Microtexture determination by electron back-scatter diffraction

This review describes the use of an experimental technique known as electron back-scatter diffraction (EBSD) to measure microtexture, that is, spatially specific texture measured on an individual orientation basis. Other methods of microtexture determination are briefly described and compared with EBSD. The EBSD technique itself is described in considerable detail including recent developments such as on-line automation. Those EBSD-based microtexture studies which have been reported in the literature are summarized, including those which provide a direct comparison with macrotexture measurements obtained by X-ray diffraction. The concept of microtexture as the texture of individual grains leads naturally to the idea of mesotexture as the texture of grain boundaries. Mesotexture data can be computed from the EBSD-generated microtexture measurements and this is demonstrated and examples are given. Examples of microtexture/mesotexture studies in multiphase materials are also shown. Finally, because EBSD allows the simultaneous determination of microtexture/mesotexture and microstructural information, it is pertinent to discuss ways of displaying this sort of data, and so the review is completed by a discussion of the representation of microtextures and mesotextures, including the use of Rodrigues-Frank space and orientation mapping.     

Representation of electron backscatter diffraction data

Abstract

Current methods of data representation for electron backscatter diffraction (EBSD) measurements are reviewed. Obtaining diffraction data from microstructures using EBSD has become a relatively straightforward procedure, and EBSD software packages are used to represent these data as qualitative statistics in the form of ideal orientations, pole figures, inverse pole figures, Euler space, and Rodrigues–Frank space. Quantitative statistics in the form of secondary computations allow full microtextural analysis. Additionally, the power of EBSD is demonstrated through positional information representation. Through experimental examples, the conversion of EBSD data to statistical information to facilitate interpretation of results is demonstrated.

MST/3678     

Structure studies of non-crystalline materials by electron diffraction

The ability of electron diffraction to determine the structure of non-crystalline materials has been critically revised on the basis of the main sources of error: the premature temination of the experimental intensity curve and the problems associated with the elimination of the inelastically scattered intensity. A method of studying the problem is presented and the efficacy of the most representative procedures for reducing errors in the final correlation functions is checked.     

Femtosecond electron diffraction: 'making the molecular movie'

Femtosecond electron diffraction (FED) has the potential to directly observe transition state processes. The relevant motions for this barrier-crossing event occur on the hundred femtosecond time-scale. Recent advances in the development of high-flux electron pulse sources with the required time resolution and sensitivity to capture barrier-crossing processes are described in the context of attaining atomic level details of such structural dynamics-seeing chemical events as they occur. Initial work focused on the ordered-to-disordered phase transition of Al under strong driving conditions for which melting takes on nm or molecular scale dimensions. This work has been extended to Au, which clearly shows a separation in time-scales for lattice heating and melting. It also demonstrates that superheated face-centred cubic (FCC) metals melt through thermal mechanisms involving homogeneous nucleation to propagate the disordering process. A new concept exploiting electron-electron correlation is introduced for pulse characterization and determination of t=0 to within 100fs as well as for spatial manipulation of the electron beam. Laser-based methods are shown to provide further improvements in time resolution with respect to pulse characterization, absolute t=0 determination, and the potential for electron acceleration to energies optimal for time-resolved diffraction.     

An ultra-high vacuum reflection scanning electron diffraction system

A reflection-mode scanning electron diffraction system which operates with ultra-high vacuum is described. The instrument was developed for the study of surfaces and their reactions with admitted gases, and specimen processing facilities are incorporated. An electron diffraction pattern formed using electrons in the energy range 20–50 kV is measured by magnetically scanning it across a defining aperture. A retarding-field energy filter is incorporated to remove inelastically scattered electrons. The intensities are measured using a scintillator and a photomultiplier tube. The accuracy of the measurement is ±2 per cent at a filter potential of 5 V, and a resolution of 1 × 10⁻⁴ radians is achieved. Typical results are presented.     

Review Grain and subgrain characterisation by electron backscatter diffraction

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 m. 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.     

Quantification of partially recrystallized polycrystals using electron backscatter diffraction

A new experimental method of determining the volume fraction of recrystallized structure in a polycrystalline material has been developed. This method is based upon the comparison of electron backscatter diffraction patterns from adjacent locations in a polycrystal. Positions along a line are tested to determine whether adjacent points have the same crystal lattice orientation indicating that no dislocation structure exists between the two locations. If the distance between the two positions is relatively small, it is assured that the two points lie within a region of essentially defect-free lattice. Scanning through a region of material in this manner yields the lineal fraction of recrystallized material which directly correlates with the volume fraction. This automatic measurement eliminates the subjectivity and human error always associated with the quantitative determination of fraction recrystallized in a material.     

Characterisation of high temperature oxidation using electron backscatter diffraction

Abstract

Electron backscatter diffraction (EBSD) in the scanning electron microscope (SEM) has been used to characterise the oxide scales formed on a low alloy steel. The technique provides a powerful combination of local phase information and orientational relationships both within and between phase layers. It has revealed that hematite grain growth occurs almost exclusively along the 0001 direction for the entire range of samples examined. Wüstite and magnetite grains were also found to grow preferentially along orientations close to the 001 direction.

EBSD is also well suited to characterising more complex scales such as those formed during hot working (e.g. millscale), and those formed on Fe–Ni alloys. In the latter complementary chemical information from energy dispersive spectroscopy (EDS), which was acquired simultaneously with the EBSD, enables the identity of crystallographically similar phases to be distinguished. EDS also shows that no nickel exists in the external scale and that it instead accumulates at the interface with the scale and at adjacent grain boundaries.     

Transmission electron microscopy and X-ray diffraction studies of quantum wells

A series of In _x Ga_1?x As (x=0·47) quantum wells with InP barrier layers have been grown on InP substrates by metalorganic vapour phase epitaxy (MOVPE) at 625°C. The nominal well widths were defined during growth at (i) 25 Å, 39 Å, 78 Å and 150 Å for one sample and (ii) 78 Å for all 4 wells in another sample. The InP barrier widths have been kept constant at 150 Å. These layers have been characterized by X-ray diffraction (XRD) which from simulation gave the nominally 78 Å well width as 84 Å and the nominally 150 Å barrier width as 150·5 Å. Transmission electron microscopy (TEM) and high resolution TEM (HRTEM) have been carried out on etched and ion-milled samples for direct measurement of well and barrier widths. The well widths found from TEM are 25 Å, 40 Å, 75 Å and 150 Å. TEM micrographs revealed that, while the InP barrier layer is of good quality and the growth is confirmed to be epitaxial, dipoles are detected at the interface and the quantum well has some small disordered regions. These thickness measurements are in good agreement with earlier photoluminescence (PL) and secondary ion mass spectrometry (SIMS) studies.     

Some observations on thin β-tin films by electron diffraction

Forbidden “110-type” reflections are present on some patterns of tin single-crystal films. Their intensities may vary in a large range and these reflections do not appear with polycrystalline samples. The other forbidden “002-type” reflections which were observed by other authors and ourselves are probably due to secondary elastic scattering, but the kind of “forbidden” spots studied here can be explained by dynamic interactions.     

Computer-aided prediction and analysis of electron and x-ray diffraction patterns

Using a personal computer, electron and x-ray diffraction patterns are analytically generated and printed to scale. The structure factor equation is used to determine which crystalline planes diffract according to the supplied crystal lattice type, atoms present and their positions in the unit cell, lattice dimension(s), crystal orientation, and radiation wavelength. Single crystal spot and Kikuchi electron diffraction patterns as well as polycrystalline ring patterns can be generated. Powder x-ray diffraction patterns are also generated using this code. Complex composite patterns are generated by overlaying multiple patterns resulting from crystallographic orientation effects. A user friendly program with a direct approach to generate diffraction patterns for any defined crystal aids in the analysis and indexing of these patterns.     

Calibration of a scanning electron microscope with a diffraction grating

We examine the problem of high accuracy SEM measurement. We have developed a calibration method that eliminates the operator from the procedure and thereby removes the main source of random error.Translated from Izmeritel'naya Tekhnika, No. 9, pp. 33–35, September, 1995.     

In-situ scanning electron microscopy and electron backscatter diffraction investigation on the oxidation of pure iron

Abstract

α-iron samples, with a 4 to 6 nm oxide layer on top of the surface due to preparation, were oxidised in situ within a scanning electron microscopy equipped with electron back-scatter diffraction. At 773 and 973 K, two different growth mechanisms during the initial stage of iron oxide formation could be observed, layer-by-layer growth at 773K and island growth at 973 K. The growth mechanism at 973K is correlated to the formation of a thin layer of Fe_1–XO (wüstite) prior to Fe₃O₄ (magnetite) crystallisation whereas at 773K only magnetite was found as newly grown oxide. Magnetite showed a strong epitaxial relationship to α-iron with its {111} plane growing preferentially on the {110} plane of iron.     

Computer simulation of electron diffraction studies of GdCo crystalline compounds

The interference function, the diffraction intensity and the radial distribution function for Gd₁₀Co₉₀, Gd₂Co₁₇ and GdCo₅ compounds are studied by computer simulation. It is found that the position of the maxima of the radial distribution function in all the cases are same and the curves nearly coincide with the experimental curves. The first peak in all the cases are nearly equal at r ≈ 3.8 Å. The effect of strain on the interference function is studied.     

Triple junction distribution profiles as assessed by electron backscatter diffraction

The connectivity between the boundaries is very important because the defect character of triple junctions is expected to have a significant influence on the bulk properties of materials, particularly mechanical behaviour. The investigation of triple junction investigations presented here indicates that a restricted Coincidence Site Lattice (CSL) model was found to be the most relevant and practicable for the characterisation of triple junctions in Grain Boundary Engineered materials. The triple junction character distribution was measured using the automated Electron Back-Scatter Diffraction (EBSD) application of Crystal Orientation Mapping (COM) for a series of thermomechanically processed copper and alpha-brass specimens. Triple junction character statistics were determined from COM data, automatically using a custom-built computer program, utilising the CSL model. The alpha-brass data were then compared with a series of previously acquired triple junction data for a series of strain-recrystallisation copper specimens. The main aim of the investigation was to determine the relationship between the grain boundary and triple junction character distributions, which was found to be essentially a linear relationship.