Showing papers on "Electron backscatter diffraction published in 2005"

Electron diffraction of tilted perovskites

Abstract: Simulations of electron diffraction patterns for each of the known perovskite tilt systems have been performed. The conditions for the appearance of superlattice reflections arising from rotations of the octahedra are modified to take into account the effects of different tilt systems for kinematical diffraction. The use of selected-area electron diffraction as a tool for perovskite structure determination is reviewed and examples are included.

Image Quality Analysis: A New Method of Characterizing Microstructures

Abstract: Polycrystalline aggregates are comprised of three microstructural features: grain centers, grain boundaries, and regions affected by grain boundaries. It is these features that determine the mechanical properties, and any advanced understanding of microstructure-property relations requires their quantitative description. Traditionally, descriptions of microstructures have been based on visualization, i.e., how grains appear in the optical or scanning electron microscope (SEM). While this may lead to classification systems that permit differentiation, it does not allow for quantification, especially in complex microstructures, and does not lend itself to either developing or applying structure-property relationships. The goal of this paper is to present a new approach to the characterization of complex microstructures, especially those found in advanced modern high strength steels. For such steels, the new approach employs the fact that different types of ferrite formed at different transformation temperatures have different dislocation or sub-grain boundary densities. Hence, measuring the degree of lattice imperfection of the grain centers of the ferrite is one way of first identifying, then grouping, and finally quantifying, the different types or forms of ferrite. The index chosen in this study to distinguish the degree of lattice imperfection is the image quality (IQ). Finally, as part of the new approach a procedure has been developed to improve the accuracy of applying IQ measurements.

Deformation mechanisms responsible for the high ductility in a Mg AZ31 alloy analyzed by electron backscattered diffraction

Abstract: The microstructural evolution during tensile deformation of an AZ31 alloy with grain size ranging from 17 to 40 µm, at intermediate temperatures, has been studied using electron backscattered diffraction (EBSD) and optical microscopy (OM) as the main characterization tools. Two deformation regimes could be distinguished. In the high-strain-rate regime, the stress exponent was found to be about 6, and the activation energy is close to that for Mg self-diffusion. These values are indicative of climb-controlled creep. In the lower strain rate range, elongations higher than 300 pct were measured. In this range, significant dynamic grain growth takes place during the test, and thus, the predominant deformation mechanisms have been investigated by means of strain-rate-change tests. It was found that the stress exponent varied during the test between 1.7 and 2.5, while the activation energy remains close to that for grain-boundary diffusion. The EBSD analysis revealed, additionally, the appearance of low to moderately misoriented boundaries that tend to lay perpendicular to the tensile axis. The enhanced ductility of this AZ31 alloy in this regime is attributed to the operation of a sequence of deformation mechanisms. Initially, grain-boundary sliding governs deformation; once dynamic grain growth occurs, dislocation slip becomes gradually more important. Dislocation interaction gives rise to the appearance of new interfaces by continuous dynamic recrystallization (CDRX).

Electron backscattered diffraction study of ε/α' martensitic variants induced by plastic deformation in 304 stainless steel

Abstract: The electron backscattered diffraction (EBSD) technique has been used to assess crystallographic features of the residual γ phase and the strain-induced e/α′ martensites in a 304 stainless steel, tensile tested to 10 pct strain at T=−60 °C. The martensitic transformation rate varies according to the γ-grain orientation against the applied stress and the γ-grain size. The α′-transformation textures as well as the γ-misorientation spreads observed in specific γ-grain orientations have been analyzed. Large misorientation spreads are observed in the less-transformed γ grains. This reveals an important crystallographic slip activity, even if less strain-induced martensite has been formed. A strong γ → α′ variant selection was detected in the cube- and Goss-oriented γ grains for which the transformation is less developed. For the {110} 〈1–11〉 and copper-oriented γ grains, the amount of α′ martensite is significantly higher and the γ → α′ variant selection is less pronounced. This variant selection is then analyzed on at a local scale and is related to the presence of {111} γ localized deformation bands on which further e/α′ martensites have nucleated.

Fabrication of copper oxide nanofluid using submerged arc nanoparticle synthesis system (SANSS)

Abstract: The optimal parameters are found for preparing nanofluid in our submerged arc nanoparticle synthesis system (SANSS) using a copper electrode. A suspended copper oxide nanofluid is thus produced at the current of 8.5–10 A, voltage of 220 V, pulse duration of 12 μs, and dielectric liquid temperature of 2°C. The CuO nanoparticle are characterized by transmission electron microscopy (TEM), field emission scanning electron microscope (FESEM), X-ray diffraction (XRD), electron diffraction pattern (SAD) and electron spectroscopy for chemical analysis (ESCA). The equality volume spherical diameter of the obtained copper oxide particle is 49.1 nm, regular shape and narrow size distribution.

Study by EBSD of the development of the substructure in a hot deformed 304 stainless steel

Abstract: The EBSD technique is a useful tool to characterize deformation microstructures. In this study a 304 stainless steel was hot deformed to different strains in torsion mode. The application of the above-mentioned technique focused on the characterization of the high angle boundary distribution evolution under strain. Special emphasis was given to the Σ3 boundaries. It was observed that as the deformation increased the twin character was progressively lost and at the same time, some of the twins appear to be curved in the microstructure. The orientation analysis carried out inside the grains led to a classification of the grains according to the way that in-grain orientation spreads develop.

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.

Estimation of dislocation densities in cold rolled Al-Mg-Cu-Mn alloys by combination of yield strength data, EBSD and strength models

Abstract: Al-Mg-Cu-Mn alloys have been developed for the packaging industry, in which large cold-working deformations are normally applied that can produce high dislocation densities. In this study, we present a simplified model for the yield strength contributions and apply that to obtain the dislocation densities by determining the orientation factors, which can be obtained via the crystal information of electron backscatter diffraction (EBSD). One alloy subjected to three cold-rolling reductions (10%, 40% and 90%) has been analysed by EBSD, and the density of dislocations are estimated using the strengthening model. This assessment suggests that dislocation densities by the Taylor model are roughly consistent but slightly lower than those determined by transmission electron microscopy.

Microstructural evolution in copper processed by severe plastic deformation

Abstract: The mechanisms of microstructural evolution in copper subjected to equal channel angular pressing (ECAP) have been investigated after successive passes. The first few passes are the most efficient in grain refinement while the microstructure becomes gradually more equiaxed as the number of passes increases. The texture evolution is discussed based on electron back scattered diffraction (EBSD) results. These experimental results are interpreted in terms of a preliminary model with four successive stages: homogeneous dislocation distribution; elongated sub-cell formation; elongated subgrain formation; break-up of subgrains into equiaxed units; sharpening of grain boundaries and final equiaxed ultrafine structure.

Determination of a Mean Orientation in Electron Backscatter Diffraction Measurements

Abstract: The average orientation of an electron backscatter diffraction (EBSD) map is calculated by the quaternion method and is compared with nonlinear solving by the Hill Climbing and Barton-Dawson methods. An automated EBSD system acquires orientations on a regular grid of pixels based on indexation of Kikuchi patterns and the orientation is described by one of the crystal symmetry-related equivalents. In order to calculate the quaternion average, it is necessary to make a cloud for a set of pixels in a grain. A cloud consists of the representative orientations with small misorientation between each and every pair of points. The position criterion says that two adjacent pixels have a smaller misorientation than with all others. With this, the proper equivalent orientation, or representative orientation for the cloud, can be selected from among all the crystal symmetry-related equivalents. The orientation average is the quaternion summation divided by its norm. The instant average or cumulative average is useful for dealing with polycrystalline grains or orientation discontinuity and is also useful for selection of the proper orientation of EBSD map with large scattering. The quaternion, Hill Climbing, and Barton-Dawson nonlinear methods are tested with a Gaussian distribution around the ideal texture component, Brass {110}〈112〉. The accuracy of the three results is similar but the nonlinear methods are associated with longer computation times than the quaternion method. The quaternion method is adapted for characterization of a partially-recrystallized interstitial-free (IF) steel and randomly distributed Brass, S, and cube texture components according to several different orientation spreads.

Comparison of the microstructure and thermal stability of an AZ31 alloy processed by ECAP and large strain hot rolling

Abstract: The aim of this work is to compare the microstructure, the texture, as well as the thermal stability of an AZ31 Mg alloy processed via two different severe plastic deformation processing techniques, namely large strain hot rolling (LSHR) and equal channel angular pressing (ECAP). The microstructure was characterized by optical microscopy and the texture was measured both by X-ray diffraction and electron backscatter diffraction (EBSD). The microstructure obtained via LSHR has average grain sizes around 3 μm, but it is quite heterogeneous. Additionally, a well-defined basal texture develops. ECAP gives rise to a more homogeneous and slightly coarser microstructure, with an average grain size of 7 μm and a shear type texture. The higher resistance of the extruded sample to secondary recrystallization after severe post-deformation annealing is attributed to a texture effect.

The reduction of planar anisotropy by texture modification through asymmetric rolling and annealing in AA5754

Abstract: Asymmetric rolling (ASR) was applied to aluminum alloy AA5754 under various processing conditions. The deformation texture and the texture after subsequent recrystallization were studied by X-ray diffraction and electron back scattering diffraction (EBSD) in a scanning electron microscope (SEM). After unidirectional ASR, the deformation texture was rotated by around 10° about the transverse direction from the f.c.c. plane strain compression texture (β-fibre texture), whereas it was close to the ideal β-fibre texture after reverse-ASR. The recrystallization texture was randomised with a reduced cube {0 0 1}〈1 0 0〉 component, and the randomising effect is more prominent under high velocity ratio between the upper and lower rolls and reverse rolling conditions. Based on crystal plasticity theory calculations and measurements of cup earing profiles, the sheet planar anisotropy is predicted to decrease after ASR and annealing, but the mean r-value is not enhanced.

The characterization of low-angle boundaries by EBSD.

Abstract: A method of accurately measuring misorientations by electron backscatter diffraction (EBSD), which is an extension of that proposed by Wilkinson and based on the comparison of diffraction patterns, is described. The method has been applied to linescans, and found to improve the angular resolution by a factor of more than 30. The consequent improvement in determining misorientation axes is also analysed. Small changes of orientation very close to some low-angle boundaries were investigated and found to be artefacts of the analysis. Measurements of the area from which diffraction patterns are generated show this to be much larger than the effective spatial resolution of EBSD, and it is concluded that this may be a limiting factor in the use of EBSD for microstructural characterization.

Application of the EBSD technique for the characterisation of deformation zones in metal cutting

Abstract: The Electron Back Scattering Diffraction (EBSD) technique was applied for mapping the extent of plastic deformation zones of single and two-phase steels subjected to orthogonal cutting using the quick-stop method. The analysis of the EBSD patterns obtained indicates that the method appears to be applicable for investigating deformation in a chip root and is well suited for the analysis of the deformation zones ahead of the tool tip in the vicinity of the primary deformation zone Z 1 . Though the EBSD patterns (EBSPs) could not be resolved clearly in the chip in the case of materials displaying continuous chips (Remko iron, carbon steels), the analysis was more successful for materials exhibiting shear-localised chips such as austenitic steel, due to the presence of alternating low and high deformation zones. The presence of slip lines within the deformed grains in the austenitic chip material was clearly noticeable and also evidence of twinning deformation was found. The EBSD analysis of the machined surfaces indicated that the deepest heavily deformed area was found for the Remko iron material and the smallest for the austenitic variant. These results were also confirmed by microhardness profiling in the machined surfaces.

In-situ observations of Widmanstätten ferrite formation in a low carbon steel

Abstract: An in situ laser-scanning confocal microscopy study has been undertaken into Widmanstatten ferrite formation in an Fe–C alloy, in combination with electron backscattered diffraction It has been found for the first time that the sympathetic nucleation of Widmanstatten ferrite on grain boundary allotriomorphs can exhibit a step wise change in orientation and growth direction until the most favourable growth conditions are achieved

Structure of framboidal pyrite: An electron backscatter diffraction study

Abstract: The detailed crystallography of natural pyrite framboids has been determined for the first time using electron backscatter diffraction techniques. The crystallographic ordering of microcrystals correlates positively with morphological ordering; the crystallographic orientations are random in morphologically disordered framboids and are almost ordered in morphologically ordered framboids. Morphologically ordered framboids involve two types of systematic misorientations across the microcrystal boundaries: low-angle (ca. . Thus, the crystallographic orientation of microcrystals is not uniform, even in highly ordered framboids. This suggests that the self-organization of microcrystals in pyrite framboids is not crystallographically controlled, for example by sequential replication of existing microcrystals, since this would not result in high lattice misorientation angles between adjacent microcrystals. Presumably, the self-organization process is a consequence of the aggregation of multiple equidimensional and equimorphic microcrystals that have nucleated in a fixed volume. We suggest that the regular arrangement of microcrystals occurs by the physical rotation (reorientation) of individual microcrystals, driven by the reduction in surface free energy between neighbors.

Diffraction by DNA, carbon nanotubes and other helical nanostructures

Abstract: This review discusses the diffraction patterns of x-rays or electrons scattered by fibres of helical biological molecules and by carbon nanotubes (CNTs) from the unified point of view of the Fourier–Bessel transform of an atomic helix This paper is intended for scientists who are not professional crystallographers X-ray fibre diffraction patterns of Pauling's protein α-helix and of Crick and Pauling's protein coiled-coil are revisited This is followed by a non-technical comparison between the historic x-ray diffraction patterns of the A and B conformations of DNA, which were crucial for the discovery of the double helix The qualitative analysis of the diffraction images is supported by novel optical simulation experiments designed to pinpoint the gross structural informational content of the patterns The spectacular helical structure of the tobacco mosaic virus determined by Rosalind Franklin and co-workers will then be described as an early example of the great power of x-ray crystallography in determining the structure of a large biomolecular edifice After these mostly historical and didactic case studies, this paper will consider electron diffraction and transmission electron microscopy of CNTs of great current interest, focusing particularly on recent data obtained for single-wall, double-wall and scrolled nanotubes Several points of convergence between the interpretations of the diffraction patterns of biological helices and CNTs will be emphasized