Showing papers on "Electron backscatter diffraction published in 2001"

Review Grain and subgrain characterisation by electron backscatter diffraction

Abstract: The application of automated Electron Backscatter Diffraction (EBSD) in the scanning electron microscope, to the quantitative analysis of grain and subgrain structures is discussed and compared with conventional methods of quantitative metallography. It is shown that the technique has reached a state of maturity such that linescans and maps can routinely be obtained and analysed using commercially available equipment and that EBSD in a Field Emission SEM (FEGSEM) allows quantitative analysis of grain/subgrains as small as ∼0.2 μm. EBSD can often give more accurate measurements of grain and subgrain size than conventional imaging methods, often in comparable times. Subgrain/cell measurements may be made more easily than in the TEM although the limited angular resolution of EBSD may be problematic in some cases. Additional information available from EBSD and not from conventional microscopy, gives a new dimension to quantitative metallography. Texture and its correlation with grain or subgrain size, shape and position are readily measured. Boundary misorientations, which are readily obtainable from EBSD, enable the distribution of boundary types to be determined and CSL boundaries can be identified and measured. The spatial distribution of Stored Energy in a sample and the amount of Recrystallization may also be measured by EBSD methods.

Transmission electron microscopy and diffractometry of materials

Abstract: Diffraction Geometry and the X-Ray Powder Diffractometer.- The TEM and Its Optics.- Scattering.- Inelastic Electron Scattering and Spectroscopy.- Diffraction from Crystals.- Electron Diffraction and Crystallography.- Diffraction Contrast in TEM Images.- Diffraction Lineshapes.- Patterson Functions and Diffuse Scattering.- High Resolution TEM Imaging.- Dynamical Theory.- Appendix.

A process model for friction stir welding of age hardening aluminum alloys

Abstract: In the present investigation, a numerical three-dimensional (3-D) heat flow model for friction stir welding (FSW) has been developed, based on the method of finite differences. The algorithm, which is implemented in MATLAB 5.2, is provided with a separate module for calculation of the microstructure evolution and the resulting hardness distribution. The process model is validated by comparison with in-situ thermocouple measurements and experimental hardness profiles measured at specific time intervals after welding to unravel the strength recovery during natural aging. Furthermore, the grain structure within the plastically deformed region of the as-welded materials has been characterized by means of the electron backscattered diffraction (EBSD) technique in the scanning electron microscope (SEM). Some practical applications of the process model are described toward the end of the article.

Experimental study of porosity and its relation to fatigue mechanisms of model Al–Si7–Mg0.3 cast Al alloys

Abstract: The microstructure and fatigue properties of three model AS7G03 cast aluminium alloys containing artificial pores have been studied. Synchrotron X-ray tomography has been used to characterise in three dimensions the pore population in the alloys. The development of fatigue cracks in relation with local crystallography has been studied by means of electron back scattered diffraction (EBSD). Both the average number of cycles to failure and the lifetime scatter depend on the pore content specially at high stress level. The mechanism leading to the initiation of a crack from a pore has been identified. The crack propagation at high stress level appears to be quite insensitive to microstructural barriers and can be reasonably well described by a Paris type law. At low stresses, however, short cracks are often observed to be stopped at grain boundaries and the fatigue life is no longer predicted by a simple propagation law.

Orientation averaging of electron backscattered diffraction data

Abstract: The use of data averaging to improve the angular precision of electron backscattered diffraction (EBSD) maps is discussed. It is shown that orientations may be conveniently and rapidly averaged using the four Euler-symmetric parameters which are coefficients of a quaternion representation. The processing of EBSD data requires the use of an edge preserving filter and a modified Kuwahara filter has been successfully implemented and tested. Three passes of such a filter have been shown to reduce orientation noise by a factor of approximately 10. Application of the method to deformed and recovered aluminium alloys has shown that such data processing enables small subgrain misorientation (< 0.5 degrees ) to be detected reliably.

Correlation of the M23C6 precipitation morphology with grain boundary characteristics in austenitic stainless steel

Abstract: The relationship between grain boundary characteristics and the formation of grain boundary carbides in AISI 304 stainless steel have been investigated by using the electron backscattered diffraction (EBSD) technique. It was observed that an increase in the misorientation between two adjacent grains resulted in a change in the carbide morphology from a plate-like to an acute triangular form, where carbides preferentially maintained coherency with the grain for which the {111} planes made the smallest angle with the grain boundary plane. The carbides grew into the other grain at a later stage, having the lowest interfacial energy, which subsequently resulted in the triangular carbide morphology. After low cycle fatigue with a hold time at tensile peak strains, it was observed that cavity formation was more pronounced at random boundaries than at coincidence site lattice (CSL) boundaries. This result provides a good explanation that acute triangular carbides, which predominantly precipitate at random boundaries, are more likely to lead to cavity nucleation than the plate-like carbides precipitate at CSL boundaries.

In Situ X-ray Diffraction Measurements of the Self-Preservation Effect of CH4 Hydrate

Abstract: In situ observations of CH4 hydrate dissociation using X-ray diffraction were carried out at atmospheric pressure and at both 168 and 189 K. Dissociation rates of the hydrate and the rate of transformation into hexagonal ice were measured using time-resolved energy-dispersive X-ray diffraction. The dissociation of CH4 hydrate had an initially fast regime followed by slower dissociation. Thus, the data support a previously suggested two-step process. In addition, we observed dynamic behavior of the X-ray diffraction intensities of ice Ih, which implies a transient crystal structure at the beginning of the dissociation. Our analyses indicates that the first step, which lasted several tens of minutes, was the formation of an ice Ih layer around the CH4 hydrate, and the second step was relatively slow because the CH4 had to diffuse through the thickening ice layer. This second step determined the hydrate lifetime. The resulting diffusion coefficients were estimated at 2.2 × 10-11 m2/s at 189 K and 9.6 × 10-12...

Residual‐Stress Predictions in Polycrystalline Alumina

Abstract: Microstructure-level residual stresses arise in polycrystalline ceramics during processing as a result of thermal expansion anisotropy and crystallographic disorientation across the grain boundaries. Depending upon the grain size, the magnitude of these stresses can be sufficiently high to cause spontaneous microcracking during the processing of these materials. They are also likely to affect where cracks initiate and propagate under macroscopic loading. The magnitudes of residual stresses in untextured and textured alumina samples were predicted using object oriented finite (OOF) element analysis and experimentally determined grain orientations. The crystallographic orientations were obtained by electron-backscattered diffraction (EBSD). The residual stresses were lower and the stress distributions were narrower in the textured samples compared to those in the untextured samples. Crack initiation and propagation were also simulated using the Griffith fracture criterion. The grain boundary to surface energy ratios required for computations were estimated using AFM groove measurements.

Formation of ultra-fine ferrite in hot rolled strip: potential mechanisms for grain refinement

Abstract: A novel single-pass hot strip rolling process has been developed in which ultra-fine (<2 μm) ferrite grains form at the surface of hot rolled strip in two low carbon steels with average austenite grain sizes above 200 μm Two experiments were performed on strip that had been re-heated to 1250°C for 300 s and air-cooled to the rolling temperatures The first involved hot rolling a sample of 009 wt%C–168Mn–022Si–027Mo steel (steel A) at 800°C, which was just above the Ar3 of this sample, while the second involved hot rolling a sample of 011C–168Mn–022Si steel (steel B) at 675°C, which is just below the Ar3 temperature of the sample After air cooling, the surface regions of strip of both steel A and B consisted of ultra-fine ferrite grains which had formed within the large austenite grains, while the central regions consisted of a bainitic microstructure In the case of steel B, a network of allotriomorphic ferrite delineated the prior-austenite grain boundaries throughout the strip cross-section Based on results from optical microscopy and scanning/transmission electron microscopy, as well as bulk X-ray texture analysis and microtextural analysis using Electron Back-Scattered Diffraction (EBSD), it is shown that the ultra-fine ferrite most likely forms by a process of rapid intragranular nucleation during, or immediately after, deformation This process of inducing intragranular nucleation of ferrite by deformation is referred to as strain-induced transformation

A combinatorial approach for efficient mapping of phase diagrams and properties

Abstract: A methodology is developed which extends the combinatorial approaches to structural materials research and development. This high-efficiency methodology employs diffusion couples and “diffusion multiples” to create large variations (libraries) of compositions in bulk samples for fast and systematic surveys of bulk properties. These composition libraries coupled with microanalytical techniques such as electron probe microanalysis, electron backscatter diffraction analysis, and nanoindentation tests can be used for efficient surveys of phases, equilibria, diffusion coefficients, precipitation kinetics, properties, and composition–phase–property relations (such as solution hardening and strengthening effect) for accelerated design of multicomponent alloys.

Electron Diffraction Using Transmission Electron Microscopy.

Abstract: Electron diffraction via the transmission electron microscope is a powerful method for characterizing the structure of materials, including perfect crystals and defect structures. The advantages of electron diffraction over other methods, e.g., x-ray or neutron, arise from the extremely short wavelength (≈2 pm), the strong atomic scattering, and the ability to examine tiny volumes of matter (≈10 nm(3)). The NIST Materials Science and Engineering Laboratory has a history of discovery and characterization of new structures through electron diffraction, alone or in combination with other diffraction methods. This paper provides a survey of some of this work enabled through electron microscopy.

Quantitative Analysis of the Electrostatic Potential in Rock-Salt Crystals Using Accurate Electron Diffraction Data

Abstract: Very accurate electron structure factors measured by a significantly improved transmission electron diffraction technique for polycrystalline samples were used in a high-resolution quantitative study of the electrostatic potentials in LiF, NaF, and MgO crystals. The spatial electrostatic potential distribution was obtained using an analytical structural -model adapted for electron diffraction. A topological analysis of the electrostatic potential, defining the features of the electrostatic field and the Coulomb force field in a crystal was developed. In addition to the topological analysis of the electron density, this approach provides a more complete description of the atomic interactions. The application of this approach to the characterization of bonding in a crystal has been demonstrated. The suitability of electron diffraction for determination of the core-electron binding energy is discussed.

Microstructure and damage initiation in duplex stainless steels

Abstract: The damage nucleation of a duplex stainless steel is investigated. Electron Back Scatter Diffraction (EBSD) technique is used to correlate local phase morphology with crystallographic properties. In situ tensile tests are performed to characterize strain fields and to monitor sites of damage nucleation. These observations are correlated with crystallographic orientations and finite element calculations.

A methodology for determining average lattice orientation and its application to the characterization of grain substructure

Abstract: Lattice orientation data often are available at a collection of locations within a grain, and an average of these lattice orientations is useful in data analysis. We define an average orientation based on misorientation angles, which leads to a nonlinear least-squares problem that may be solved numerically. This average orientation then serves as the basis for several related quantities that characterize the misorientation structure within a grain. The spread of a grain in (mis)orientation space is determined, as is a spatial correlation of the misorientation parameters. The method is applied to experimental data from electron backscattered diffraction (EBSD) scans and to simulation data from finite-element analyses of the deformation of a polycrystal. The quantities characterizing the grain substructure are found to evolve with strain and to illuminate trends in the microstructural evolution.

Combined application of electron backscatter diffraction and stereo-photogrammetry in fractography studies

Abstract: The main aim of this paper is to report on recent experimental developments that have succeeded in combining electron back-scatter diffraction (EBSD) with stereo-photogrammetry, compared with two other methods for study of fracture surfaces, namely visual fractography analysis in the scanning electron microscope (SEM) and EBSD directly from facets. These approaches will be illustrated with data relating to the cleavage plane orientation analysis in a ferritic and C-Mn steel. It is demonstrated that the combined use of EBSD and stereo-photogrammetry represents a significant advance in the methodology for facet crystallography analysis. The results of point counting from fractograph characterization determined that the proportions of intergranular fracture in C-Mn and ferritic steels were 10.4% and 9.4%, respectively. The crystallographic orientation was determined directly from the fracture surface of a ferritic steel sample and produced an orientation distribution with a clear trend towards the [001] plane. A stereo-photogrammetry technique was validated using the known geometry of a Vickers hardness indent. The technique was then successfully employed to measure the macroscopic orientation of individual cleavage facets in the same reference frame as the EBSD measurements. Correlating the results of these measurements indicated that the actual crystallographic orientation of every cleavage facet identified in the steel specimens is [001].