Aluminum 7xxx series alloys have been a major focus for lightweight automotive structural components to achieve the targeted weight reductions by auto industries and, in turn, to increase the vehicle fuel efficiency. AA 7xxx series wrought alloy components have been widely used by the aerospace and automotive industries for many decades due to their low density and high strength. However, although near net shape casting of these alloys has many benefits versus wrought alloys; this processing route has been a challenge for the researchers and the auto industry because of limitations due to castability issues such as like hot tearing and die soldering. One possible mitigation strategy involves the addition of iron (Fe) as one of the major alloying element and then subsequently optimizing the alloy chemistry and solidification parameters in terms of die soldering. The objective of this thesis is to determine the microstructural evolution of any Al-Fe intermetallic phases with respect to cooling rate during solidification for a range of AA 7xxx series alloy compositions. Fe was added at three different levels in a total of nine alloy composition developed from a Taguchi experimental matrix based on the interaction of three composition levels for four alloying elements. The alloys were cast using a custom built casting rig while the cooling rates were measured along the length of a directionally solidifying sample. The thermocouple measurements were analyzed to determine the velocity of the solid/liquid interface, overall cooling rate and thermal arrest points for later correlation to variations in the microstructural development of any Al-Fe intermetallic phase particles present in the experimental alloys. Metallographic samples were taken at locations with iv known cooling rates to determine the resultant microstructure. Scanning electron microscopy (SEM) and energy dispersive X-Ray spectroscopy (EDS) were performed to obtain elemental analyses of the Al-Fe intermetallic phases for present in the samples. The Fe maps obtained by EDS were processed and analyzed using Image-J software to determine the size distribution and area fraction of the Al-Fe intermetallic phases as a function of alloy composition and solidification rates. Also, a regression analysis was used to develop a statistical model to predict the variation of intermetallic particle size and area fraction of the Al-Fe intermetallic phases as a function of alloy composition and cooling rate. Based on the experimental investigation and analysis of the nine Al 7xxx-Fe alloys the results can be summarized as follows: (1) Cooling rate has a strong influence on the chemistry and morphology of the Fe intermetallic particles: It was determined that the dominant intermetallic species changes from the equilibrium Al3Fe to the metastable Al6Fe alloys for cooling rates in excess of approximately10 °C/s. (2) Alloy cooling rate does significantly affect the area fraction of the Fe intermetallic particles. It was determined that the morphology of the Al-Fe particles transitions from a relatively low aspect ratio particles to a high aspect, needle-like particles for cooling rates less than approximately 10 °C/s. (3) Alloying elements such as Zn, Cu, and Mg does not influence the Fe intermetallic chemistry and the area fraction of the intermetallics. / Thesis / Master of Applied Science (MASc)

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Made available in DSpace on 2014-06-11T19:27:11Z (GMT). No. of bitstreams: 0 Previous issue date: 2006-06-30Bitstream added on 2014-06-13T18:55:39Z : No. of bitstreams: 1 souza_ga_me_guara.pdf: 3512611 bytes, checksum: e67ec93171ad6e8015c80fa5a52bf34e (MD5) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Este trabalho investiga os constituintes de um aço médio carbono e baixa liga com estrutura bainítica/martensítica, obtida através de tratamento isotérmico em baixas temperaturas, a 336 ºC e 286 ºC (próximas à temperatura de transformação martensítica), utilizando-se da microscopia óptica e de processamento metalográfico com os reagentes nital, LePera, LePera Modificado e solução aquosa à base de metabissulfito de sódio, para análise qualitativa e quantitativa das fases presentes. O ataque químico com nital foi eficiente na identificação da fase bainita, revelada com coloração marrom escuro ou preta, porém tornou difícil a distinção entre as fases martensita e austenita retida. Da mesma forma, através do ataque químico LePera foi possível a observação da fase bainita na coloração marrom escuro ou preta e o constituinte martensita-austenita retida com tonalidade clara. O reagente LePera Modificado mostrou-se mais eficiente do que o reagente LePera, principalmente para estruturas constituídas quase que totalmente pela estrutura bainítica, apresentando maior nitidez na identificação da fase bainita, revelada na coloração marrom, e do constituinte formado pelas fases martensita e austenita retida (em tonalidade clara). O ataque químico com reagente à base de metabissulfito de sódio permitiu a caracterização da austenita retida (observada em tonalidade branca). Também utilizouse o reagente Contorno de Grão para revelar o contorno de grão da estrutura austenita, formada previamente na zona de austenitização. Uma combinação do reagente Contorno de Grão e reagente nital permitiu a visualização da nucleação e crescimento das ripas da bainita a partir dos contornos de grãos austeníticos. PALAVRAS-CHAVE: caracterização microestrutural, bainita, martensita, AISI 4340. / This work investigated the microstructures of the medium carbon and low alloy steel with mixed structure bainite/martensite, obtained through isothermal treatment in low temperatures (close to the temperature of martensitic transformation) to 336 ºC and 286 oC, by optical microscopy and metallographic methods with the etchings nital, LePera, LePera Modified and sodium metabisulfite reagent for qualitative and quantitative analysis of the present phases. The etching with nital was efficient in the identification of the bainite, revealed with brown darkness or black coloration; however it turned difficult the distinction between the martensite and retained austenite. In the same way, through the color tint etching with LePera reagent was possible the observation of the bainite in the brown darkness or black coloration and the martensite-austenite in the clear or straw-colored. Etching technique LePera Modified was more efficient than the LePera, mainly for microstructure constituted almost that totally for the bainitic structure, producing better color contrast in the identification of the bainite, revealed in the brown coloration, and the martensiteretained austenite (in white). The etching with sodium metabisulfite reagent allowed the characterization of the retained austenite (appears as small bright white particles). The Austenite Grain Boundary reagent was used to reveal the grain boundaries prioraustenite. A combination of the Austenite Grain Boundary reagent and nital reagent allowed the observation of the nucleation and growth of the laths of the bainite from the grain boundaries prior-austenite.

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Made available in DSpace on 2014-06-11T19:34:58Z (GMT). No. of bitstreams: 0 Previous issue date: 2007-09-10Bitstream added on 2014-06-13T19:24:02Z : No. of bitstreams: 1 anazawa_rm_dr_guara.pdf: 10651017 bytes, checksum: 16ae1ffa0b957dba762c69ee5a3bb352 (MD5) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Os aços multifásicos são de grande interesse comercial e têm atraído a atenção, principalmente, da indústria automobilística devido a combinação de alta resistência e ductilidade. A fase austenita retida, presente nestes aços, tem sido objeto de especial interesse devido ao efeito TRIP, que contribui para a melhoria da tenacidade. Neste trabalho é utilizado o aço aeronáutico 300M e, com o objetivo de estudar a influência das fases sobre as propriedades mecânicas são realizadas as seguintes etapas: a otimização de rotas de tratamentos térmicos para a obtenção de uma microestrutura multifásica; caracterização microestrutural por microscopia óptica, microscopia eletrônica de varredura e microscopia de força atômica e avaliação do envelhecimento de deformação à temperatura ambiente. A principal contribuição é a determinação da fração volumétrica da austenita retida por meio de medidas de saturação magnética através de curvas de histerese magnética. As frações volumétricas de austenita retida obtidas por medidas de magnetização são comparadas com as obtidas por difratometria de raios X e microscopia óptica após ataques químicos de metabissulfito de sódio. Os resultados obtidos por microscopia óptica e pelo método magnético apresentam boa concordância com os valores encontrados na literatura. A alteração microestrutural devido aos tratamentos térmicos aplicados permite melhorias no limite de escoamento e resistência. Analisa-se, ainda, o efeito do envelhecimento por deformação à temperatura ambiente. Os resultados são surpreendentes, ocorrendo um aumento no limite de escoamento em até 87%, devido à associação do envelhecimento por deformação com o efeito TRIP. / The multiphase steels are of great commercial interest and have attracted attention mainly of the automobile industry due to the combination of their high strength and ductility. The retained austenite phase present in these steels has been object of special interest due to TRIP effect contribution for the improvement of the toughness. In this work the influence of the phases of the aircraft 300M steel is carried out by the optimization of heat treatments routes to obtain multiphase microstructure. Microestructural characterization is evaluated by optical microscopy, scanning electronic microscopy, atomic force microscopy and analysis of the deformation ageing at room temperature. The main contribution is the determination of retained austenite volume fractions from magnetic hysteresis curves. These values are compared with x-ray diffraction data and optical microscopy after sodium metabisulfite etching, presenting good agreement with the literature. The microstructural alterations due the applied heat treatments also affect the yield strength and strength. The association of the deformation ageing and the TRIP effect resulted in a surprising increase of up to 87% in the yield strength.

Resumo: Este trabalho investiga os constituintes de um aço médio carbono e baixa liga com estrutura bainítica/martensítica, obtida através de tratamento isotérmico em baixas temperaturas, a 336 ºC e 286 ºC (próximas à temperatura de transformação martensítica), utilizando-se da microscopia óptica e de processamento metalográfico com os reagentes nital, LePera, LePera Modificado e solução aquosa à base de metabissulfito de sódio, para análise qualitativa e quantitativa das fases presentes. O ataque químico com nital foi eficiente na identificação da fase bainita, revelada com coloração marrom escuro ou preta, porém tornou difícil a distinção entre as fases martensita e austenita retida. Da mesma forma, através do ataque químico LePera foi possível a observação da fase bainita na coloração marrom escuro ou preta e o constituinte martensita-austenita retida com tonalidade clara. O reagente LePera Modificado mostrou-se mais eficiente do que o reagente LePera, principalmente para estruturas constituídas quase que totalmente pela estrutura bainítica, apresentando maior nitidez na identificação da fase bainita, revelada na coloração marrom, e do constituinte formado pelas fases martensita e austenita retida (em tonalidade clara). O ataque químico com reagente à base de metabissulfito de sódio permitiu a caracterização da austenita retida (observada em tonalidade branca). Também utilizouse o reagente Contorno de Grão para revelar o contorno de grão da estrutura austenita, formada previamente na zona de austenitização. Uma combinação do reagente Contorno de Grão e reagente nital permitiu a visualização da nucleação e crescimento das ripas da bainita a partir dos contornos de grãos austeníticos. PALAVRAS-CHAVE: caracterização microestrutural, bainita, martensita, AISI 4340. / Abstract: This work investigated the microstructures of the medium carbon and low alloy steel with mixed structure bainite/martensite, obtained through isothermal treatment in low temperatures (close to the temperature of martensitic transformation) to 336 ºC and 286 oC, by optical microscopy and metallographic methods with the etchings nital, LePera, LePera Modified and sodium metabisulfite reagent for qualitative and quantitative analysis of the present phases. The etching with nital was efficient in the identification of the bainite, revealed with brown darkness or black coloration; however it turned difficult the distinction between the martensite and retained austenite. In the same way, through the color tint etching with LePera reagent was possible the observation of the bainite in the brown darkness or black coloration and the martensite-austenite in the clear or straw-colored. Etching technique LePera Modified was more efficient than the LePera, mainly for microstructure constituted almost that totally for the bainitic structure, producing better color contrast in the identification of the bainite, revealed in the brown coloration, and the martensiteretained austenite (in white). The etching with sodium metabisulfite reagent allowed the characterization of the retained austenite (appears as small bright white particles). The Austenite Grain Boundary reagent was used to reveal the grain boundaries prioraustenite. A combination of the Austenite Grain Boundary reagent and nital reagent allowed the observation of the nucleation and growth of the laths of the bainite from the grain boundaries prior-austenite. / Orientador: Marcelo dos Santos Pereira / Coorientador: Tomaz Manbu Hashimoto / Banca: Olvério Moreira de Macedo Silva / Banca: Rosinei Batista Ribeiro / Mestre

<p> Microstructural as well as structural analysis of amygdules and groundmass of the Catoctin F·ormation located in the overturned western limb of the South Mountain anticline permit interpretation pertaining to the nature of cleavage development. Various -metliods haye been employed in the structural analysis of amygdules, in order to ascertain their original shapes and also the nature of the tectonic strain ellipsoid. A deformation path has been constructed in logarithmic co-ordinates. This appears to be an improvement over the arithmatic graph of Wood (1974). Microstructural investigation allows recognition of two distinct domains within the groundmass and amygdules : i)an early schistosity and ii)a crenulation cleavage. Microstructural and structural analysis provide a logical explanation of this history. An initial flattening episode forming the early schistosity was followed by approximately plane strain deformation creating the crenulation cleavage. </p> / Thesis / Bachelor of Science (BSc)

Various geotechnical-related energy applications, such as geothermal piles, subject soils to temperature changes. Recorded temperature variations around thermo-active infrastructure and within the active layer of the permafrost reveal the cyclic and transient nature of these temperature changes. Previous studies on the thermo-mechanical behavior of soils did not consider the effect of the temperature change rate on such behavior. Since it is widely accepted nowadays that soil behavior is rate-dependent, evaluating soil behavior under more realistic, transient temperature changes is crucial. In this dissertation, a method to calibrate triaxial cells used to expose soil samples to transient thermal loads is developed. This calibration is critical to ensure reliable thermally induced pore water pressure measurements and estimates of thermally induced volumetric strains of tested specimens. Then, thermally induced water flow and pore pressure generation under partial drainage conditions are formulated to account for the effect of temperature change rate on the thermal consolidation of cohesive soils. The formulation is performed by coupling Darcy's law for water flow in porous media with existing relations estimating thermally induced fully-drained volumetric strains. The resulting partial differential equation—the thermal consolidation theory—is solved and validated against experimental results that used the calibration from the first task. Using this newly developed theory, it was found that temperature-dependent properties of the pore water and the soil's hydraulic conductivity have a significant role in thermal consolidation. Lastly, a microstructural analysis is performed to assess the evolution of the microstructure of a normally consolidated clay under a full thermal cycle consisting of a freezing (F), thawing (T), heating (H), and cooling (C) thermal path. This microstructural investigation aims to explain the observed macroscale responses of cohesive soils under such a thermal path. After each step along the considered thermal path, the microstructure evolution was assessed using measurements of the specific surface area and pore size distribution. In the end, the variations of specific surface areas and pore size distributions were used to explain the macro-scale thermo-mechanical behavior of cohesive soils. / Doctor of Philosophy / Temperature can impact the properties of the soil, such as strength and stiffness. Besides the alterations in the strength, temperature change can cause volume change in the ground. In cold regions such as Alaska, the soil is frozen all year (i.e., permafrost) or experiences freezing-thawing cycles throughout the year. Freezing strengthens the soil but causes expansion of its volume, destroying infrastructures, including roads, runways, and buildings. Also, geothermal energy applications that utilize the ground as a heat exchanger medium may increase or decrease the surrounding soil temperature. Increasing the ground temperature changes the strength of the soil and also causes settlements. Climate change is also aggravating the situation. The temperature rises due to climate change alters the temperature pattern worldwide. Furthermore, global warming exposes frozen grounds to longer thawing stages at higher temperatures, deteriorating the permafrost. Consequently, such thermal cycles make cold regions' infrastructures susceptible to damage. Measurements of the temperature variations in the ground show that they are cyclic in nature, with different rates, maximums, and minimums. Therefore, it is essential to study the thermal behavior of soils under cyclic thermal loads. For this purpose, a new method for accurately measuring soils' response to more realistic temperature changes is developed in this dissertation. Then a model is developed to predict thermally induced volume changes and water pressures that account for the rate of temperature change. The model is then used for a sensitivity analysis to study the most important parameters controlling the deformation induced by temperature changes. It was found that variations of pore water properties with temperature and the ability of the soil to retain or drain water are the two most critical parameters that control thermally induced deformation in soils. Finally, the microstructure evolution of cohesive soils with temperature is also investigated to explain the observed alterations in soil behavior with temperature. This microstructural assessment suggests that the microstructure of soils reacts to temperature by changing the pore size distribution, shape, and number of pores.

After the invention of the high-efficiency blue light-emitting diode (LED) at the end of the twentieth century, a new generation of light-emitting devices based on III-nitrides emerged, showcasing the capabilities of this semiconductor family. Despite the current limitations in the fabrication of III-nitrides, their optical and electronic properties still place them as some of the most promising semiconductors to continue the development of optoelectronic devices. To take full advantage of the versatility offered by these materials, the fabrication of novel III-nitride-based devices demands rigorous control of all of its stages. From the initial deposition of the materials, which involves controlling the composition and size of often complex heterostructures, up to the microfabrication processing used to create a final device, any deficiency occurring will negatively impact the performance of the device. Most of the time, these deficiencies reflect in microscopic defects, hindering their detection and identification of their origin. Without such knowledge, the deficiencies cannot be fixed, stalling the improvement of the device fabrication process and, consequently, its performance. This dissertation presents a variety of methodological approaches to characterize, from a microstructural point of view, different properties of novel III-nitride-based heterostructures and devices. The characterizations include studying the structure, interface, composition, and crystalline defects of different heterostructures and evaluating the microfabrication quality of microscopic LEDs. The results of the different characterizations contributed to developing novel LED and photocatalytic devices, for example, a single-quantum-well InGaN-based red LED with a high color purity, a monolithic phosphor-free white LED, microscopic green LEDs with a size smaller than 5×5 μm$^2$, and metal oxide/GaN-based photocatalysts with improved resilience to photocorrosion. The analyses and results presented in this dissertation strongly relied on the analytical capabilities offered by transmission electron microscopy, which proved to be a convenient and versatile tool for the characterization of many aspects related to the fabrication of III-nitride-based optoelectronic devices.

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