Selected Area Electron Channeling Research Articles

Ultra-high spatial resolution selected area electron channeling patterns

An approach for producing ultrahigh spatial resolution selected area electron channeling patterns (UHR-SACPs) using the FEI/Thermo Elstar electron column is presented. The approach uses free lens control to directly assign lens and deflector values to rock the beam about precise points on the sample surface and generate the UHR-SACPs. Modification of the lens parameters is done using a service application that is preinstalled on the microscope or using the iFast scripting interface to run a short program to assign lens and deflector currents. Using the approach outlined here, the UHR-SACPs are collected at normal instrument scanning rates and pixel densities, resulting in rapid collection times and sharp patterns with simple push button changes in instrument mode. UHR-SACPs with spatial resolutions of 300 nm with angular ranges of 20° are demonstrated, as are patterns approaching 125 nm spatial resolution with angular ranges of 4°. Such spatial resolution/angular range combinations are significantly better than any reported previously. This approach for rapidly collecting high accuracy crystallographic information greatly enhances the ability to carry out electron channeling contrast imaging (ECCI) for a broad range of materials applications.

Determination of the crystallographic parameters of a fcc metal by means of electron channelling patterns

AbstractIndexing and mapping of selected area electron channelling patterns (SAECPs) of an Al‐Zn‐Mg alloy have been performed. Although, channelling patterns are similar for crystals having the same system, since the interplanar spacing and the angular band width depend on lattice parameter, the details of the map will differ for every material. Angles between the poles and the angular separations of channelling bands were measured on the micrographs and calculated with the well known equations and found to be well in agreement within the error limits. Also the channelling line widths were calculated with the help of extinction distances of the reflecting planes. Structure factors and atomic scattering factors for selected reflections were also calculated for the present alloy whose a = 4.054 Å.

Microcrack Nucleation and Crack Propagation in Equiaxed γ TiAl

ABSTRACTTo gain a better understanding of the ductility limitations in TiAl alloys, the mechanisms involved in deformation strain transfer and/or microcrack initiation at grain boundaries have been examined in an equiaxed near-γ alloy. These studies have been carried out on both in-situ and ex-situ deformed bulk samples using scanning electron microscopy (SEM) techniques for both orientation analysis and deformation defect imaging. Selected area electron channeling patterns (SACPs) have allowed determination of grain orientations, eliminating ambiguity between the a and c axes. Deformation twins and dislocations have been imaged in the bulk samples using electron channeling contrast imaging (ECCI). A combination of ECCI contrast analysis and trace analysis based on orientations determined from SACP has allowed identification of the active deformation systems. Microcracks have been found to initiate at γ-γ boundaries as a result of an inability to adequately transfer twin strain from grain to grain. Once initiated, cracks propagate through cleavage and re-nucleation of grain boundary microcracks in front of the advancing crack. A geometric based predictive factor has been developed that accounts for microcrack initiation at γ-γ boundaries based in deformation twinning and strain accommodation by ordinary dislocations.

Spatial Resolution of Electron Backscatter Diffraction in a FEG-SEM

Crystallographic information can be determined for bulk specimens in a SEM by utilizing electron backscatter diffraction (EBSD), which is also referred to as backscatter electron Kikuchi diffraction. This technique provides similar information to that provided by selected area electron channeling (SAEC). However, the spatial resolutions of the two techniques are limited by different processes. In SAEC patterns, the spatial resolution is limited to ˜2 μm by the motion of the beam on the specimen, which results from the angular rocking of the beam and the aberration of the probe forming lens. Therefore, smaller incident probe sizes provide no improvement in spatial resolution of SAEC patterns. In contrast, the spatial resolution for EBSD, which uses a stationary beam and an area detector, is determined by 1) the incident probe size and 2) the size of the interaction volume from which significant backscattered electrons are produced in the direction of the EBSD detector. The second factor is influenced by the accelerating voltage, the specimen tilt, and the relative orientation of scattering direction and the specimen tilt axis.

Etching characteristics of Ni 3Al

Etched surfaces of Ni 3Al (γ′ phase) and mixtures of Ni 3Al and Ni (γ phase) have revealed a large variation in gray level for different grains depending upon their crystallographic orientation. A selected area electron channeling pattern study in the scanning electron microscope has shown that grains oriented near 〈001〉 etch light, while those oriented near 〈111〉 etch dark with a gradual transition for intermediate orientations. It is shown that the etching characteristics result from formation of {001} facets on the etched surface, which produces a nearly planar surface on {001} faces and microfaceted arrays of very small pyramidal projections having {001} facet planes on the {111} faces.

Measurement of plastic zones associated with small fatigue cracks by selected area channeling patterns

The selected area electron channeling pattern method has been used to measure the plasticity associated with “growing” small fatigue cracks in a 7000 series aluminium alloy, allowing study of plastic zone development as well as investigation of the mechanisms by which small fatigue cracks and their plastic zones interact with grain boundaries. It was found that both plastic zone size and shape were dependent on both crack length and growth morphology. Naturally initiated Stage I fatigue cracks were predominantly crystallographic in nature and were accompanied by a relatively long, slender plastic zone shape. However, during subsequent growth over 1–2 grain diameters, this shape evolved, first to a semicircular shape, and finally to the lobed configuration typically found associated with long cracks.

The formation of wither tongue- or ribbon-like extrusions in fatigued copper polycrystals

The surface morphology of polycrystalline copper fatigued at temperatures of 107, 298 and 533 K was investigated by scanning electron microscopy (SEM). Both ribbon- and tongue-like extrusions were observed except after fatigue at 533 K, where ribbon-like, complex and needle-like surface structures were apparent. The dependence of the formation of either ribbon- or tongue-like extrusions on the crystallographic orientation of the grains in the direction of the stress axis was investigated by selected area electron channeling patterns in the SEM. The results suggest strongly that the morphology of extrusions formed at emerging PSBs is controlled by the type and the Schmid factors of the activated slip systems. A model suggested by Polák et al. [ Mater. Sci. Engng 74, 85 (1985)] to explain the formation of the tongue-like extrusions was checked for 20 different grains and found not to be valid in general.

Enhanced grain growth in polycrystalline CuInSe2 using rapid thermal processing

Rapid thermal processing of precursor small-grained (less than or equal to 1 μm) CuInSe2 thin films can result in enhanced grain growth on the order of 100 μm under certain processing conditions. Crystal growth was accompanied by large increases (×10) in the X-ray diffraction intensity of the (112) peak, indicating near-perfect crystal growth in the [221] direction. Single-crystal quality was verified by selected-area electron channeling. However, at present, this effect occurs in conjuction with excessive void and pinhole formation between single-crystal platelets. These voids are often decorated with relatively selenium-deficient phases.

Texture evolution in the presence of precipitates in Fe−3% Si alloy

Texture evolution by grain growth in the presence of MnS or MnS and AIN precipitates in Fe−3% Si alloy was investigated by X-ray diffraction (vector method) and ECP (selected area electron channelling pattern). Texture evolved by grain growth varied with the intensity of the inhibitors in the following manner: when an inhibitor intensity is higher than a critical value, the orientation N having the highest I C Σi value in the matrix becomes major orientation after grain growth. Here the I C Σi value denotes an intensity of orientations with Σi type coincidence orientation relationship with a given orientation N (except Σ1 and Σ3 type coincidence orientations). When an inhibitor intensity is weaker than the critical value, the orientation ( N) having the highest P CN Σi value in the matrix becomes major orientation after grain growth. Here the P CN Σi value denotes an product of a intensity of the given orientation ( I N) and the I C Σi value. With an weaker inhibitor level, I N being more dominant and with a stronger inhititor level, I C Σi being more dominant under the same P CN Σi value. The inhibitor effect of P CN Σi1 value increases with increasing in the inhibitor intensity, where P CN Σ1 is a product of the intensity of the given orientation ( I N)

Growth and characterization of iron aluminide films on compound semiconductors

Transmission electron microscopy (TEM), selected area electron channeling patterns (SACP), and reflection high-energy electron diffraction (RHEED) are used to investigate the growth and quality of epitaxial iron aluminide films on GaAs(100) substrates. These films are observed to grow layer-by-layer even when the constituents are codeposited. During growth the lattice relaxation is determined fromin situ RHEED measurements of the separation of two diffracted beams. After growth TEM measurements of Moire fringes are used to determine the local residual strain and the Burger's vector of the iron aluminide film. Selected area electron channeling is used to determine the residual strain in the films without thin specimen artifacts. The results indicate that strain in the FeA1 films relaxes by the formation of misfit dislocations with anα Burger's vector. The relaxation does not occur as quickly as the Matthews equilibrium model predicts. Since the FeAl and GaAs have different slip systems, the dislocations inducing the relaxation must nucleate in the epilayer. The defects present suggest growth by nucleation and subsequent climb of dislocation half-loops.

Strains and Relaxations Near metal Alumlnide/Semiconductor Interfaces

AbstractMechanisms and phenomena of strain relaxation at bi-material interfaces have been studied for over half a century. The details, however, and limiting kinetics, thermodynamics and mechanics are still being sorted out - particularly for large misfit systems. Three techniques are required to accurately portray strain distributions during and after epitaxial growth: RHEED, TEM and SACP. Reflection high energy electron diffraction (RHEED) is used to measure the lattice parameter during growth. Both transmission electron microscopy (TEM) and selected area electron channeling pattern (SACP) analysis are necessary to identify the defects and the strain distribution. These techniques have been applied to NiAl and FeAl grown by MBE on GaAs with a thin AlAs buffer layer. It is shown that both island and layer by layer growth can occur with the corresponding defects being remarkably similar in character. From a combined Moiré, HREM and computer simulation, the dislocation character is assessed. Both <100> dislocations from half-loops or island edges may occur providing only partial relaxation of the film-substrate systems. The impact of the remaining elastic strain distribution on kinetic measurements of dislocation velocities is discussed.

Low temperature MOCVD of Hg1−xCdxTe on 311, 511, 711 and shaped GaAs

The influence of six new GaAs substrate orientations on the surface morphology and defect hillocks of epitaxial Hg1−xCdxTe (MCT) layers, grown by low temperature MOCVD, is investigated using Nomarski interference contrast microscopy and SEM. Smoother epitaxial layers are obtained on (311), (511) and (711) substrates of A or B polarity compared with growth on (100) or (100) 2° off toward (110) substrates. The quality and orientation of the layers was monitored by X-ray techniques, Rutherford backscattering/channeling and selected area electron channeling patterns. Absolute polarity determination by X-ray diffraction confirmed that the epilayers adopted the polarity of the substrate. Conformal growth on contoured substrates revealed preferential facet formation and growth rate variation which leads to local composition changes in the interdiffused multilayer process. The growth hillocks commonly observed in epitaxy of MCT are related to the oval defects in III–V compounds, and dissolve on annealing.

Characterization of active slip systems in Hg0.8Cd0.2Te

The compound semiconductor Hg0.8Cd0.2Te is commonly used in the production of infrared detection devices. In bulk growth, due to low Hg-Te bond strengths, high dislocation densities are often encountered. These dislocations, which are generated during the crystal growth process, significantly impair the performance of the semiconductor device. To best control this dislocation generation during growth, a knowledge of the active slip systems within the crystal lattice is desirable.Microhardness tests were performed on two distinct crystal surfaces of Bridgman grown Hg0.8Cd0.2Te crystals. Selected area electron channeling was used to determine trie crystallographic orientations of the surfaces that were tested. The slip lands that were introduced during the indentation process were observed using both the scanning electron and optical microscopes. From the data obtained, using elementary crystallographic techniques, the dominant slip systems active in Hg0.8Cd0.2Te were characterized.

Large-area HgTe–CdTe superlattices and Hg1−xCdxTe multilayers on GaAs and sapphire substrates grown by low-temperature metalorganic chemical vapor deposition

Specular HgTe–CdTe superlattice epilayers have been obtained on two 2-in. diam GaAs or sapphire wafers per growth run using a horizontal metalorganic chemical vapor deposition (MOCVD) reactor in which the pyrolysis of the organometallics is induced by a cracking susceptor suspended above the substrates. This enables growth of superlattices below 300 °C using the standard precursors dimethyl cadmium, diethyl telluride, and elemental mercury. Annealing the superlattices at 350 °C converts them to homogeneous Hg1−xCdxTe. Compositional profiles obtained by Rutherford backscattering, sputter Auger depth profiling, and e-beam analysis of ultramicrotomed thin cross sections, were used to study interdiffusion of the CdTe and HgTe layers during growth and annealing. Arrays of metal–semiconductor field effect transistors (MESFETS) and photoconductive detectors with room temperature peak responsivity between 1.3 and 1.6 μm have been prepared from incompletely interdiffused material grown on semi-insulating GaAs without thick buffer layers. Device processing used mesa technology and a KI:I2:HBr etchant developed for its selectivity with respect to Hg1−xCdxTe . Capping layers of CdTe and HgTe deposited at low temperatures have been investigated as dielectric and ohmic contacts, respectively. Electrical analyses by Hall and C–V measurements are presented. Room temperature electron mobilities up to 27 500 cm2 /V s and resistivities of 1–2 Ω cm have been observed for the HgTe layers. Information on crystallinity and defect structure of the epilayers was obtained by selected area electron channeling patterns, high-resolution transmission electron microscopy (HRTEM), Rutherford backscattering spectroscopy (RBS) and four-circle, double-crystal, and triple-crystal x-ray diffraction. Compositional profiles in the 100 cm2 deposition area were obtained by wavelength dispersive x-ray analysis (WDX), Fourier transform infrared (FTIR) spectrometry and inductively coupled plasma atomic emission spectrometry (ICPAES).

Surface versus bulk deformation in fatigue

Selected area electron channeling was used to detect the plastic strain gradient from bulk to surface grains which developed during low cycle fatigue. After an applied strain of 5%, the surface grains have undergone approximately three times as much accumulated strain as the bulk.

The crystallography of fatigue crack initiation in coarse grained astroloy at 20°C

The effects of crystallographic orientation on fatigue crack initiation has been examined for coarse-grained Astroloy at 20°C. Specimens were cycled by three-point bending at stress ranges between 5 and 95% of the proportional limit until fatigue cracks were detected. The crystallographic orientation of individual grains within which fatigue cracks initiated was determined by use of selected area electron channeling. Grains forming cracks were found to have surface normals near the 〈100〉, 〈011〉, and 〈113〉 directions. Conversely, grains which did not initiate cracks were not similarly grouped in orientation. Calculations of the Taylor factor using the Bishop-Hill approach revealed that fatigue crack initiation in Astroloy occurred at grains with low values of the Taylor factor.

Coincidence grain boundary and texture evolution in Fe-3%Si

Orientation and distribution of coincidence boundaries in primary and secondary recrystallized Fe-3%Si were investigated by X-ray diffraction (vector method) and ECP (selected area electron channeling pattern). {110}〈001〉 oriented grains were not among the largest grains nor were they present as a colony in the initial primary texture. Orientation of the extraordinarily large grains in the initial primary texture were among the major texture component in the initial primary texture. The ideal Goss orientation had the cusp of frequency of coincidence oriented grains in relation to the ideal Goss orientation and had a very low frequency of Σ1 boundary in the initial primary matrix, while the extraordinarily large grains had rather higher frequency of Σ1 boundary in the initial primary matrix. The frequency distribution of Goss secondaries were proportional to the frequency distribution of coincidence oriented grains in the initial primary texture and secondary grains of the orientations of extraordinarily large grains in the initial primary matrix did not exist in the secondary recrystallization texture. Based on these findings, a new mechanism of the sharp Goss texture evolution was proposed.

Cleavage fracture in an Al3Ti-based alloy having the Ll2 structure

Selected area electron channeling patterns were used to identify the cleavage planes in a polycrystalline Al–23Ti–6Fe–5V alloy, which is an Al3Ti-based alloy having the Ll2 structure. Alloys like this one are of scientific and technological interest because they fracture by brittle transgranular cleavage, despite the availability of more than five independent slip systems in the Ll2 structure and their relatively low hardness when compared to ductile Ll2 alloys like Ni3Al. Homogenized bars of the levitation zone melted and directionally solidified alloy were fractured at room temperature in three point bending. The fracture surfaces, which consisted almost entirely of brittle transgranular cleavage facets, were examined in a scanning electron microscope (SEM) equipped to take channeling patterns. An optical microscope with a short depth of focus was inserted in the SEM column just below the objective lens, and by focusing it on several points on the cleavage facets, it was possible to orient the facets normal to the optic axis of the SEM prior to taking the channeling patterns. Of the eight cleavage facets examined in this study, six were of the {110} type and two were of the {111} type. Although it cannot be said that these are the only two cleavage planes in this alloy, the availability of more than one plane with low cleavage strength contributes to the brittleness of this alloy. These results are examined in the light of theoretical treatments of cleavage fracture, and comparisons are made with earlier studies on other Ll2 materials.

SAECP Combined With Quantitative Image Analysis—an Improved Tool for Texture Analysis

A selected area electron channelling pattern (SAECP), which allows the determination of the orientation of individual grains in polished steel specimens, can be obtained with a scanning electron microscope. The different methods to determine the orientation of the grains from the measured pattern are summarized. A minimum grain diameter of about 7μm is required for orientation determination and the maximum error for the determination of (hkl) is about 2°, and about 4° for [uvw]. The total time needed for the determination of the orientation of 400 grains is in the range of 60 hours. Different representations of results are discussed. An example shows that the measurement of single orientations can give more information on the mechanisms of recrystallization than an investigation by x-rays only.

A determination of the texture of a directionally solidified sample of high-purity copper

A study making a combined use of neutron diffraction and selected-area electron channelling to determine the solidification texture in a high-purity copper sample is described. Good correlation between the techniques is shown with both demonstrating a strong [100] fibre texture in the directionally solidified rod.