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No description available.

Structures and growth kinetics of oxide films formed on molten aluminium have been studied to provide. information in the context of melt losses and melt quality. Consideration of water vapour potentials at the boundaries of oxide films predicts that. in air-of normal humidity, a slightly hydrated transitional alumina (n - A1203) is stable at the oxide/atmosphere interface, and the dehydrated a- A1203 is stable at the metal/oxide interface. Structures observed in oxide films lifted from, the-liquid metal are consistent with this concept. The morphology of alpha-A1203 growing at the metal/oxide interface depends on the impurities incorporated within the oxide, and according to circumstances it can appear as degenerate dendritic shapes or plate like structures. The oxidation kinetics, in moist-oxygen, are characterised by a rapid initial rate during which n- A1203 plays a dominant but diminishing role, followed by much-slower kinetics after the alpha- Al2O3 subfilm is fully established and exercises rate control. Oxidation kinetics in dry. oxygen of samples obtained from high purity aluminium conform to theiparabolic growth law after an initial period during which linear kinetics are observed. The kinetics of samples obtained from commercial purity aluminium are much simpler and conform to the parabolic growth law.

Metal surfaces, and particularly steels, may be modified by processes which include; plastic-deformation, chemical changes and heating. These layers are often characterised by high hardness and a pronounced resistance to chemical etching. This latter characteristic giving rise to the generically descriptive term "White Layer", which is often applied to such features. Processes which may result in the formation of "White Layer" can be broadly separated into three groups; thermal, chemical and mechanical. In practice, "White Layers" observed on materials removed from service environments have generally experienced a combination of these processes. In this work white-etching layers formed on engineering steels have been characterised, using a variety of electron beam and X-ray analytical techniques, to establish the chemical nature and structural properties of "white layer" material. Specimens drawn from such diverse service applications as; high-pressure tank gun barrels, digger-teeth from gravel extraction plant and adiabatically-sheared armour steel have been compared with samples produced in the laboratory by conventional pin-on-disc wear testing apparatus, specialised machining techniques and laser surface-hardening heat-treatment. The presence of a hard white-etching layer on each of the samples was first established using metallographic examination by light microscopy and microhardness testing to confirm the etch-resistance and high hardness of the white-layer. The chemical composition of the whiteetching layer was then compared with the bulk steel composition using scanning electron microscopy and conventional microprobe analysis ( Z > 11 ), and no significant differences were observed. Light element microprobe analysis and SIMS showed an increase in H, C, N, 0 in the white-etching layers formed within gun barrels, and on abusively turned steel. The levels detected were not considered to be significant in terms of white-layer formation but may well have an influence on strain aging and embrittlement phenomena. A technique for the preparation of cross-sectional thin foils was developed which allowed the structure of the white-etching layers to be compared with the underlying matrix by TEM and electron diffraction. The results of this study showed clear similarities between white-etching layers formed by wear and abusive machining and those formed by adiabatic shear. From these results it is concluded that " white layer " is a fine grain martensite which forms on steel as a result of thermo-mechanical transformation.

A survey of the methods of observing Barkhausen noise was made and a suitable system of measurement Identified for laboratory measurements to be carried out on steel samples. A coil system to measure the Barkhausen noise from samples was constructed. The noise was sampled and recorded to microcomputer by means of a transient recorder. The experimental parameters were selected to optimise the data. A Fast Fourier Transform was performed on the noise data by means of a library routine on a mainframe computer and the results plotted as the power spectra of the data. The recorded voltage profile of the noise was also plotted. The system was verified by use of grain oriented silicon iron samples. The power spectra and voltage profiles were plotted and, when compared with the results of other workers, were found to be favourable. Tests were performed on constructional steel samples. The power spectra and voltage profiles from the constructional steels were considered with the coercivity and remanence of the steels and a relationship sought. No simple general relationship was identified although empirical relationships were suggested for some of the data. A knowledge of the microstructure of the steel is required to explain some of the results. The power spectra and voltage profiles were assessed in terms of the hysteresis loop shape and influence of microstructure on the value of coercivity. It was found that these were the influencing factors in the Barkhausen noise. The effect of plastic deformation on power spectra was investigated. Results of an industrial investigation were reported.

Chrome residue, the disposal product of a chrome ore pyrometallurgical extraction process, contains a number of valuable metals in oxide form. The bulk extraction of the main metals in residue is investigated by acid dissolution, followed by solvent extraction techniques for the selective recovery of the metals from solution. Efforts have been focused on screening for Cr(III) extractants, and parameter optimization for maximizing extraction efficiency and selectivity of suitable extractants. The implications for a potential process are discussed. This thesis presents laboratory results of studies to determine optimum dissolution and selective metal extraction conditions and reagents.

This thesis describes an investigation of the magnetic properties of high tensile steels typical of those produced for the high pressure gas pipe-line industry. Results are presented for both bulk and small section samples, and the observed variations in magnetic behaviour as a function of orientation and position within the steel are described qualitatively by changes in steel metallography. The development of an automated double-crank Vibrating Sample Magnetometer, required for determining the magnetic characteristics of small samples, is also described which, without signal amplification, has a moment detectability limit of 10(^-4) e.m.u. The representation of the full magnetization loop by a Fourier series is investigated and the variations in harmonic amplitudes found for the range of steels considered here are compared to those predicted by theoretical models. The successful parameterization of the initial magnetization curve is also reported using a two parameter model (LnB = (k – H(^-1))LnA)), and linear relationships between the coercive field (H(_c)) and these parameters (k,LnA) are presented which permit the prediction of the initial magnetization curve of any similar steel from a knowledge of H(_c). Although the latter may be determined accurately by direct measurements of small samples, further linear relationships are indicated which allow the determination of the coercive field from knowledge of either the steel chemistry or metallography. The compatibility of the observed ferrite grain size dependent contribution to the coercive field with grain boundary domain wall pinning models is also investigated.

A consideration of pertinent aspects of general electro- chemistry and electrodeposition was followed by a review of the relevant literature relating to hard chromium plating technique and understanding (and problems and proposed improvements therein). Following the establishment and evaluation (using a Standard Solution at 55 0 C) of a reproducible technique for the measurement of (low current density) partial cathodic polarization data during plating, the Tafel regions of the resulting curves were analysed using the relationship - dn Cp -2.303 kT to provide an indication as to the nature of the rate-determining step in the plating and hydrogen evolution (cathodic) reactions. Information thus generated was used with a modified Arrhenius- type expression and further (experimentally determined) data to provide values of the thermodynamic constants of activation for the plating reaction. A series of experiments were undertaken to determine the effects upon various (physical) plate and plating parameters of three new inorganic additions (at selected concentrations) to a Standard Solution at 55 0 C. The results of these tests were commented upon with regard to potential industrial applications. The electrochemical investigative techniques referred to earlier were then employed with each new solution tested to determine the effect (if any) of the new additions upon those aspects of the deposition mechanism yielded, and to relate these effects to the observed (corresponding) changes in plate and plating parameters. The new additions were found to have profound effects upon a number of (physical) plate and plating parameters, and upon experimentally determined theoretical aspects of the deposition mechanism (the relationship between the two is discussed). Whilst no definite conclusion has been reached with regard to the precise mechanism of chromium plating, the techniques employed and the data generated have been used to provide significant pointers with regard to the direction future investigations might take.

No description available.

No description available.