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In the pursuit of more efficient gas turbine engines, components are required to operate for longer times at elevated temperatures. This increased time in service, together with a complex loading regime, can expose the material to environmental attack. This work has demonstrated that the interaction of stress, NaCl and a sulphur-containing environment is critical to cause crack initiation in the early stages of the exposure and accelerated corrosion rates in CMSX-4 at 550°C. The effect of having small concentrations of moisture in the gaseous environment or as water crystallisation in the salt is still to be investigated. A working hypothesis is that the interaction of alkali chlorides with a sulphur-containing atmosphere is the trigger to a self-sustaining cycle where metal chloride formation, vaporisation and oxidation lead to high amounts of hydrogen injection in a rapid manner and, therefore, hydrogen embrittlement.

Single crystal material, of CMSX-4® alloy composition, was cast and secondary orientation was controlled at the machining stage, to produce c-ring cross-section tubes with known crystallographic orientations. The c-ring tubes were coated with NaCl before being subject to loading up to 700MPa and heated for durations of up to 2 hrs at 550°C in flowing environments containing air and SO2. No cracking was observed in short term tests that were run in the absence of either NaCl, or SO2, indicating a symbiotic interaction is required to initiate cracking. Experiments confirm the presence of oxygen, chlorine and sulphur at the crack tips, formed along {001} crystallographic planes, however, they were distributed discretely, with several oxide and sulphide phases observed. In this work, we image, analyse and identify the phases formed during the cracking and corrosion of CMSX-4® superalloy and hypothesise on the complex chemical interactions that take place during crack initiation.

Sintered rods of stoichiometric superconductor were transformed into quasi-single crystals by partial remelting in a temperature gradient. Each rod was positioned in the furnace so that one end remained solid while the rest was heated into the liquid plus two-phase region. The rods were then directionally solidified by pulling them slowly out of the furnace. Quenching experiments during growth showed that the growth front of the quasi-single crystals was at a temperature of only , significantly below the pseudo-binary peritectic temperature of at which liquid, solid and solid are in equilibrium with each other. It is proposed that the growth of quasi-single crystals occurs primarily via the solidification reaction followed by a variant eutectic reaction rather than by the pseudo-binary peritectic reaction and that quasi-single-crystal growth breaks down when the invariant four-phase reaction occurs.

The influence of sulphur on the early stages of Ni(100) oxidation has been studied by STM and LEED. It has been observed that sulphur concentrations as low as 16% of a monolayer have a marked effect on oxidation behaviour. Oxidation of terraces is completely suppressed and a Langmuir-Hinshelwood process is not observed to occur. Oxide nucleation is observed to occur on step edges, which leads to NiO(111) formation during room temperature oxygen adsorption. Strong oxide growth anisotropy and the occurrence of semi-coherent oxide growth along nickel step edges results in the formation of oxide laths orientated perpendicular to the step edges. The presence of these laths and the preferential growth of an oxide across the lower terrace results in the formation of a characteristic, irregularly edged, triangular oxide morphology. On annealing the oxidised sample in a temperature range of 270–290°C, LEED indicates that NiO(111) transforms to NiO(100). Increasing the annealing temperature to above 500°C causes the oxide spots to become fainter and more diffuse, which is consistent with oxygen diffusion into the bulk and oxide disproportionation. This was consistent with the behaviour observed by STM.

Sulphur segregation levels on various grains of a polycrystalline nickel sample have been measured. If the high-energy nickel Auger signal is used as a reference, variations of up to 30% in the S/Ni ratio are seen. This large change in sulphur level results from crystallographic effects, as evidenced by the different variation of the high- and low-energy nickel Auger signals as the specimen tilt angle is varied. Of the two processes responsible for such effects, primary beam channelling and Auger electron diffraction, the latter is found to be important. Use of the background as a reference reduced the variation of the sulphur signal to within the error bars of the measurement of ∼10%.

Melt texturing is a well-developed, if not fully understood, technique for the production of single crystals of . Although this technique can produce single crystals of up to 2 mm diameter by 25 mm in length, problems remain. For example, it is difficult to control the orientation of the crystal relative to the rod in which it grows. It is important that the crystal is oriented so that the a - b planes lie along the bar, thus maximizing the current-carrying capability of the material. This paper reports on experiments in the use of oriented seed crystals to achieve the correct single-crystal orientation relative to the bar and highlights the problems associated with the use of seed crystals.

AlTiC grain-refining master alloys are of increasing importance in aluminium casting because they are believed to introduce a smaller volume fraction of insoluble particles into the melt than conventional AlTiB master alloys are. The main microstructural features of an Al-6wt.%Ti-0.02wt.%C master alloy have been examined using X-ray diffraction metallic phases were identified, namely titanium aluminide and titanium carbide. The titanium aluminides were found to contain small amounts of dissolved vanadium and to possess a range of morphologies from well faceted to rough and irregular. A combination of microanalysis and electron diffraction was used to show that clusters of submicron particles, rich in Ti but with virtually no Al, were TiC crystals. Individual particles were found to be single crystals with an octahedral morphology. A simple thermodynamic analysis of the stability of TiC in the Al-rich corner of the AlTiC phase diagram is presented which confirms the stability of TiC during master alloys synthesis. Using standard test procedures, the AlTiC alloy proved to be an effective grain refiner when added to commercial purity aluminium, and it is proposed that undissolved TiC particles were most likely to have been the heterogeneous nuclei.

The p(2×2) and c(2×2) sulfur overlayers on Ni(100), and the clean Ni(100) surface itself, have been imaged in an ultrahigh-vacuum scanning tunneling microscope and topographic and spectroscopic information was obtained from all three surfaces. These data are compared to theoretical calculations of the surfaces derived using the Green's-function method of Pendry, Pretre, and Krutzen, where the sample Green's function is calculated in a multiple-scattering formalism based on the layer Korringa-Kohn-Rostoker method. Our simple model has proved reasonably successful in predicting the corrugation heights observed on the sulfated nickel surfaces, and confirmed the fact that the difference in height observed between the p(2×2) and the c(2×2) phases is electronic in origin

Some of the problems of using high spatial resolution microanalysis in the vicinity of particles and voids in metal grain boundaries are discussed. New analytical results are presented for nickel based alloys, which suggest that elemental distributions in the region immediately adjacent to growing grain boundary particles and voids are often anisotropic. © 1993 Wiley-Liss, Inc.

The grain boundary composition of long-term aged Nimonic PE16 has been investigated by both Auger electron spectroscopy (AES) and scanning transmission electron microscopy-energy dispersive spectroscopy (STEM-EDS) analysis, and a detailed comparison made of the quantification routines developed for the two techniques. The investigation showed that Mo, Cr and P segregate to the grain boundaries of the alloy during the standard pre-service heat treatment, and that on extended ageing, at 450°C for 10 000 h and 550°C for 5000 h, the concentration of these elements at the grain boundaries increases. Furthermore, it was shown that there was good agreement between the two quantification routines developed to interpret the EDX and Auger spectra, allowing for the errors inherent in each method.

YBaCuO rods have been textured using a moving thermal gradient method. Micrographs show well aligned (123) grains and 211 inclusions. The electrical resistivity transition is sharp, but broadens under an applied magnetic field. The thermoelectric power presents a smooth rise above the critical temperature. The AC susceptibility real part is smooth, but the imaginary part presents some fine structure. All properties are those of quasi single crystal materials. The critical current density is high (7000 A/cm2 at 80 K, 5 kG). It increases logarithmically with frequency.

Textured YBaCuO rods have been grown using a moving thermal gradient method. Electrical resistivity and precise AC magnetic susceptibility data are presented for the samples. Fine structure in the imaginary part of the susceptibility plots is related to features of the microstructure. Following generalization of the Beam critical state model, the authors calculate the flux profile and penetration depth, and deduce the critical current density. They also give values for the activation energy in the flux creep model. It is shown that the critical current density increases logarithmically with frequency, as expected, but that care is needed when comparing values of Jc obtained by different methods.

We studied by magnetic and microscopic techniques a disk (2.0 mm diam and 1.0 mm thick) sliced from a quasi single‐crystalline rod prepared by very slow solidification in a small temperature gradient. The sample presents features very similar to a typical millimetric single crystal obtained by the standard flux methods. We deduced a high critical current density Jc from magnetic measurements: 2×106 A/cm2 at 4.2 K in a 3T field parallel to the c axis. The transport critical current density along the rod axis (i.e., ∼45° off the c axis) was found to be about 7000 A/cm2 at 77 K in zero applied field. It is comparable to that deduced from the hysteresis cycle at the same temperature for the same current direction.

Equilibrium segregation to grain boundaries has been studied for many years, but only recently have the techniques to quantify such segregation at high spatial resolution become widely available. These include, for example, the Auger surface analysis of fractured samples, or the study of boundaries edge-on in a field emission gun STEM equipped with energy dispersive X-ray analysis or electron energy loss spectroscopy. The key question, however, is whether these different techniques yield the same results when applied to a specific set of samples, and this paper reviews some of the successes and problems associated with such studies.For STEM analysis of grain boundary segregation, several instruments now routinely give probe sizes of ∼1nm. Hence line profiles across grain boundaries at this level of resolution are quite feasible. However a larger probe with greater probe current and better X-ray counting statistics is often more useful, especially if sample drift is a potential problem.

Electron microscopy and diffraction have been used to study twins in annealed single crystals of α-N2. It is shown that although in principle two different types of twins may occur, only one type (a type II twin) is seen in practice. Calculations of the energies of such twin configurations and stacking faults, using lattice relaxation methods, also indicate that the energy of a type II twin which is observed in α-N2 is less than the energy of a type I twin. It is also less than the energy of various stacking faults which are not observed.

A study of the oxidation of Fe–5Al alloys with additions of silicon and yttrium has shown that high stresses can develop within the oxide scale. These stresses occur after very short oxidation times (

Recent transmission electron microscope observations of MoSe2 containing hohlstellen (spherical or disc-shaped voids) have revealed the presence of 'superlattices' in the diffraction patterns according to Agarwal et al. (1977). A new explanation for this effect is suggested and contrasted with observations of in situ intercalation of other layer structures in the electron microscope.

The many-beam dynamical theory of electron diffraction is used to simulate zone axis patterns in molybdenite. It is shown that all the main features of the complex [0001] patterns can be generated with a 61 beam calculation, and that marked changes in the patterns with accelerating voltage and specimen thickness can be explained by the form of the dispersion surfaces.

Electron diffraction has been used to study the formation of a variety of complex superlattices in the IT, 2H and 4Hb polytypes of TaS2 during intercalation with hydrazine gas (N2H4), in situ in the electron microscope. Results are presented over the temperature range 15 K-360 K and are interpreted in terms of scattering from periodic lattice distortions linked to charge density waves. Comparisons are also made with similar effects in both the as-grown crystals and TaS2 samples containing other adducts.

The oxidation and corrosion resistance of superalloys in present use is often enhanced by the application of MCrAlY overlay coatings, where M is a combination of cobalt and nickel. The properties of the basic coating may be further improved by suitable elemental additions. In this work, standard CoNiCrAlY coatings were augmented by the addition of 4·5 or 3 wt-%Ta. The coatings were deposited onto two different substrates, IN 100 and MAR M002, then oxidised at 1000 and 1050°C for up to 500 h. It was found that the addition of tantalum to the coating had a significant effect upon oxidation resistance only if the superalloy substrate was present. The tantalum within the coating was found to react, with titanium and carbon moving into the coating from the substrate with the formation of (Ta,Ti) carbides of the MC type. This reduced the amount of titanium in solid solution within the alumina scale and thus improved the oxidation resistance. The morphologies of particles within the coatings and oxide sc...

A combination of selected-area electron diffraction and the new technique of `real-space crystallography' has been used to monitor the electron-induced decomposition of tin (IV) sulfide. In contradiction to the results of other workers, the final product of the decomposition was found to be SnS, while at an intermediate stage, platelets of SnS were formed with (010)SnS parallel to (001)SnS2.

In the course of a programme of investigating van der Waal's solids by low temperature transmission electron microscopy, dislocations, stacking faults and twins have been observed in the rare gas solids, β-O2 and α-N2. Calculations of the relative stability of different crystal structures of these solids have also been performed. Stacking faults have been observed in solid xenon and the width of dislocations and extended nodes used to estimate both the stacking fault energy and the relative energy difference between the h. c. p. and f. c. c. forms of xenon. A theory of this energy difference has been given which is in order of magnitude agreement with the observations. The structure of β-O2 is rhombohedral, and can be visualised as an ABC packing of parallel O2 molecules. Twins and stacking faults on the basal (0001) plane are frequently observed, which arise because of the low energy difference between ABC and ABAB packing of the molecules. Twins on (1014) are also observed frequently and their energy is estimated. The low temperature phase α-N2 has been shown to have the Pa3 cubic structure, in which the structure is f. c. c., except that at each of the four lattice points, the molecular axis points in a different direction. Twins, but not stacking faults, are readily formed, and electron diffraction and microscopy experiments indicate that these are probably type II twins. Calculations have indicated that the structures of β-O2 and α-N2 may be understood qualitatively in terms of dispersion plus quadrupole forces, and extension of these calculations to understand defect configurations is underway.

IN this letter we report further observations made during our development of a new electron diffraction technique, which was introduced under the title of ‘;Real Space Crystallography’;1. This technique involves the use of bend contour patterns, which are produced in thin domed or saucer-shaped foil specimens when an aperture is inserted in the back focal plane of the objective lens of an electron microscope. Since the symmetry of these patterns is used to reveal the symmetry elements of crystal structures, we have now examined the effect of faults on the contour patterns. The sensitivity of the pole figures to changes in specimen thickness and accelerating voltage has also been studied.

We have discovered an image plane technique of crystallographic analysis which achieves spatial resolution of structural defects and diffraction information, reveals a lack of a centre of symmetry, requires only minute crystals and is rapidly executed.

Still higher resolution and miniaturization were the goals in instrumental development for the transmission electron microscope (TEM). This was the message of the speakers at the sixth European congress on electron microscopy which was held in Jerusalem last September.