Pure aluminum deformed in torsion (shear) at elevated temperatures reaches a broad "peak" stress and then undergoes about a 17% decrease in flow stress with deformation to roughly 1-2 equivalent uniaxial strain. Beyond this strain the flow stress is approximately constant. The sources for this softening are unclear. The suggested basis includes texture softening, microstructural softening, and enhanced dynamic recovery. Experiments were performed where specimens were deformed in torsion to various strains within the softening regime followed by compression tests at ambient and elevated temperature. Analysis of the compressive yield strengths indicate that the softening is at least substantially explained by a decrease in the average Taylor factor due to the development of texture. / Graduation date: 2002

M. S.

M. S.

This investigation was involved with the combustion of 0.020" diameter aluminum in various mixtures of industrial oxygen and nitrogen at a total pressure of one atmosphere. A combustion chamber was constructed similar to that which was utilized in previous investigations involving titanium, zirconium, iron and molybdenum wires. Prior investigations of the combustion of metal wires were made using conventional high speed motion picture photography to burning rate data. Because combustion of aluminum evolves oxide smoke which obscures motion picture vision, a method utilizing an electronic counting circuit and infrared sensitive cadmium sulfide photo conductive cells in lieu of motion picture photography was developed. Burning rates of O.020" diameter titanium wire were determined for comparison with published information from the previous investigations. This data compared favorably with the published information. It was found that sustained combustion of the O.020" diameter aluminum test wire occurred only in an atmosphere of 90 per cent to 100 per cent industrial oxygen. Burning rates for this mixture range are presented. With mixtures between 70 per cent and 90 per cent industrial oxygen, it was found that the melting rate of aluminum exceeded the burning rate, drastically limiting sustained combustion and causing rapid extinguishing of any flame which did occur. Below an atmosphere mixture or 70 per cent industrial oxygen ignition could not be obtained. / Master of Science

Orientador: Nilson Cristino da Cruz / Resumo: O alumínio é um dos materiais mais utilizados em vários setores da indústria. Devido ao baixo custo, se comparado ao cobre, e considerando sua abundância de oferta, o interesse nas propriedades térmicas do alumínio tem aumentado. Neste contexto, a proposta desta pesquisa é avaliar as propriedades térmicas de amostras de alumínio tratadas por oxidação eletrolítica plasmática (PEO), em eletrólitos de silicato de sódio, utilizando espectroscopia fotoacústica. A rugosidade, morfologia e a composição química dos revestimentos foram analisados, respectivamente, por perfilometria, Microscopia Eletrônica de Varredura (MEV) e Espectroscopia de Energia Dispersiva (EDS). A porosidade da camada superficial do revestimento foi estimada pelo software de processamento de imagem digital, utilizando imagens da análise de MEV, enquanto difração de Raios X (DRX) foi utilizada para determinar a estrutura cristalina das superfícies tratadas. Os resultados das análises demonstraram que os revestimentos, predominantemente constituídos por γ-Al2O3, são rugosos, com superfícies porosas e apresentam boa resistência mecânica. A espectroscopia de absorção de UV-VIS demonstrou que a amostra tratada absorveu em média 18% mais radiação na região infravermelha quando comparada com amostras sem tratamento. Em consequência, como revelado pela espectroscopia fotoacústica a difusividade térmica das amostras tratadas é pelo menos 30% maior se comparada ao alumínio não tratado e 700% maior do que a alumina conven... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: Aluminum is one of the most widely used materials in several areas of the industry. Owing to lower cost, if compared to copper, and considering its abundance, the interest on the thermal properties of aluminum has increased. In this context, the proposal of this research is to evaluate the thermal properties of aluminum samples treated by Plasma electrolytic oxidation (PEO), in sodium silicate electrolytes, using photoacoustic spectroscopy. Roughness, morphology and chemical composition of the coatings have been analyzed, respectively, by profilometry, scanning electron microscopy (SEM) and X-ray energy dispersive spectroscopy (EDS). The porosity of the surface layer was estimated by digital image processing using SEM micrographs while X-ray diffraction (XRD) was used for determining the crystalline structure of the treated surfaces. The results of the analyses have shown that the coatings, predominantly constituted by γ-Al2O3, are rough, with porous surfaces and present good mechanical resistance. UV-Vis absorption spectroscopy has shown that the treated sample absorbed on average 18% more radiation in the infrared region than the pristine samples. In consequence, as revealed by photoacoustic spectroscopy the thermal diffusivity of the treated samples is at least 30% larger if compared to untreated aluminum and 700% larger than that of conventional Alumina / Mestre