which arise from the epitaxial contact forces that are inevitable whenever nanocrystals are prepared on a substrate4. When strain is present, the diffraction copies at different Bragg peaks are no longer identical and contain additional information, appearing as broken local inversion symmetry about each Bragg point. Here we show that one such pattern can nevertheless be inverted to obtain a ‘complex’ crystal density, whose phase encodes a projection of the lattice deformation. A lead nanocrystal was crystallized in ultrahigh vacuum from a droplet on a silica substrate and equilibrated close to its melting point. A three-dimensional image of the density, obtained by inversion of the coherent X-ray diffraction, shows the expected facetted morphology, but in addition reveals a real-space phase that is consistent with the three-dimensional evolution of a deformation field arising from interfacial contact forces. Quantitative three-dimensional imaging of lattice strain on the nanometre scale will have profound consequences for our fundamental understanding of grain interactions and defects in crystalline materials4. Our method of measuring and inverting diffraction patterns from nanocrystals represents a vital step towards the ultimate goal of atomic resolution single-molecule imaging that is a prominent justification for development of X-ray free-electron lasers5,6,7.

High resolution mapping of strains and rotations using electron backscatter diffraction

Abstract: The angular resolution of electron backscatter diffraction (EBSD) measurements can be significantly improved using an analysis based on determination of small shifts in features from one pattern to the next using cross-correlation functions. Using pattern shift measurements at many regions of the pattern, errors in the best fit strain and rotation tensors can be reduced. The authors show that elements of the strain tensor and small misorientations can be measured to ± 10−4 and ±0·006° for rotations. We apply the technique to two quite different materials systems. First, we determine the elastic strain distribution near the interface in a cross-sectioned SiGe epilayer, Si substrate semiconductor heterostructure. The plane stress boundary conditions at the sample surface are used to separate every term in the strain tensor. Second, the applicability to structural materials is illustrated by determining the lattice curvature caused by dislocations within the plastic zone associated with the wake and ...

Piezoelectric Properties of BaTiO3 Ceramics with High Performance Fabricated by Microwave Sintering

Abstract: Hydrothermally synthesized BaTiO3 powders with nanoscale-sized particles were densified by microwave sintering. A sintered sample of the nanopowder fabricated by hydrothermal synthesis has a high piezoelectric constant d33 due to fabrication by microwave sintering. The maximum value of the piezoelectric constant d33 of a specimen fabricated by microwave sintering was approximately 350 pC/N for a small grain size of 2.1 µm. Detailed microstructures of the samples were observed by transmission electron microscopy (TEM) and scanning electron microscopy/electron backscattered diffraction analysis/orientation imaging microscopy (SEM/EBSD/OIM). The size of ferroelectric domains in the samples showing superior piezoelectric properties was less than 50 nm. SEM/EBSD/OIM observations revealed that the fraction of random boundaries was higher by approximately 10% in microwave sintered samples than in conventionally sintered ones. It is suggested that the small size of domain and the higher fraction of random boundaries might be responsible for the excellent piezoelectric properties of small grains, which can partially be attributed to domain size.

Observations on the faceted initiation site in the dwell-fatigue tested Ti-6242 alloy : Crystallographic orientation and size effects

Abstract: A novel use of the electron backscattered diffraction (EBSD) characterization technique for study of fracture has been demonstrated. This new approach has been employed for characterization and analysis that contribute to the understanding of crack initiation in Ti-6242 under dwell-fatigue loading conditions. A faceted crack initiation site is typically observed on the dwell-fatigue fracture surface of Ti-6242. The level of microtexture has a major influence on the dwell-fatigue failures in near-α titanium alloys, such as Ti-6242. In this study, serial sectioning and EBSD techniques were used to obtain the orientation images of almost the entire specimen cross section at different depths below the fracture surface. The orientation images are color coded on three different bases: the angle between the loading axis and basal plane normal, the Schmid factor for prism slip, and the Schmid factor for basal slip. The aim was to determine the important aspects of the crystallographic orientation and the size of the microtextured region that is associated with the faceted initiation site. The results of this study are used to explain the possible locations of crack initiation in a test specimen under dwell-fatigue loading condition. These results are also used to better understand the role of size of microtextured regions in determining which crack will outgrow the other cracks (for the case of multiple cracking typically observed in the alloy of current study under the dwell-fatigue loading conditions) to become the dominant crack that leads to eventual specimen failure. This understanding has important practical implications because the dominant crack effectively determines the specimen life.

Crystal Structures of Thermoelectric n- and p-type Ba8Ga16Ge30 Studied by Single Crystal, Multitemperature, Neutron Diffraction, Conventional X-ray Diffraction and Resonant Synchrotron X-ray Diffraction

Abstract: Comprehensive single-crystal structural investigations of n- and p-type Ba8Ga16Ge30 have been carried out using multitemperature neutron and conventional X-ray diffraction as well as resonant synchrotron X-ray diffraction. The data show that the guest atom positions and dynamics are very similar in the two structures, although the barium atoms are slightly more displaced from the cage centers in the p-type structure than in the n-type structure (Deltad = 0.025 A). For both structures Fourier difference maps calculated from very high-resolution neutron diffraction data (sin theta/lambda > 2 A-1) show that the Ba nuclear density at lowest temperatures (15 K) is distributed in a torus around the crystallographic 6d site with maxima in the 24j positions. At room temperature the maxima have shifted to the 24k position. Analysis of atomic displacement parameters give Einstein temperatures of approximately 60(1) K for both structures. Thus, the fundamental difference in the low temperature thermal conductivity observed for p- and n-type Ba8Ga16Ge30 appear not to be directly related to the guest atom behavior as is commonly assumed in thermoelectric research. The neutron data and the resonant synchrotron X-ray data facilitate refinement of Ga/Ge framework occupancies. The Ga atoms have a clear preference for the 6c site with the preference being somewhat stronger for the n-type structure.

Direct synthesis and structure characterization of ultrafine CeO2 nanoparticles

Abstract: A new method to directly synthesize single-crystalline CeO2 nanoparticles has been developed. The advantages of the method are rapid synthesis, at normal atmosphere, 100% productive ratio and low cost, with a great potential for scale-up. X-ray diffraction (XRD) spectra showed unusual peak width versus particle size, compared with Scherrer equation predictions. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), electron diffraction and ultraviolet (UV) absorption were used to examine the particle size and microstructure to find out the cause. As a result, ultrafine particles with a size less than 6 nm were found to be self-assembled into a 'coherent interface', so that a large group of particles behave like a large single particle in XRD.

Fatigue strength, microstructural stability and strain localization in ultrafine-grained copper

Abstract: Fatigue lifetime under stress control of ultrafine-grained Cu of 99.9% purity prepared by equal channel angular pressing is shown to exceed that of conventionally grained cold worked counterparts by a factor of 1.7 in the low-, high- and very-high-cycle region. The electron back scattering diffraction technique did not reveal changes of bulk microstructure due to cyclic loading. Minor changes of dislocation microstructure were detected by transmission electron microscopy. Qualitative change from moderate cyclic hardening to cyclic softening was observed with increasing stress amplitude. Comparison of S–N data with those available in literature shows substantially higher lifetime of the material studied in this work in the high- and very-high-cycle region. This effect is attributed to the high stability of the grain structure and lower purity of the examined ultrafine-grained copper.

Metal precipitation at grain boundaries in silicon: Dependence on grain boundary character and dislocation decoration

Abstract: Synchrotron-based analytical microprobe techniques, electron backscatter diffraction, and defect etching are combined to determine the dependence of metal silicide precipitate formation on grain boundary character and microstructure in multicrystalline silicon (mc-Si). Metal silicide precipitate decoration is observed to increase with decreasing atomic coincidence within the grain boundary plane (increasing Σ values). A few low-Σ boundaries contain anomalously high metal precipitate concentrations, concomitant with heavy dislocation decoration. These results provide direct experimental evidence that the degree of interaction between metals and structural defects in mc-Si can vary as a function of microstructure, with implications for mc-Si device performance and processing.

Application of EBSD to analysis of microstructures in commercial steels

Abstract: Electron backscatter diffraction (EBSD) is a very powerful technique for microstructural characterisation and analysis of crystalline materials. Many of the structural parameters that control the properties and performance of the material can be derived from EBSD data, e.g. grain size, phase constituents, mechanical anisotropy and residual strain. This should make EBSD a valuable tool to control and develop microstructures of commercial metallic materials but technique has mainly been used in basic research at universities and not to the same extent for research and development in industry. The development in scanning electron microscopes and EBSD equipment in recent years makes it possible to measure 'difficult' structures, e.g. with higher dislocation content, but can it manage the complexity of the structures of commercial materials and achieve reliable data? During 2004 an EBSD round robin test on industrial metallic materials was coordinated by KIMAB to test the current status of the techniqu...

Crystallography of fracture facets in a near-alpha titanium alloy

Abstract: A faceted initiation site is observed in Ti-6242 alloy for both the cyclic and static-loading test conditions. In this experimental study, the crystallographic orientation of the facets has been determined using the electron backscattered diffraction (EBSD) technique in conjunction with the quantitative tilt fractography in a scanning electron microscope (SEM). Quantitative tilt fractography analysis has been used to determine the spatial orientation of fracture facets. The results indicate that the normal-fatigue (no-dwell) fracture facets are oriented at ∼5 deg with respect to the basal plane; the dwell-fatigue fracture facets are oriented at ∼10 to 15 deg with respect to the basal plane and the static-loading fracture facets are oriented at ∼20 deg with respect to the basal plane. These crystallographic orientation determinations of the fracture facets at the crack-initiation site can be used to obtain an idea about the type of loading that produced them.

Crystal-plastic deformation of zircon: A defect in the assumption of chemical robustness

Abstract: Orientation contrast imaging and quantitative electron backscatter diffraction analysis of a zircon collected from an Indian Ocean gabbro reveal intragrain crystallographic misorientations (up to 14°) and low-angle orientation boundaries concentrated in the zircon tips. These features represent the formation and migration of dislocations and provide the first evidence of crystal-plastic deformation of zircon under crustal conditions. Panchromatic and wavelength cathodoluminescence (CL), combined with quantitative rare earth element (REE) ion microprobe analyses, demonstrate modification of zircon REE chemistry within the areas of crystal plasticity. These data indicate that the enhanced diffusion of REEs into the zircon is spatially linked to the presence of dislocations that behave as high-diffusivity pathways, increasing bulk diffusion rates and effective diffusion distances in the zircon by several orders of magnitude. In addition, discrete ∼2 μm zones of reduced panchromatic CL correspond exactly to the position of low-angle orientation boundaries and demonstrate a defect dependence on CL signal at high dislocation densities. The presence of deformation-related crystal-plastic microstructures in zircon, and their role in modifying elemental diffusion, questions the commonly made assumption of chemical robustness and has fundamental implications for the interpretation of zircon trace-element and isotopic data.

Misorientation mapping for visualization of plastic deformation via electron back-scattered diffraction.

Abstract: The ability to map plastic deformation around high strain gradient microstructural features is central in studying phenomena such as fatigue and stress corrosion cracking. A method for the visualization of plastic deformation in electron back-scattered diffraction (EBSD) data has been developed and is described in this article. This technique is based on mapping the intragrain misorientation in polycrystalline metals. The algorithm maps the scalar misorientation between a local minimum misorientation reference pixel and every other pixel within an individual grain. A map around the corner of a Vickers indentation in 304 stainless steel was used as a test case. Several algorithms for EBSD mapping were then applied to the deformation distributions around air fatigue and stress corrosion cracks in 304 stainless steel. Using this technique, clear visualization of a deformation zone around high strain gradient microstructural features (crack tips, indentations, etc.) is possible with standard EBSD data.

Influence of annealing on the microstructure of commercial Mg alloy AZ31 after mechanical forming

Abstract: The microstructure of commercial rolled magnesium alloy AZ31B (nominal composition Mg–3Al–0.9Zn–0.15Mn in wt.%) was investigated with the help of light microscopy, electron backscatter diffraction (EBSD) and X-ray diffraction technique after annealing in the temperature range from room temperature (RT) to 400 °C. Tensile tests at RT were performed to show the influence of the microstructure on mechanical properties. Static recrystallization (SRX) was observed during annealing of as-received alloy at and above 150 °C. Twins play an important role during SRX and serve as nucleation sites and preferred paths for growth of grains. The strong basal texture caused by rolling was weakened by SRX. Significant differences in the stress strain curves were observed for as-received and annealed specimens.

Two- and three-dimensional microstructural characterization of a super-austenitic stainless steel

Abstract: The microstructure of AL-6XN, a commercial super-austenitic stainless steel, was investigated using novel two- and three-dimensional (3D) analysis and characterization techniques. The austenite matrix, the second-phase σ precipitates, and the relationships between them have been analyzed, with particular emphasis on the true 3D microstructure of the material, including grain boundary character, grain morphologies and connectivity. A combination of serial sectioning with electron backscatter diffraction (EBSD) analysis allowed for reconstruction of individual grains, and definition of all five degrees of freedom of the grain boundary planes. Second-phase σ particle size and morphology, crystallography, composition, potential formation mechanisms, orientation relationships, and coherency with the matrix have been analyzed. These results provide boundary conditions for atomistic calculations of specific grain boundary structures as well as the basis for mesoscale image-based models of mechanical behavior of the microstructures.

Time-resolved diffraction profiles and atomic dynamics in short-pulse laser-induced structural transformations: Molecular dynamics study

Abstract: The diffraction profiles and density correlation functions are calculated for transient atomic configurations generated in molecular dynamics simulations of a $20\phantom{\rule{03em}{0ex}}\mathrm{nm}$ Au film irradiated with $200\phantom{\rule{03em}{0ex}}\mathrm{fs}$ laser pulses of different intensity The results of the calculations provide an opportunity to directly relate the detailed information on the atomic-level structural rearrangements available from the simulations to the diffraction spectra measured in time-resolved x-ray and electron diffraction experiments Three processes are found to be responsible for the evolution of the diffraction profiles During the first several picoseconds after the laser excitation, the decrease of the intensity of the diffraction peaks is largely due to the increasing amplitude of thermal atomic vibrations and can be well described by the Debye-Waller factor The effect of thermoelastic deformation of the film prior to melting is reflected in shifts and splittings of the diffraction peaks, providing an opportunity for experimental probing of the ultrafast deformations Finally, the onset of the melting process results in complete disappearance of the crystalline diffraction peaks The homogeneous nucleation of a large number of liquid regions throughout the film is found to be more effective in reducing long-range correlations in atomic positions and diminishing the diffraction peaks as compared to the heterogeneous melting by melting front propagation For the same fraction of atoms retaining the local crystalline environment, the diffraction peaks are more pronounced in heterogeneous melting A detailed analysis of the real space correlations in atomic positions is also performed and the atomic-level picture behind the experimentally observed fast disappearance of the correlation peak corresponding to the second nearest neighbors in the fcc lattice during the laser heating and melting processes is revealed

Dynamic recrystallization of NI-base alloys-Experimental results and comparisons with simulations

Abstract: The dynamic recrystallization of the Ni-base alloy Bohler L306 VMR (Alloy 80A) in a transient state was investigated both by light microscopy and electron backscatter diffraction (EBSD), and the experimental results were compared with those from simulations. Subgrain structures of the size of the recrystallized grains were observed close to the grain boundaries of the original grains. With increasing strain a texture developed in the deformed fraction. Strong twinning was found in the recrystallized fraction, with area fractions of the twinned grains of around 80% for higher strains. Thus the measured grain sizes strongly depend on the handling of the twins. A pronounced increase in the average grain size of the recrystallized fraction with increasing strain (time) was only observed after twin removal. There was generally good agreement between the measured and the simulated results.