The [111] zone axis critical voltage has been found to change by about 130kV through the II-VI semiconducting alloy series CdxHg1−xTe. Measurements of the [111] zone axis critical voltage can determine the composition to ±1%. Theoretical calculations of the variation of the [111] zone axis critical voltage with composition are similar in form but different in detail to the experimental results. Determination of composition by measurement of FOLZ line separations at [100] orientation has been found to be rather insensitive due to the large Debye-Waller factor associated with Mercury.

Summary The relationship between the crystallography of intergranular fracture and phosphorus segregation has been investigated in a Fe−0.06wt%P−0.002wt%C alloy aged for 1 h at temperatures between 600 °C and 1000 °C. Two novel techniques were devised for the investigation: first, electron back‐scatter diffraction (EBSD) across the reconstructed fracture surface and, second, a combination of Auger electron spectroscopy, stereophotogrammetry and microscopy to measure phosphorus and carbon on fracture facets combined with EBSD measurements direct from the fracture surface. In total, 700 misorientations were measured from across the reconstructed fracture surface and in ‘control’ areas away from the fracture. It was found that Σ 3s were in general more resistant to brittle fracture than were random boundaries, and it was suggested that alloys of this type could be grain boundary engineered to improve fracture resistance by a short anneal in the austenite region to increase the final proportion of Σ 3s. Sixteen fracture facets yielded combined Auger/EBSD data. The combined Auger/EBSD methodology to acquire joint crystallographic and segregation information from facets was shown to be feasible, although laborious. There were significantly more {110} planes than any other type in the sample population of facets from which combined segregation/crystallography data had been collected. The data suggested that there was on average lower phosphorus segregation on fracture facets that were near {110} than on other intergranular fracture facets.

Submerged arc weld materials have been employed in a study of the effects of manganese and carbon on phosphorus segregation and intergranular embrittlement. The equilibrium grain boundary segregation behaviour of these elements during aging has been studied in two different boiler shell weld materials, which differ mainly in the manganese concentration and operating temperature during service. The materials have seen temperatures above 300 C during operation of the boiler in service, at which temperature thermally induced segregation and embrittlement occurs. A new co-segregation model has been compared with the existing site competition model. The microstructure shows fine and coarse grained regions. The effects of manganese and carbon on the grain boundary segregation of phosphorus have been examined. Thermally induced grain boundary segregation during full service life (up to 50 years) as a function of temperature is described. To evaluate the free matrix concentration of a given element, equilibrium thermodynamic software was used to allow for the tendency to form precipitates within the alloy matrix. The predicted results reveal the dependence of the grain boundary concentration on temperature and show that manganese and carbon decrease the phosphorus segregation by site competition. The final segregation consists of non-equilibrium and equilibrium segregation, which occur during quenching after welding, post-weld heat treatment, and service. The microstructure has been investigated by optical microscopy and transmission electron microscopy to show carbide formation at the grain boundaries and intergranular precipitation of MnS. Preliminary analysis of the grain boundary has been made and the results compared with theoretical segregation predictions.

The present paper investigates the distribution of grain boundary types and fracture surface crystallography in an Fe–C–P alloy. It is shown that electron backscatter diffraction (EBSD) is an effective technique with which to conduct these investigations. The proportions of both Σ1 and particularly Σ3 (in coincidence site lattice notation)present after various heat treatments were higher than would have been expected for random generation. There was limited evidence that both higher annealing temperatures and longer annealing times promoted generation of Σ3 type boundaries. The standard EBSD technique was modified and extended to encompass both the novel ‘matched fracture’ specimen approach and direct mapping from fracture surfaces to provide crystallographic information. A correlation was noted between higher aging temperatures and proportions of cleavage fracture. Furthermore, there was a strong correlation between cleavage fracture surfaces exhibiting river markings and an {001} surface orientation.

An important contribution to the life extension of Magnox nuclear power stations comes from the assessment of microstructural changes in reactor pressure vessel (RPV) weld metal after extended in-service exposure to a neutron Aux. A project was undertaken to remove through-thickness weld samples from an RPV at a power station undergoing decommissioning. Sampling was carried out in a region of the reactor where the RPV was exposed to a high fast neutron irradiation dose at a low operating temperature. Each sample was cut into several specimens for mechanical tests to establish fracture toughness, chemical analysis and dosimetry data.This paper describes some of the preliminary results obtained from a microstructural characterisation programme. Samples were cut in various positions along the weld and at different depths through the vessel wall. These samples were examined by a number of techniques to evaluate the macro and microstructure after different amounts of neutron irradiation. In-situ micro-analysis and fractography was carried out on specimens after mechanical testing. (C) 2000 Published by Elsevier Science Ltd.

Analytical investigations employing field-emission gun scanning transmission electron microscopy (FEGSTEM) have indicated that the degree of chromium depletion at grain boundaries is less pronounced in AISI type-316 when compared to type-304 stainless steel after neutron irradiation in Boiling Water Reactor (BWR) conditions to a fluence of 1026 n.m-2 (E > 1 MeV). An explanation of this observation is proposed, which is based on the non-equilibrium segregation (NES) theory of grain boundary segregation. NES model predictions for the radiation-induced segregation of chromium and molybdenum in an austenitic matrix are presented together for the first time. The predicted segregation profiles are compared with the FEGSTEM data, whilst the predicted trends as a function of neutron dose are discussed in terms of possible microstructural evolution in a BWR environment. In particular it will be demonstrated how, according to NES theory, the binding of molybdenum to self-interstitial atoms (SIAs) will take precedence over chromium. In turn, the subsequent migration of molybdenum-SIA “complexes” retard the migration of chromium away from the grain boundary, thus moderating the magnitude of radiation-induced chromium depletion.

Microstructural characterisation has proven an essential part of a Post Irradiation examination (PIE) programme for ex-service Magnox Reactor Safety Control Rods material. Clad ductility of the component is a key parameter and this can be assessed microstructurally by Transmission Electron Microscope examination. After service, one specific control rod was found to have suffered damage to its outer ferritic steel sheath. Using an understanding of the mechanisms for copper precipitation and dislocation loop formation under reactor operating conditions, it was possible to make an estimate of the operating temperature of the component. This was calculated to be 285+/-15 degrees C, considerably lower than the nominal operating temperature of similar to 350 degrees C. In support of this measurement, direct comparison with work on dislocation loop formation in ferrovac iron, irradiated to a comparable neutron dose over a range of temperatures, suggests an irradiation temperature of similar to 280 degrees C, derived from the comparison of mean dislocation loop size.

The acquisition of a series of energy-filtered TEM images over the energy-loss range of interest creates a three-dimensional data-set containing both spatial and spectral information. Such an image-series contains energy-loss information not available with conventional two- or three-window methods, allowing standard EELS analysis techniques to be applied to extracted 'image-spectra'. In this work, the advantages of this approach are outlined by reference to recent applications, namely quantitative elemental mapping of grain-boundary segregation in an AlZnMgCu alloy, and identification of an amorphous inter-phase at an aluminium-silica interface using an image-spectrum line-trace.

The majority of pressurized water reactors internals are fabricated from the 300 series stainless steels. As a result of neutron irradiation (1x10(21) to 3X10(22) n/cm(2), E>0.1MeV) at service temperature, it has been shown that the stainless Steels in this research program experience changes in measurable macroscopic properties as well as susceptibility to stress corrosion cracking. This paper presents the results of the microstructural work performed on internal materials using TEM, FEGSTEM/EDX/PEELS, and AES and compares the microstructural changes to mechanical property changes.

Control rods from the Magnox reactors are subjected to a wide ranging post irradiation examination programme. The current paper concentrates on microstructural and microchemical studies of control rod stainless steel sheath and weld metal.Changes in grain boundary microchemstry with irradiation are monitored using field emission gun scanning transmission electron microscopy (FEGSTEM). Molybedenum is show to have a beneficial effect in inhibiting chromium depletion at grain boundaries. Samples with low levels of Mo become sensitised and parallel electron energy loss spectroscopy (PEELS) reveals the lack of a passivating oxide layer on the surface in a narrow region around the boundaries.Sheath and weld metal has been characterised microstructurally before and after irradiation. Mechanical testing shows a much greater change in properties with irradiation for sheath than for weld. The microstructures are compared and the differences in mechanical properties accounted for.

The active electron optics suite at Magnox Electric`s Berkeley Centre has a VG HB501 FEGSTEM and a FE-sourced Auger Microprobe (a Fisons Microlab-F) which are routinely used for grain boundary chemistry studies in a variety of highly active austenitic and ferritic steels and nickel base alloys. In a number of investigations sufficient grain boundary data have been obtained to allow a meaningful comparison between the two instruments to be made. It has been established that for the overwhelming majority of materials and conditions there exists a reasonable correlation between the two sets of compositional (on-boundary) data. This paper presents this correlation and gives a simple theoretical description justifying the relationship between the raw data from the instruments. Examples are given of materials and conditions for which the correlation cannot be established and this leads to a discussion of the usefulness of the availability of the correlation.

Behaviour of structural materials during service can depend on nanometre-scale variations in composition. We describe current work at Magnox Electric on the segregation of Mn to grain boundaries in irradiated ferritic steels. The ability to detect Mn by Energy Dispersive x-ray Spectroscopy (EDS) is inhibited by Mn x-rays emitted by Irradiated steels. However, the less routine technique of Parallel Electron Energy Loss Spectroscopy (PEELS) is not affected by sample radioactivity. Segregation of both Mn and P to the grain boundaries has been demonstrated in a Magnox Reactor Pressure Vessel surveillance weld material using EDS. PEELS has also been applied to measure the presence of Mn in the same material allowing a comparison of the two micro-analytical techniques.

Field Emission Gun Scanning Transmission Electron Microscopy and Auger Electron Spectroscopy have been used to characterize grain boundaries in unirradiated and neutron-irradiated type 304 stainless steel. Both techniques are used to give compositional information with nanometer-scale spatial resolution at and around grain boundaries. Irradiation induced changes in grain boundary nanochemistry from the solution treated starting condition are described. Initial segregation of Cr at boundaries is seen to develop through an intermediate ``side-lobe`` distribution, seen clearly at an intermediate dose of {approximately}10{sup 21}n/cm{sup 2}, to Cr depletion at higher dose of {approximately} 10{sup 22}n/cm{sup 2}. Thin foil analysis suggests a considerably higher grain boundary phosphorus level in the intermediate dose material than is measured by fracture surface analysis. For the high dose material the two techniques produce consistent phosphorus levels when comparison is made using experience gained from dual examinations of other steels. It is suggested that in the medium dose material fracture occurs along the plane of minimum chromium arising from the ``side-lobe`` Cr distribution so that the surface exposed by fracture is several nanometers away from the true grain boundary.

The addition of a Gatan imaging parallel electron-energy loss spectrometer (IPEELS) to a Hitachi HF 2000 cold field emission TEM has allowed us to produce high quality energy-filtered coherent electron diffraction patterns and electron holograms from a wide variety of materials. In this paper we review the recent achievements and make an assessment of the use of coherent electron diffraction in solving problems at high spatial resolution in materials science.

Thickness extinction fringe profiles are analysed under zone-axis diffraction conditions to characterize strain in Si/SiB multilayers. For the exact (110) zone-axis orientation, three branches of the dispersion surface dominate the 250 keV electron wavefunction, leading to a simplified analysis. Thickness fringe extinction periodicities and shifts associated with strains are interpretable in terms of two dominant interbranch extinction distances.

Now that coherent electron diffraction can be performed routinely on appropriately chosen specimens in a standard commercial instrument there are a number of applications of the technique which are in progress.One of these is in crystal structure determination. For sufficiently thin crystals, the fringes in the overlap between adjoining reflections n and n+1 give the relative phases (ϕn + 1 - ϕn+δ) of their complex structure factors, apart from an unknown phase factor, δ, which depends on the probe position. In a systematic row of reflections, the next overlap between reflections n + 1 and n + 2 gives (ϕn + 2 - ϕn + 1+δ) and so the difference between these two phasings becomes (2ϕn + 1 - ϕn - ϕn + 2), eliminating the unknown factor δ. Now that coherent diffraction has been achieved in the zero order Laue zone of a zone axis pattern it is possible to phase the whole two dimensional mesh relative to the central beam (Figure 1).

In a previous publication, it was shown that interference fringes due to the coherent addition of amplitudes were observed in the overlap regions of adjacent discs in convergent beam electron diffraction (CBED) patterns recorded with a defocused probe in a conventional transmission electron microscope equipped with a cold field-emission source (Hitachi HF-2000). This research has been extended to describe coherent large-angle CBED (LACBED) patterns from 6H SiC, where the angular field of an overlap region is expanded by use of a small (1 mum) selected-area aperture that transmits only a restricted set of beams. The number of fringes visible within an overlap is increased to over 100, where the fringe shifts and rotations are directly related to angular variations in the relative phase of the two beams. In a direct analogy with processing of images produced by electron holography, it is shown that coherent LACBED patterns encode the amplitude product and the phase difference of the overlapped beams. Examples are discussed where the glide plane in 6H SiC produces a pi phase shift of fringes across the Gjonnes-Moodie dark bar, equivalent to a direct representation of the glide operator. The minimum demagnified source size in the object plane is 3.5 angstrom, suggesting that fringes with 5 angstrom period should be visible in the diffraction plane.

Commercial transmission electron microscopes equipped with a field emission gun now produce a focused coherent probe smaller than the projected cell of many crystals. When the probe is slightly defocused, coherent fringe contrast is observed on the overlaps between discs in convergent beam patterns. Some recent results and future prospects are discussed, including coherent large angle patterns, the absence of coherent contrast in inelastic patterns and the phasing of zone axis patterns to recover the image wavefunction.