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Thesis (Ph. D.)--Oregon Graduate Center, 1988.

In this investigation, the resistivity of vanadium nitride at elevated temperatures has been measured using a resistance bridge based on the Dauphinée/Mooser (1955) chopper system. In general, an almost linear increase of resistivity with temperature was observed for VN, but at certain temperatures, deviations from linearity (anomalies) were observed, these have been classified into three types. Attempts have been made to correlate the anomalous behaviour and temperature coefficients of resistivity with the ambient atmospheres and with the impurities, (especially oxygen), stoichiotmetry and density of this material. Oxygen and excess nitrogen appear to be electron donors in their effects on VN and result in a decrease in the slope of resistivity vs temperature curves. Oxygen and excess nitrogen also appear to increase the overall resistivity due to an increase in defect concentration. No effects due to the ambient atmosphere have been observed in the case of hot-pressed samples. lndicating that reaction can only occur if the reacting gases can reach the interior of the samples.

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

Call number: LD2668 .T4 1950 L44 / Master of Science

The ternary Pt-Al-Cr system was investigated as part of the continued development of a thermodynamic database for the Pt-Al-Cr-Ru system. Scanning electron microscopy with energy dispersive X-ray spectroscopy and X-ray diffraction analyses were used to obtain phase equilibria data. The alloys were studied in the as-cast condition, as well as after annealing at 600°C and 1000°C respectively. A solidification projection was constructed and a liquidus surface derived. Isothermal sections at 600°C and 1000°C were also determined. It was concluded that all phase regions were identified correctly since the results were selfconsistent. Three ternary phases were found and 19 ternary invariant reactions identified. A thermodynamic database was developed for the Pt-Al-Cr system using Thermo-Calc. Phase relations could be reasonably accurately predicted between 600°C and 1000°C, and even up to temperatures close to the melting point. However, the match between the calculated and experimental diagrams could be improved. As with the Pt-Cr-Ru system, problems with the constituting binary systems seemed to be the major cause for problems encountered in the modelling. Only once the Al-Pt and especially the Cr-Pt and Cr-Ru binary phase diagrams are confirmed more rigorously, the calculated ternary phase diagrams could be worked on with more confidence. More than half of the alloys investigated had hardnesses in excess of 600 HV10 regardless of their state of heat treatment. Based on the examination of hardness indentations, alloys in the Pt-Al-Cr system were also often brittle due to the presence of hard intermetallic compounds. Alloys containing ~Pt3Al showed better behaviour with regard to toughness which was encouraging for the Pt-based alloys that are being developed by Mintek.

A thesis submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, in fulfilment of the requirements for the degree of Doctor of Philosophy in Engineering / This study investigated the effect of microstructure on properties of selected Pt-based alloys. Six alloys of different compositions were analysed after heat treatment at 1500°C for 18 hours, followed by quenching in water; then annealed at 1100°C for 120 hours and air cooled, equivalent to a potential industrial specification. Microstructural characterisation utilised OM, SEM, AFM, TEM and EDX. Further characterisation was carried out using a nanoindentation hardness tester for nanohardness and elastic modulus measurements. The research focus was to characterize the different morphologies of γ′ ~Pt3Al precipitates during a single heat treatment, and to understand the nano-mechanical properties of the γ′ precipitates and γ (Pt) matrix, taking their proportions into account. In the present work, the samples were successfully etched, (which was not possible before) allowing optical microscopy and SEM to give much clearer microstructures than previously. The precipitate volume fractions were measured from SEM and AFM images, and agreed well. The γ′ volume fraction (expressed as percent) of nominal Pt78:Al11:Cr6:Ru5 (at.%) alloy was 51 ± 6% (SEM) and 57 ± 10% (AFM), while for nominal Pt85:Al7:Cr5:Ru3 (at.%) it was 45 ± 6% (SEM) and 48 ± 8% (AFM). A comparison of the γ′ volume fractions obtained from TEM showed that, compared to SEM, as the γ′ volume fraction observed with SEM increased, the γ′ volume fraction measured in TEM increased, although the TEM volume fraction results are believed to have considerable error due to TEM only revealing the microstructure of relatively small regions compared to SEM. Comparing with Pt-Al-Cr-Ni alternatives with γ′ volume fractions of 51-57%, the nominal Pt78:Al11:Cr6:Ru5 and nominal Pt85:Al7:Cr5:Ru3 (at.%) alloys have comparable γ′ volume fractions within, experimental error, and are considered as promising. From a microstructural viewpoint, these alloys were identified as the most promising. TEM revealed that at the specific heat treatment there were multiple size ranges of γ′ precipitates. The ~Pt3Al precipitate structure was found to be cubic L12, rather than tetragonal. The orientation relationship between the γ matrix and γ′ precipitates was found to be [114]M||[114]P; [001]M||[001]P; [103]M||[103]P. The nano-mechanical properties of the γ matrix and γ′ precipitates of Pt-Al-Cr-Ru alloys were investigated for the first time. At 2.5mN, it was possible to measure mechanical properties inside the individual γ′ precipitates and γ matrix channels, and in all six alloys the γ′ precipitates were the harder phase. The hardness of γ´, γ and the overall alloy was a function of the Pt content, and the hardness of the overall alloy was also a function of the Al content. The overall alloy hardness for nominal Pt85:Al7:Cr5:Ru3 (at.%) was 9.0 ± 0.3GPa and 9.2 ± 0.3GPa for nominal Pt78:Al11:Cr6:Ru5 (at.%). The new findings on image analysis showed that the precipitate volume fractions of nominal Pt78:Al11:Cr6:Ru5, nominal Pt85:Al7:Cr5:Ru3 and nominal Pt78:Al11:Cr8:Ru3 (at.%) were comparable to commercial nickel-based superalloys (NBSAs). TEM has shown that the precipitate morphology was similar to that of NBSAs, while nanoindentation studies indicated that the Pt-Al-Cr-Ru alloys’ overall, γ and γ phase nanohardnesses and elastic moduli were also similar to NBSAs. These results were encouraging, since the NBSAs already have commercial applications. Thus, more research efforts are encouraged on the Pt-Al-Cr-Ru alloys in order to further improve the properties of these alloys.

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