Ultraviolet photoemission spectroscopy has been employed to investigate the electronic density of states of Heusler and Cu3Au-type intermetallic compounds. Cooper minima and resonant photoemission effects have been utilised to determine the contribution of the constituent elements to the valence band spectra. For the Mn-based Heusler alloys Co2MnSn, Cu2MnAl and Pd2MnSn, the Mn 3d-derived states disperse across the full width of the valence band.

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M.Ing. (Civil Engineering) / Please refer to full text to view abstract

The effect on microstructure and strength properties of hypereutectic aluminum - 20% silicon alloys of various refining additions has been studied by means of metallographic observations, hardness tests and a quantitative determination of the interparticle spacing for the primary silicon. A mechanism has been proposed to explain the observed coarsening of the eutectic which accompanies primary silicon refinement, and the combined effect of the improvement of primary dispersion and increased eutectic coarseness on strength properties has been discussed. Refining mechanisms for the refining additions used have been suggested and the reasons for the observed effect of the various refining additions on the shape of primary silicon have been presented. An explanation is offered for the increase in primary refinement which results from an increase in the time the alloy is held molten after fluxing with phosphorus penta-chloride. The apparently anomalous behavior of phosphorus-copper refined alloys has been explained on the basis of the degree of dissemination of the phosphorus in the molten alloy. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

An investigation was conducted on the mechanisms of bond formation between alloys of nickel and single crystals of alumina. Nickel-titanium, nickel-chromium, and nickel-zirconium powder mixtures were cleaned with purified hydrogen gas at 800°C and were individually melted under vacuum (10⁻⁵ m.m. of Hg) in contact with alumina. Interfacial energy measurements at 1500°C were made by the sessile-drop method. The bond surfaces were examined by X-ray fluorescence and X-ray diffraction techniques. The bond formation in all cases appeared to involve two basic mechanisms - metal solute segregation and interfacial reaction. By interfacial measurements and X-ray fluorescence analyses, the solute atoms, titanium and chromium, were shown to be selectively adsorbed at the metal-ceramic interface. Interfacial reaction products were detected by X-ray diffraction methods. The adsorbed titanium reacted with the alumina to produce an interfacial layer of alpha titanium sesqui oxide (⍺-Ti₂0₃). Adsorbed chromium similarly reacted with the alumina to form an interfacial compound. However, this compound could not be identified. With nickel-zirconium alloys, the violence of the reaction between zirconium and alumina made experimental measurements impossible. / Applied Science, Faculty of / Mining Engineering, Keevil Institute of / Graduate

The system Zn-0.2 wt. % Al has been investigated to determine under what conditions of strain rate, grain size and temperature the phenomenon of superplasticity may be observed. The analysis and experimental conditions were based on established procedures which have been applied to known superplastic alloys. However the continually decreasing strain rate and grain growth during testing complicate the normal analysis. Based on this study the requirements for superplastic behavior are a fine-grained microstructure, grain boundaries which are relatively free of obstructions and a homolohous temperature of at least 0.42. A model incorporating grain boundary shear and non-continuous grain growth has been proposed to account for the observed superplastic behavior where grain boundary migration is the rate controlling process. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

Aluminum alloys are widely used in aircraft industry for their strength and light weight. Those alloys that are hardened by precipitation, especially the Copper-rich of the 2000 series, are prone to corrosion and are protected against it using chromate containing coatings. The primary component of these coating systems is Chromium 6+ (CrVI) that has been found to be very toxic in the environment and carcinogenic, toxic and mutagenic in humans. The Mg-rich primer development is the result of a successful multi-year project funded by the US Air-force with its objective the replacement of coatings based on CrVI with a class of coatings less toxic and with comparable protective performances. The Mg rich primer fulfilled the USAF requirements and it is currently undergoing commercial and military qualifications testing. The use of Mg as one of the active pigments in coatings allows the primer to protect the underlying Al sacrificially, not considered possible for this substrate until now. Mg is anodic to most of the other structural metals and when particulate Mg became available commercially, the concept of the primer was first developed by analogy to Zn-rich coatings for steel. When Mg and Al are in contact and immersed in a corrosive environment, magnesium corrodes preferentially and protects the aluminum.

A Dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, for the Degree of Master of Science / A simple three-dimensional model of a high y' volume fraction Ni-based alloy is developed. The model is based on the idea that a unit volume in the superalloy can be represented by a cuboidal precipitate with thin 'slabs' of matrix material 'stuck' onto its faces in such a way that coherency is maintained. Rafting is investigated by considering the changes in total energy whon the cuboidal precipitates start to flatten into plate shapes or lengthen into rod shapes. It is assumed that inelastic effects are negligible. Internal and external stresses and strains are determined. Expressions for the derivative of the total energy with respect to a shape parameter are then calculated in the absence of applied stress and in the presence of applied stress. Predictions of rafting behaviour are made for six alloys. Excellent agreement is found with experimental evidence but it is suggested that the agreement is fortuitous. / AC 2018

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