Alan G. Fox

Spinodal decomposition of Al-rich Al-Li alloys

Abstract The microstructure of a series of as-quenched and aged Al-Li base alloys has been studied by high resolution transmission electron microscopy (HREM) and by X-ray diffraction. In alloys which had been ice brine quenched and room temperature aged for 168 h the HREM studies showed positive evidence for spinodally decomposed Al-rich δ′ Al3Li (Ll2 structure) with modulated order/disorder regions of varying lithium content. The 100 and 110 superlattice reflections associated with the ordered regions were detected by X-ray diffraction, but were weak and highly broadened due to the order modulation associated with the spinodal. Appropriate X-ray intensity measurements allowed a determination to be made of the overall degree of long-range order, which was found to be about half the maximum possible for each alloy had they been fully ordered. Further HREM studies on under, and overaged, samples indicated that the ordered δ′Al3Li precipitates which develop in a disordered matrix in Al-Li alloys arise from lithium enrichment of the ordered regions and further lithium depletion of the disordered regions in the as-quenched spinodally decomposed alloys, rather than by nucleation from a supersaturated solid solution. These results are in agreement with the latest theoretical ideas on this topic, and suggest that Al-Li base alloys are intermetallic compounds rather than primary solid solutions. This view is, of course, consistent with the fact that the elastic moduli of Al-Li base alloys are significantly higher than that of pure aluminum.

Is it feasible to determine the bonding charge density of stoichiometric γ-TiAl through structure factor measurements?

Abstract All the experimental methods for the measurement of structure factors are reviewed with emphasis on determining the electron charge-density distributions of intermetallic alloys, in particular γ-TiAl. The techniques discussed include X-ray diffraction, X-ray pendellosung methods, γ-ray diffraction and high-energy electron diffraction (intersecting Kikuchi line, critical voltage and convergent-beam rocking-curve procedures). All these methods (except for electron diffraction) require high-quality single crystals which seem impossible to prepare for equiatomic γ-TiAl. However, the critical-voltage electron diffraction measurements presented in this work indicate that the 001, 110, 111, 200, 002, 220 and 202 structure factor amplitudes of stoichiometric γ-TiAl can be determined with sufficient accuracy for a charge-density study provided that great care is taken.

Hot sodium sulphate corrosion of a Nicalon silicon carbide fibre-reinforced lithium aluminosilicate glass-ceramic matrix composite

The corrosion products arising from the exposure of a Nicalon silicon carbide fibre-reinforced lithium aluminosilicate glass-ceramic matrix composite to molten sodium sulphate at 900 °C for 100 h in both oxygen and argon atmospheres were studied by X-ray diffraction (XRD) and scanning and transmission electron microscopy (SEM and TEM respectively). The microstructure of the as-received composite plates was found to be similar to that reported by other workers. The matrix consisted of grains of close to stoichiometric mullite and β-spodumene and a high silica glass with 20–50 nm wide fibre-matrix interfaces comprising a layer of turbostratic carbon and amorphous silica. The effects of hot sodium sulphate corrosion were found to be very similar in both argon and oxygen but proceeded at a much greater rate in the latter case where it had progressed 100 μm into the composite and consumed many fibres. XRD studies indicated that mullite had virtually disappeared in the corroded region and this was confirmed by SEM. TEM studies of thin sections cut from near the end of the corroded zone also showed that the matrix had become a very fine mixture of glass and β-spodumene grains and that the fibre-matrix interface region had grown to ca. 600–800 nm wide. The microstructure of this corroded interface comprised several alternating layers of turbostratic carbon, mixed carbon and amorphous silica and pure carbon, each with widths varying between ca. 100 and 200 nm. This layered structure apparently developed as a result of oxidation of the silicon carbide fibre in the presence of a gradient of oxygen partial pressure that decreased from the matrix across the interface to the fibre.

A powder x-ray diffraction study of a solution treated and ice brine quenched Al-14.25 at.% Li alloy

These results suggest that the microstructure of this as-quenched alloy comprises ordered regions of about 4 nm in size in a sea of disordered matrix with a very small amount of δAlLi present. This conclusion is in excellent agreement with recent small angle x-ray and transmission electron microscope studies on similar alloys

The low-angle atomic scattering factors and charge density of zinc

Abstract The 0002 and low-angle X-ray atomic scattering (form) factors of zinc have been accurately measured by the systematic critical voltage (V c) method in high-energy electron diffraction. An indication of the value of the form factor was also obtained by this technique. These results are in good agreement with recent X-ray Pendellosung data for this element and charge difference maps based on this information indicate that bonding in zinc is achieved by the asymmetric depletion of electrons from atomic sites in 〈210〉 directions and redistribution such that there is a maximum build-up of charge density around the mid-point between third nearest-neighbour atoms in the basal plane. This charge distribution is entirely consistent with the long c/a of zinc (1·856), and a comparison with the equivalent charge densities for beryllium (c/a = 1·58) suggest that there is a simple correlation between bonding mechanism and c/a ratio for these hexagonal elements.

Low-angle atomic scattering factors and charge density of aluminum

Abstract All the published experimental and theoretical atomic scattering (form) factors for aluminum have been carefully reviewed and only the low angle 111, 200 and 220 crystal form factors appear to be significantly different from the best free atom values. The best powder X-ray diffraction and high accuracy 111 and 200 electron diffraction form factors for Al are in good agreement with each other and with the best theoretical calculations. The equivalent scattering factors measured by X-ray Pendellosung methods seem somewhat high and are not a good representation of the charge density of aluminum. A (110) deformation charge density map based on the best experimental form factors indicates that bonding in aluminum is achieved by the spherical depletion of electrons from atomic sites and redistribution so that there is a build-up between nearest-neighbor (n.n.), second n.n. atoms, etc; this, of course, is the traditional plcture of a metallic bond. A comparison of the aluminum (110) deformation density map with the equivalent map for copper shows that the bonding schemes for the two elements follow an almost identical pattern. This indicates that even though copper and aluminum have large differences in atomic number and valency, their bonding schemes are primarily determined by the face-centered-cubic crystal structure.

An Investigation of Hardness Homogeneity and Microstructure in Pure Titanium Processed by High Pressure Torsion

High-pressure torsion (HPT) was conducted on disks of commercial purity Ti under applied pressures of 3 and 6 GPa. Measurements of the Vickers microhardness showed improving hardness homogeneity with increasing numbers of HPT turns. Transmission electron microscopy demonstrated that a higher HPT pressure leads to a smaller grain size after straining and these grains contain a high dislocation density with arrays of twins. This is consistent with the higher hardness of the Ti samples processed by HPT under 6 GPa pressure.

On the determination of the Debye-Waller factor and structure factors of NiAl by X-ray powder diffraction

A conventional powder X-ray diffraction experiment was performed on close-to stoichiometric β-NiAl (B2 structure) using a Cu target. From the integrated intensity data the lattice parameters and room temperature Debye-Waller factors of β-NiAl were determined. During the analysis of the integrated intensities, great care was taken to ensure proper treatment of extinction which was found to depend on the mosaic crystal size rather than the powder particle size. Other factors such as dispersion, thermal diffuse scattering and preferred orientation were found to have less of an effect on the integrated intensity analysis. As a result of this improved analytical procedure, Debye-Waller factors for β-NiAl were determined which are in close agreement with the accurate values obtained by single crystal X-ray and electron diffraction. This is in contrast with previous powder X-ray diffraction work on β-NiAl, where poor values of the Debye-Waller factor were usually obtained due to the failure to correct for extinction. The effects of bonding in the solid state were also examined and it was found that the use of accurate crystal structure factors for β-NiAl in the analysis as opposed to those calculated from free atom scattering factors had no significant effect on the experimental Debye-Waller factor values.

The effect of inter-layer diffusion on magnetic exchange spring behaviour

The effect of inter-layer diffusion between the magnetically hard and soft layers in magnetic exchange spring systems is examined, using 1D and 2D models. It is shown that diffusion across the hard/soft interfaces leads to an increase in the bending field BB. This increase eventually saturates when the bending field BB and the coercivity BC merge. Moreover, if the increase in the bending field BB is large enough, the nature of the magnetic reversal can be affected. This behaviour is illustrated using a YFe2 dominated YFe2/DyFe2 exchange spring system. In this case the 1D model predicts that inter-layer diffusion can drive a magnetic phase change, from negative to positive coercivity. Discrete 2D model calculations of inter-layer diffusion are also presented and discussed. The latter support the predictions of the 1D model. Finally, while the emphasis is on atomic diffusion, some comments are made concerning interface roughness.

Magnetic anisotropy basis sets for epitaxial (110) and (111) REFe2 nanofilms

Magnetic anisotropy basis sets for the cubic Laves phase rare earth intermetallic REFe2 compounds are discussed in some detail. Such compounds can be either free standing, or thin films grown in either (110) or (111) mode using molecular beam epitaxy. For the latter, it is useful to rotate to a new coordinate system where the z-axis coincides with the growth axes of the film. In this paper, three symmetry adapted basis sets are given, for multi-pole moments up to n = 12. These sets can be used for free-standing compounds and for (110) and (111) epitaxial films. In addition, the distortion of REFe2 films, grown on sapphire substrates, is also considered. The distortions are different for the (110) and (111) films. Strain-induced harmonic sets are given for both specific and general distortions. Finally, some predictions are made concerning the preferred direction of easy magnetization in (111) molecular beam epitaxy grown REFe2 films.