Microstructural comparison of the isothermal oxidation performance of an experimental Ni-5Cr-15Al overlay coating applied to CMSX-4 and CMSX-486 was performed at 1100 ℃. High temperature oxidation was carried out in a box furnace for a maximum duration of 192 hrs. Samples were periodically removed and the oxide, coating and substrates microstructurally analyzed using SEM, EDS and XRD equipment. Uncoated CMSX-4 and CMSX-486 were also oxidized using the same conditions in an attempt to understand how the coating affected oxidation performance based on microstructural changes. Results show that both CMSX-4 and CMSX-486 have unacceptable oxidation characteristics in the un-coated condition, where CMSX-486 had significantly better performance than CMSX-4. However, in the coated condition, both CMSX-4 and CMSX-486 had no significant difference in oxidation performance. It was found, in the un-coated and coated conditions, that the role of Hf in CMSX-486 played a significant role in determining oxidation performance of the material.

This thesis will introduce the study of methane as a mineral. Along with ammonia and water, methane is one of the main planetary-forming materials in the outer solar system. The topic of `new mineralogy of the outer solar system' is outlined and introduced, and previous studies in the area are discussed. This review identities a lack of highpressure structural knowledge on methane when compared to ammonia and water. The significance of this knowledge for the study of the planets Neptune and Uranus is discussed. The crystal structures of methane above 5.2 GPa were, prior to this thesis, unknown. To tackle this long-standing problem an integrated approach of high-pressure diffraction techniques had to be used. The dominance of hydrogen within the structures of methane necessitated the use of neutron diffraction. The difficulties and limitations of highpressure neutron powder diffraction are presented. It will be shown that the complexity of the subsequent structures required the use of single-crystal x-ray diffraction. Using a combination of x-ray and neutron diffraction the structures of methane phase A (5.2 - 10 GPa) and B (10 - 25 GPa) were solved. The structure of phase A, was shown to conform to an indexing from literature [Nakahata 99] of a rhombohedral unit cell with α ≈ 89.3° and a ≈ 8.6 Å. Powder data were insufficient to determine atomic positions for this phase, and a single-crystal xray diffraction study was undertaken. The process of growing samples for this study is described as well as data collection. As a result of these studies the carbon atoms were located within methane phase A, and the density of the structure confined. The heavy atom structure, of phase A, was refined against neutron powder diffraction data, enabling positions of hydrogen atoms to be found. Preliminary powder diffraction studies of methane phase B found that the structure did not conform to the unit cell described within the literature. The phase was instead assigned to a cubic unit cell with a ≈ 11.73 Å. Similarly to the studies of phase A, a single-crystal x-ray diffraction study was undertaken. This was complicated by the presence of a contaminant within the sample area. This contaminant was shown to have no effect on the structural results. From a single-crystal study the heavy atom structure of phase B was found. The thesis charts the attempt, but ultimate failure, to obtain neutron powder diffraction on this phase. Comparisons of phase B with the higher pressure phase HP (25 GPa +) led to the conclusion that there would still be some disorder within the hydrogen atoms of phase B. Other studies have been carried out on the methane phase diagram. A Raman spectroscopy study, in the literature, on the low-temperature and high-pressure region of the phase diagrams (20 K up to 30 GPa) had suggested the existence of 3 additional phases of methane. A low-temperature, high-pressure neutron diffraction experiment was undertaken to try and characterise these phases. It was found that the phase A structure persisted under all conditions (to 20 K and 5 GPa) throwing the original results into question. During the growth of single-crystals for the above studies on phase A and B, a high-temperature solid-solid phase transition was observed. This transition line was mapped out and the phase resulting from it characterised with high-temperature single-crystal x-ray diffraction.

The single crystal superalloy PWA 1484 is used in hot section turbine blade applications due to its performance at high temperatures. In practice, the turbine blades are often coated in order to protect them from environmental degradation. However, under repeated cyclic loading, the coating may serve as a site for crack initiation in the blades. Fundamental out-of-phase (OP) thermo-mechanical fatigue (TMF) studies, primarily using uncoated solid cylindrical test samples, have previously examined both crack initiation and propagation in PWA 1484. In this work, mechanical strain-controlled OP TMF tests were performed on coated and uncoated specimens of a hollow cylindrical geometry in order to study the effects of both geometry and coating on the TMF crack initiation behavior. To accomplish this, it was necessary to create and analyze a modified gripping mechanism due to the unique geometry of the test samples, and as predicted by hand calculations and finite element analysis, these modifications proved to be successful. The TMF test results for the uncoated material were compared to those from previous studies under the same testing conditions, and it was found that the differences in geometry had a minimal impact on fatigue life. Comparisons of the results for the coated and uncoated material suggested that the coating may have offered a slight improvement in life, although insufficient results were available to determine whether these differences were statistically significant. Damage mechanisms resulting from different test conditions were also observed through microscopy on failed specimens.

Microstructural comparison of the isothermal oxidation performance of an experimental Ni-5Cr-15Al overlay coating applied to CMSX-4 and CMSX-486 was performed at 1100 ℃. High temperature oxidation was carried out in a box furnace for a maximum duration of 192 hrs. Samples were periodically removed and the oxide, coating and substrates microstructurally analyzed using SEM, EDS and XRD equipment. Uncoated CMSX-4 and CMSX-486 were also oxidized using the same conditions in an attempt to understand how the coating affected oxidation performance based on microstructural changes. Results show that both CMSX-4 and CMSX-486 have unacceptable oxidation characteristics in the un-coated condition, where CMSX-486 had significantly better performance than CMSX-4. However, in the coated condition, both CMSX-4 and CMSX-486 had no significant difference in oxidation performance. It was found, in the un-coated and coated conditions, that the role of Hf in CMSX-486 played a significant role in determining oxidation performance of the material.

A machining process is one of the most common techniques used to remove material in order to create a final product. Most studies on mechanisms of cutting are performed under the assumption that the studied material is isotropic, homogeneous and continuous. One important feature of material- its anisotropyis linked to its crystallographic nature, which is usually ignored in machining studies. A crystallographic orientation of a workpiece material exerts a great influence on the chip-formation mechanism. Thus, there is a need for developing fundamental understanding of material's behaviour and material removal processes. While the effect of crystallographic orientation on cutting-force variation is extensively reported in the literature, the development of the single crystal machining models is somewhat limited.

A presente dissertação consta de quatro capítulos, sendo que nos dois primeiros apresentamos alguns tópicos dos métodos diretos para determinação de estruturas e nos dois últimos, o desenvolvimento experimental e os resultados finais obtidos na determinação da estrutura cristalina e molecular de um produto natural extraído de E. Nitens, o (2R, 3S) - 2-hidroxi-3-(2&#8217 - hidroxi-isopropil) - 5-hidroximetil-8-metoximetil-1-ceto-1, 2, 3, 4-tetrahidronaftaleno, conhecido pelo nome vulgar Emotina B. A Emotina B, C16H22O5, cristaliza no sistema triclínico, grupo espacial P1. Os parâmetros da cela unitária encontrados foram: a=7,474(2), b=8,922(5), c=12,405(4)&#197, &#945=87,68(4), &#946=78,96(3), &#947=67,40(4)° V=715,042޵ dcalc=1,37g/cm3; Z=2 moléculas por cela unitária. Foram coletadas em 1667 reflexões únicas, utilizando o difratômetro automático CAD-4, com radiação monocromatizada de MoK&#945, das quais foram mantidas 1010 reflexões, consideradas observadas segundo o critério I&#62 2&#948(I). A estrutura foi resolvida por métodos diretos (MULTAN-80) e por síntese de Fourier-diferenças sucessivas e refinadas por mínimos quadrados (SHELX-76) até um índice de discordância, R=0,054 para apenas as reflexões observadas e 0,067 para todas as reflexões.As moléculas são aproximadamente planas com variações conformacionais significativas apenas na parte alifática. Relacionam-se através de um pseudo-centro de inversão parcial localizado em (0,0846; 0,0622; 0,4174). Os esquemas de ligação hidrogênio são diferentes nas moléculas A e B e consistem de ligações intra e intermoleculares. / This dissertation consists of four chapters. Chapters 1 and 2 present some theoretical aspects of direct methods for crystal structure determination. Chapters 3 and 4 present the description of the experimental work and the crystal structure determination of a natural product obtained from E. Nitens, the (2R, 3S)-2-hydroxy-3-(2&#8217-hydroxy-isopropyl)-5-hydroximethyl-8-methoxymethyl-1-ceto-1, 2, 3, 4-tetrahydronapghtalen (Emmotin - B). Emmotin - B, C16H22O5, crystallizes in the triclinic system, space group P1. Cell dimensioned are: a=7,474(2), b=8,922(5), c=12,405(4)&#197, &#945=87,68(4), &#946=78,96(3), &#947=67,40(4)° V=715,042޵ dcalc=1,37g/cm3; Z=2 molecules/unit cell. The intensities of 1667 unique reflexions were collected using a CAD-4 automatic diffractometer with monochromated MoK&#945 radiations of which only 1010 with I&#62 2&#948(I) were considered observed. The structure was solved by direct methods (MULTAN-80) and successive applications of difference-Fourier calculations. It was refined by least square methods (SHELX-76). The final agreement index was R=0.054, considering only the observed reflexions and 0.067, considering all reflexions. The two independent molecules are almost identical showing discrepancies only in conformation of the aliphatic side chains. They are mutually related by a partial pseudo center of symmetry located at (0.0846, 0.0622, 0.4174). The hydrogen bonding schemes are different in both A and B molecules and consist of intra and intermolecular bonds.

Les revêtements barrières thermiques sont de plus en plus employés dans l'industrie afm d'accroître lesperformances thermiques et de diminuer les émissions de NOx des turbines aéronautiques et de générationd'électricité. Au cours de cette étude, le système constitué d'un substrat en superalliage base nickelmono cristallin AMI, d'une sous-couche NiAlPt et d'une barrière thermique en zircone yttriée élaborée par voieEBPVD (Electron Bearn Phase Vapor Deposition) a été étudié. Le but de cette thèse était de caractériser lesmodes d'endommagements en conditions représentatives de celles rencontrées en service sur le profil des aubesde turbines hautes pressions. Pour pallier au manque de données dans la littérature ouverte, des essais à différentsniveaux de température et de chargement mécanique ont été réalisés avec pour objectif de découpler les diversmodes d'endommagement qui entrent en jeu dans la ruine des barrières thermiques. Ces essais mécaniques detype fluage isotherme, fluage cyclé thermiquement, fatigue oligocyclique et fatigue avec temps de maintien ontété réalisés dans la gamme de température 950°C-1200°C avec des moyens d'essais classiques de laboratoire. Ilsont permis de mettre en évidence l'impact du chargement mécanique sur la cinétique de croissance de la couched'alumine inter faciale à llOO°C(notamment en fluage) mais aussi le délaminage à l'interface sous-coucheinterne/sous-couche externe en conditions de forte vitesse de déformation viscoplastique du substrat et le rôlemajeur de l'interdiffusion entre sous-couche et substrat sur la durée de vie en fatigue oligocyclique à 950°C. Desessais technologiques ont aussi été réalisés sur le banc MAATRE. / Thermal barrier coatings are widely used in the industry to enhance thermal performances and to decrease NOxemissions of aeronautic gas and electricity generating turbines. During this study, the system consisting of asingle crystal nickel based superalloy AMI (substrate), of a NiAIPt bondcoat and an yttria-stabilized zirconiatopcoat made by columnar EBPVD (Electron Bearn Phase Vapor Deposition) was studied. The aim ofthis thesiswas to characterize the damage mechanisms in conditions representative of those encountered in service on theprofile of high pressure turbine blades. To overcome the lack of data in the open literature, tests at differentlevels oftemperature and mechanicalloading has been made with the aim to decouple the various damage modesthat are involved in degradation of thermal barrier coatings. Thermomechanical testings such as isothermal creeptests, thermally cycled creep tests, low cycle fatigue with or without holding time have been made in thetemperature range 950°C-1200°C with conventional laboratory testing methods. They allowed to demonstratethe impact of the mechanical loading on the oxidation kinetics of the thermally grown oxide at IIOO°C(particularly during creep tests) but also a new delamination mode in the bond coat (near process defaults) in caseof high viscoplastic deformation rate of the substrate and the major role of interdiffusion between bondcoat andsubstrate on the low cyclic fatigue lifetime at 950°C. Finally, technological testings have also been realized onthe MAA TRE test bench.

Aircraft engine components can be subjected to a large number of thermo-mechanical loading cycles and to long dwell times at high temperatures. In particular, the understanding of creep in single crystal superalloy turbine blades is of importance for designing more reliable and fuel efficient aircraft engines. Creep tests on single crystal superalloy specimens have shown greater creep strain rates for thinner specimens than predicted by current theories. Therefore, it is necessary to develop a more predictive description of creep processes in these materials for them to be used effectively. Experimental observations have shown that the crystals have an initial porosity and that the progressive growth of these voids plays a major role in limiting creep life. In order to understand void growth under creep in single crystals, we have analyzed the creep response of three dimensional unit cells with a single spherical void under different types of isothermal creep loading. The growth behavior of the void is simulated using a three dimensional rate dependent crystal plasticity constitutive relation in a quasi-static finite element analysis. The aim of the present work is to analyze the effect of stress traixiality and Lode parameter on void growth under both constant true stress and constant engineering stress isothermal creep loading.

Neste trabalho desenvolvemos processos de crescimento e caracterização de fibras monocristalinas de SrTiO3 e Sr2RuO4 puros e como soluções sólidas através da técnica LHPG (laser heated pedestal growth). Esses processos são descritos desde as sínteses dos compostos, preparação dos nutrientes e sementes necessários para aplicação da técnica. Alguns resultados inéditos foram obtidos relativos à influência de cada um desses procedimentos e das alterações de parâmetros próprios da técnica na qualidade e características das fibras monocristalinas obtidas através dos resultados da caracterização estrutural, composicional e elétrica das fibras desses compostos. Diversas fibras monocristalinas de SrTiO3 puro foram obtidas diretamente a partir de nutrientes dos reagentes SrCO3 e TiO2, enquanto que, fibras monocristalinas de Sr2RuO4, (um novo composto de grande interesse tecnológico), foram obtidas de nutrientes de SrRuO3. A importância desses dois últimos resultados, inéditos até então, será discutida em detalhes neste trabalho. Deficiência de rutênio de até 75% do valor nominal dos nutrientes, além de gradientes de composição indesejados, foram encontrados no volume das fibras monocristalinas de algumas composições da solução sólida. Neste último caso a qualidade das fibras parece depender fortemente da taxa de puxamento do nutriente e da semente, respectivamente, bem como do processo utilizado na preparação da matéria prima. / In this work we developed characterization and crystal growth processes of Sr2RuO4, pure SrTiO3 and some compounds of the solid solution SrTi1-xRuxO3 through the LHPG (laser heated pedestal growth) technique. These processes are described from the synthesis of compounds and the preparation of seeds and nutrients necessary for the application of the technique. Some new results related to change of parameters of the technique and the influence of each preparation procedure on the quality and characteristics of the fibers were obtained through the results of the structural, compositional and electrical characterization of single crystal fibers of these compounds. Several SrTiO3 single crystal fibers were directly obtained from nutrients of the reagents SrCO3 and TiO2 while Sr2RuO4 (a new compound of considerable technological interest) single crystal fibers were obtained from SrRuO3 nutrients. The importance of these last two results, that are unknown up to now, will be discussed in detail in this work. Ruthenium deficiency up to 75% of the nominal value of the nutrients, besides undesired composition gradients, were found in the bulk of solid solution single crystal fibers of some compositions. In this last case, apparently the qualities of the single crystal fibers have a strong dependence on the feeding and pulling rates of nutrient and seed, respectively, as well as of the processes used for start materials preparation.

Nesta tese estudamos as condições para o crescimento estável dos compostos óxidos de metais de transição Sr2MO4 e das soluções sólidas SrM1-xM\'xO3, (M,M\'=Ru,Ti,V) pela técnica de fusão a laser de pedestais. Os nutrientes e sementes policristalinos utilizados para produzir fibras monocristalinas crescidas por fusão a laser não foram, em geral, nem queimados nem sinterizados antes do seu uso. Dois caminhos diferentes de processamento foram seguidos para a preparação de amostras cristalinas. A melhor condição de crescimento cristalino encontrada foi a que envolve o uso de atmosfera gasosa isostática como ambiente de crescimento cristalino. Porém, enquanto que fibras monocristalinas altamente homogêneas de SrTiO3, Sr2RuO4 e SrVO3 foram obtidas desta maneira, foi muito difícil estabelecer condições aceitáveis de crescimento cristalino para as soluções sólidas. A forte influência da natureza química do ambiente de crescimento cristalino e dos reagentes químicos de partida na qualidade da fibra monocristalina foi investigada. Explicações para as principais dificuldades encontradas no crescimento tanto das composições extremas como das suas soluções sólidas foram inferidas usando argumentos termodinâmicos e cálculos semi-empíricos dos parâmetros importantes envolvidos no processo de crescimento cristalino. A caracterização das fibras monocristalinas incluiu algumas adaptações aos métodos tradicionais de caracterização da resistividade elétrica, microestrutura e composição em cristais volumétricos. / In this thesis we have studied the conditions for the stable growth of the transition metal oxide compounds Sr2MO4 and solid solutions SrM1-xM\'xO3, (M,M\'=Ru,Ti,V) by the laserheated pedestal growth (LHPG) technique. The polycrystalline seed and feed rods used to grow single crystal fibers have, in general, not been fired or sintered prior to use. Two different processing methods were followed to prepare single crystal samples. The best crystal growth condition found was one involving the use of isostatic gaseous atmosphere as the crystal growth ambient. However, while highly homogeneous SrTiO3, Sr2RuO4 and SrVO3 single crystal fibers were obtained in this way, it was very difficult to establish acceptable crystal growth conditions for the preparation of the solid solutions. The strong influence of the chemical nature of the crystal growth ambient and starting reagents on the single crystal fiber quality was investigated. Explanations for the main difficulties found to grow both the extreme compositions and their solid solutions have been inferred using thermodynamic arguments and semi-empirical calculations of important parameters involved in the crystal growth process. Single crystal fibers characterization included some adaptations to the traditional electrical resistivity, microstructural and compositional methods practised in bulk crystals.