Face milling machinability investigation of two difficult-to-machine nickel-based superalloys, namely Inconel 718 and Waspaloy, has been carried out with four different types of tungsten carbide tools under various cutting conditions. The tools comprised of one double-layer CVD-TiCN+Al2O3 coated (KC994M), two PVD-TiN coated (KC720 and KC730) and one uncoated (KMF) tungsten carbide tools. The objectives of the study include investigation of tool performance, failure modes and wear mechanisms under the cutting conditions employed. In addition, surface integrity of the machined surfaces, with regard to surface finish, subsurface microhardness and metallographic examination of the subsurface microstructure, was investigated. CVD-coated KC994M gave the best overall performance in terms of tool life at low and high cutting conditions on both workpieces. The second best-performing tool was the uncoated KMF grade which gave as high tool lives as KC994M at lower cutting speeds. However at higher cutting speeds, KMF was generally outperformed by PVD-TiN coated tools. Short tool lives were obtained at higher cutting speeds of 75 and 100 m/min due to premature failure by chipping. Tool wear at low cutting speed range was due to a combination of progressive microchipping and plucking through a fracture/attrition related wear mechanism associated with cyclic workpiece adhesion and detachment and abrasion/diffusion-related flank wear. Plucking and microchipping were the dominant wear mechanisms. Coating layers on the rake face of both CVD and PVD coated tools were almost completely removed within the first few seconds of cutting at all cutting speeds tested, thus becoming ineffective. On the flank face, however, they remained intact for a longer period and hence increasing tools performance at the medium cutting speed range. Analysis of the subsurface microstructures and microhardness measurements showed that plastic deformation was the predominant effect induced onto the machined surface, the degree of which influenced by the cutting speed, tool wear and prolonged machining. In addition surface irregularities in the form of tearing and embedded hard particles were found to occur which was mainly associated with the chipping dominated wear mode.

Les motoristes aéronautiques doivent désormais montrer que la présence de petites anomalies de surface, pouvant être introduites lors d’opérations de maintenance, ne mènent pas à la rupture des pièces, et ce sur toute la durée de vie du moteur. Cette étude concerne la caractérisation de la nocivité d’anomalies de surface de type rayure et choc sur la tenue en fatigue du superalliage à base nickel René 65.Afin de découpler les effets de géométrie des effets de contraintes résiduelles, les rayures et les chocs possèdent un profil géométrique identique en V. Une technique de suivi de potentiel 3 points a été mise en place dans le but d’améliorer la détection de l’amorçage et d’avoir une information sur la morphologie du front de fissure. Les résultats expérimentaux montrent un amorçage rapide et un fort ralentissement de la vitesse de propagation dans les premiers stades. Nous avons également observé, par le biais de marquages thermiques, une évolution particulière de la forme du front de fissure s’amorçant au fond des rayures.L’utilisation de traitement thermique de relaxation a alors montré que c’est le champ mécanique hétérogène créé lors de la fabrication de ces anomalies qui contrôle la durée de vie et que c’est le paramètre physique d’ordre un à modéliser.Une stratégie de modélisation de la propagation de fissures à partir d’anomalies de type choc a été proposée. Celle-ci est basée sur la connaissance du champ de contraintes résiduelles par des simulations numériques, et sur l’application de ce champ dans un modèle numérique de propagation. Les résultats ont permis de confirmer que les contraintes résiduelles étaient bien la source du ralentissement de la propagation et également responsables de l’évolution de la forme du front de fissure. Ils ont également permis d’identifier les paramètres qui doivent être mesurés lors des contrôles non destructifs. / Anomalies, introduced during maintenance operations, are not critical for in-service life of a component. This study was undertaken to characterise the harmfulness of scratch and dent anomalies on the fatigue behaviour of the nickel based superalloy René 65.In order to separate the effects of the geometry and the residual stresses, scratches and dents have the same V-type profile. A 3 points DCPD method has been used to improve the detection of the initiation and also to have information about the crack front morphology. Experimental results showed that the initiation fatigue life is short and a slowdown of the fatigue crack growth in the first stages. We also observed, thanks to heat tints marking, aparticular crack front morphology for cracks initiating from scratches. Heat treatment has been used and showed that the heterogeneous mechanical field induced by the fabrication of the anomalies controls the fatigue life and that it constitutes one of the parameters to be taken into account in a future modelling. A modelling strategy of the crack propagation from dent anomalies has been developed. This model is based on the knowledge of the residual stress field by finite elements simulations, and the application of the calculated stress field in a numerical crackgrowth model. The results confirmed that the residual stresses were the physical source of the fatigue crack growth slow-down and also responsible for the evolution of the crack front morphology. They also allowed us to identify the parameters which have to be measured during non-destructive testing.

Cette thèse contribue au remplacement du revêtement de Stellite 6 utilisé aux zones de frottement dans le circuit primaire du réacteur à neutron rapide. Elle comprend trois parties : 1) Une étude bibliographique afin de présider au choix des matériaux de remplacement et du procédé de dépôt. 2) Une étude paramétrique en vue d’obtenir des dépôts sains (bonne adhésion avec le substrat, peu de porosité, absence de fissure, dilution faible). 3) Une étude du comportement tribologique des dépôts réalisée pour deux valeurs de température. Ces essais tribologique ont été réalisés sous atmosphère inerte afin d’évaluer la résistance à l’usure des matériaux choisis sans l’influence d’une éventuelle couche d’oxydation. De l’étude bibliographique il ressort les choix suivants mis en oeuvre dans notre étude : * le procédé projection laser qui présente des avantages tels que :- Bonne adhésion (métallurgique)- Vitesse de refroidissement élevée- Taux de dilution faible- Large plage paramétrique *deux alliages base nickel : le Colmonoy-52 et le Tribaloy-700. Ces alliages présentent un bon comportement tribologique à sec et sont déposables par laser.Pour la partie obtention d’un dépôt sain, tout d’abord nous avons caractérisé la poudre métallique. Ensuite, une recherche paramétrique a été conduite afin de disposer d’un jeu de paramètres qui permette d’obtenir un dépôt sain de Stellite 6 (référence), de Colmonoy-52 et de Tribaloy-700. A cette occasion, les relations entre trois paramètres principaux du procédé de projection laser (puissance du faisceau laser, vitesse de balayage de la surface, débit de poudre) ont été investigués en relation avec la microstructure et la composition chimique finale du dépôt.Lors de l’étude tribologique, un tribomètre de type pion-disque a été utilisé et les essais de frottement ont été réalisés sous atmosphère d’argon, à température ambiante et à 200°C. Les mécanismes d’usure ont été identifiés pour les 3 matériaux et leur contre pièce (bille en AISI 316L). Les volumes usés ont quant à eux été analysés en fonction de la charge appliquée et l’effet de la température. / This thesis contributes to the replacement of the coating of Stellite 6 which is used in friction areas for the primary circuit of the reactor fast neutron.It contains three parts:1) A literature review for selecting the materials and the deposition process2) A parametric study to get healthy deposits (good adhesion with the substrate, little porosity, no cracks, low dilution)3) A study wear behavior of deposits obtained, at high temperature (200°C) under an atmosphere inert gas, to determine the wear resistance of materials selected without the influence of an eventual oxidation layer.From the literature review, it appears the following choices implemented in our study : * the method laser cladding with advantages such as: - Good adhesion (metallurgical) - High cooling speed - Low dilution rate - Wide parametric range * two nickel-based alloys: Colmonoy-52 and Tribaloy-700. These alloys have good dry wear behavior and could be deposited by the laser. In the manufacturing part of the healthy deposit, firstly, we characterized the metal powder. Then, a parametric study was performed to look for a good parametric range that makes us getting a healthy deposit of Stellite 6 (reference) of Colmonoy-52 and Tribaloy-700. In this case, relationships among three main process parameters laser cladding (laser beam power, surface scanning speed, rate of powder) with the microstructure and chemical composition of the deposit are studied. In study the wear behavior, a pin-on-disc type of tribological was used and tests were carried out in argon at room temperature and 200°C. We investigated the wear mechanism of the best deposition of Stellite 6, Colmonoy-52 and Tribaloy-700. The wear resistance of these materials were thourghly compared.

A necessidade de trabalhos em altas temperaturas exige o desenvolvimento de materiais com elevada resistência química e mecânica em temperaturas que podem chegar próximas ao seu ponto de fusão. Em especial, os materiais empregados devem apresentar bom comportamento em fluência, de modo a suportar a combinação de altas tensões e altas temperaturas com o mínimo de deformação. O presente trabalho visa caracterizar a superliga MAR-M247, fornecida pela empresa Açotécnica, em condições de fluência para a produção de rotores empregados em turbocompressores automotivos. Os trabalhos são divididos em duas etapas. Na primeira estuda-se a melhor combinação de tratamentos térmicos de solubilização e envelhecimento, chegando à condição de solubilização por 5 horas a 1250?C e envelhecimento por 20 horas a 980?C. Um trabalho de outro aluno (SILVA, 2011) realiza o mesmo estudo para uma variação da superliga MAR-M247 em que o tântalo é totalmente substituído por nióbio. Neste caso, a melhor condição de tratamento é solubilização a 1260?C por 8 h seguida de envelhecimento duplo por 5 h a 880?C e 20 h a 780?C. Na segunda etapa, os materiais dos dois trabalhos, nas melhores condições de tratamento para cada um, são testados em condições de fluência a 850?C a 370, 390, 410 e 430 MPa. A superliga convencional apresenta os maiores valores de vida em fluência enquanto a modificada apresenta os maiores valores de ductilidade. A análise de fratura dos materiais ensaiados a 390 e 430 MPa mostra a presença massiva de carbonetos em toda a extensão da microestrutura, apresentando, em muitos casos, uma morfologia de \"escrita chinesa\". Apesar disso, todas as amostras apresentam grande quantidade de vazios, indicando que sua nucleação e crescimento são os responsáveis pela fratura do material. / The needing for high temperature work claim for new materials capable of maintain high strength and good corrosion resistance at temperatures that could reach values near their melting point. Such materials must be creep resistant to withstand high levels of tension and temperature. The aim of this work is the characterization of MAR-M247 under creep conditions for the production of automotive turbocharger rotors made by Açotécnica. The work is split in two steps. In the first one, the material is studied to find the best conditions for solution and ageing heat treatments. The result is a solution treatment at 1250?C for 5 h followed by aging at 980?C for 20 h. A work made by another student (SILVA, 2011) makes the same with a modification of MAR-M247 in which tantalum is completely substituted for niobium. The best condition for this case is a solution treatment at 1260?C for 8 h and a double aging treatment at 880?C for 5 h and 780?C for 20 h. At the second part of the work, both the materials are tested under creep conditions at 850?C with 370, 390, 410 and 430 MPa. The conventional superalloy shows the highest values for creep strength and the modified superalloy shows the highest ductility. Fracture surfaces for 390 and 430 MPa showed brittle carbides with \"chinese script\" morphology. Conversely, holes are present on all four materials what shall mean that its nucleation and growth was the most probably fracture path.

The aim of this project was to investigate the potential of fused-slurry coatings for improving the oxidation resistance of wrought alloys. Slurry-aluminised coatings were deposited on Alloy 800H (Fe-33Ni-20Cr), Alloy HCM12A (Fe-12Cr-2W), Alloy 214 (Ni-16Cr-4Al-3Fe), Fe-27Cr-4Al and Fe-14Cr-4Al alloys. The slurry contained a cellulose-based binder in an aqueous carrier and spherical aluminium powder, with a particle size below 20 microns. The slurries were applied with a paint-brush, dried in air and heat treated in either hydrogen or argon at temperatures between 700 and 1150C. The slurries were characterised by thermogravimetry, differential scanning calorimetry and viscometry. The coatings were characterised by optical microscopy, scanning electron microscopy, energy-dispersive X-ray analysis, X-ray diffraction and Vickers hardness measurements. The oxidation resistance of selected slurry-coated specimens was assessed in air at 1000 and 1100C in tests lasting up to 1000 hours.Slurry-aluminising was found to be a simple, effective way of forming protective coatings that were similar in composition and microstructure to chemical vapour deposits. However, it was difficult to control the amount of slurry applied to the substrate and produce coatings of uniform thickness.The coatings on Alloy HCM12A and the Fe-Cr-Al alloys contained cracks in the brittle FeAl phase due to tensile stresses arising from differences in the thermal expansion coefficients of the substrates and the coatings. Rapid interdiffusion between the coatings and the ferritic substrates resulted in the appearance of Kirkendall voids.Coatings on Alloy 214 required a two-stage heat treatment to convert the brittle δ-Ni2Al3 to β-NiAl. Cracking along the coating/substrate interface was prevented by limiting the coating thickness to a maximum of 250 microns. During oxidation at 1100C, the β-NiAl was converted to γ'-Ni3Al. After 1000 h, the centre of the coating consisted chiefly of γ'-Ni3Al and bands of austenite (γ-Ni) were present at the inner and outer edges of the coating. The aluminium content at the coated surface was higher than the original aluminium content of the alloy, the protective alumina scale was improved and the oxidation life of the substrate was extended. An additional life of 1250 h at 1100C is estimated from a slurry coating before the aluminium content returns to that of the original alloy (4%), providing a potential improvement in oxidation resistance.Microstructural changes such as grain growth, sensitisation and formation of aluminium nitride particles near the coating/substrate interface, were detected in the alloy substrates after forming the slurry coatings. However, these microstructural changes did not detract from the good performance of the coatings during oxidation tests at 1100C.The work in this study has demonstrated a low-cost method of coating high-temperature alloys providing coatings with microstructures, densities and modes of degradation similar to those obtained by other coating methods. The coatings are potentially applicable to a wide range of high-temperature substrates.

No description available.

No description available.

This study focused on molten salt purification processes to effectively reduce or eliminate the corrosive contaminants without altering the salt's chemistry and properties. The impurity-driven corrosion behavior of HAYNES® 230® alloy in the molten KCl-MgCl2-NaCl salt was studied at 800 ºC for 100 hours with different salt purity conditions. The H230 alloy exhibited better corrosion resistance in the salt with lower concentration of impurities. Furthermore, it was also found that the contaminants along with salt's own vaporization at high temperatures severely corroded even the non-wetted surface of the alloy. The presence of Mg in its metal form in the salt resulted in an even higher mass-loss possibly due to Mg-Ni interaction. The study also investigated the corrosion characteristics of several nickel and ferrous-based alloys in the molten KCl-MgCl2-NaCl salt. The average mass-loss was in the increasing order of C276 < SS316L < 709-RBB* < IN718 < H230 < 709-RBB < 709-4B2. The corrosion process was driven by the outward diffusion of chromium. However, other factors such as the microstructure of the alloy i.e. its manufacturing, refining, and heat-treatment processes have also shown to influence the corrosion process. Lowering the Cr content and introducing W and Mo in the alloy increased its resistance to corrosion but their non-uniform distribution in the alloy restricted its usefulness. To slow-down the corrosion process, and enhance the material properties, selected alloys were boronized and tested for their compatibility in the molten KCl-MgCl2-NaCl salt. The borided alloys exhibited better resistance to molten salt attack, where the boride layer in the exposed alloy was still intact, non-porous, and strongly adhered to the substrate. The alloys also did not show any compensation in their properties (hardness). It was also found that the boride layer always composed of an outermost silicide composite layer, which is also the weakest and undesired layer as it easily cracks, breaks, or depletes under mechanical and thermal stresses. Various different grades of "virgin" nuclear graphites were also tested in the molten KF-UF4-NaF salt to assist in the selection of tolerable or impermeable graphites for the MSR operational purposes. It was found that molten salt wettability with graphite was poor but it still infiltrated at higher pressure. Additionally, the infiltration also depended on the pore-size and porosity of the graphite. The graphite also showed severe degradation or disintegration of its structure because of induced stresses. / Doctor of Philosophy / Molten salts are considered as potential fuel and coolant candidates in MSRs because of their desirable thermophysical properties and heat-transfer capabilities. However, they pose grave challenges in material selection due to their corrosive nature, which is attributed to the impurities and their concentration (mostly moisture and oxygen-based) in the salt. This study focused on purifying the salt to reduce these contaminants without compromising its composition and properties. The influence of purification processes on the corrosion behavior of HAYNES® 230® alloy was studied in the molten chloride salt with different purity conditions. Various nickel and ferrous-based alloys were also studied for their compatibility in the molten chloride salt. This will assist in expediting the material selection process for various molten salt applications. It was observed that several factors such as alloy composition, its microstructure, impurities in the salt attribute to molten salt corrosion. It was also quite evident that corrosion in molten salts is inevitable and hence, the focus was shifted on slowing down this process by providing protective barriers in the form of coatings (i.e. boronization). The borided (coated) alloys not only improved the corrosion resistance but also enhanced and retained their properties like hardness after exposure to molten salts. Since these studies were conducted under static conditions, a more detailed investigation is needed for the selected alloys by subjecting them to extreme flow-conditions and for longer a duration of time. To achieve this objective, a forced circulation molten salt loop was designed and fabricated to conduct flow corrosion studies for alloys in molten chloride salt. Graphite is another critical component of the MSR where it is used as a moderator or reflector. Generally, molten salts exhibit poor wettability with graphite, but they can still infiltrate (graphites) at higher applied pressures, and result in the degradation or disintegration of graphite's structure, and eventually its failure in the reactor. This study provides infiltration data, and understanding of the degradation of various 'virgin' nuclear graphite grades by the molten fluoride salt. This should assist in the selection of tolerable or impermeable graphite grades for MSR operational purposes.

Transient liquid phase (TLP) bonding has proven to be the preferred method for joining extremely difficult-to-weld advanced materials, including similar and dissimilar superalloys. In this work, an approach that combines experiments and theoretical simulations are used to investigate the effect of temperature gradient (TG) in a vacuum furnace on the temperature distribution in TLP bonded samples. When joining similar materials by this technique, the simulated results with experimental verifications show that, irrespective of where the samples are placed inside the vacuum furnace, a TG in the furnace can translate into a symmetric temperature distribution in bonded samples provided the diffusion direction is parallel to the source of heat emission. In addition, the effects of TLP bonding parameters on the joint microstructure were investigated during the joining of nickel-based IN738 and CMSX-4 single crystal (SX) superalloys. An increase in holding time and reduction in gap size reduces the width of eutectic product that forms within the joint region. It was also found that Liquid-state diffusion (LSD) can occur and have significant effects on the microstructure of dissimilar TLP bonded joints even though its influence is often ignored during TLP bonding. The occurrence of LSD produced single crystal joint when a SX and polycrystal substrate were bonded. This formation of a SX joint which cannot be exclusively produced by solid-state diffusion has not been previously reported in the literature. / February 2017

<div>Hydrous hydrazine (N₂H₄·H₂O) is a promising hydrogen carrier for convenient storage and transportation owing to its high hydrogen content (8.0 wt%), low material cost and stable liquid state at ambient temperature. Particularly, generation of only nitrogen as byproduct, in addition to hydrogen, thus obviating the need for on-board collection system for recycling, ability to generate hydrogen at moderate temperatures (20-80 °C) which correspond to the operating temperature of a proton exchange membrane fuel cell (PEMFC), and easy recharging using current infrastructure of liquid fuels make hydrous hydrazine a promising hydrogen source for fuel cell electric vehicles (FCEVs). Since hydrogen can be generated from catalytic hydrazine decomposition, the development of active, selective and cost-effective catalysts, which enhance the complete decomposition (N₂H₄ → N₂+2H₂) and simultaneously suppress the incomplete decomposition (3N₂H₄ → 4NH₃+N₂), remains a significant challenge.</div><div>In this dissertation, CeO₂ powders and various Ni-based catalysts for hydrous hydrazine decomposition were prepared using solution combustion synthesis (SCS) technique and investigated. SCS is a widely employed technique to synthesize nanoscale materials such as oxides, metals, alloys and sulfides, owing to its simplicity, low cost of precursors, energy- and time-efficiency. In addition, product properties can be effectively tailored by adjusting various synthesis parameters which affect the combustion process.</div><div>The first and second parts of this work (Chapters 2 and 3) are devoted to investigating the correlation between the synthesis parameters, combustion characteristics and properties of the resulting powder. A series of CeO₂, which is a widely used material for various catalytic applications and a promising catalyst support for hydrous hydrazine decomposition, and Ni/CeO₂ nanopowders as model catalysts for the target reaction were synthesized using conventional SCS technique. This demonstrated that crystallite size, surface property and concentration of defects in CeO₂ structure which strongly influence the catalytic performance, can be effectively controlled by varying the synthesis parameters such as metal precursor (oxidizer) type, reducing agent (fuel), fuel-to-oxidizer ratio and amount of gas generating agent. The tailored CeO₂ powder exhibited small CeO₂ crystallite size (7.9 nm) and high surface area (88 m²/g), which is the highest value among all prior reported SCS-derived CeO₂ powders. The Ni/CeO₂ catalysts synthesized with 6 wt% Ni loading, hydrous hydrazine fuel and fuel-to-oxidizer ratio of 2 showed 100% selectivity for hydrogen generation and the highest activity (34.0 h^-1 at 50 ºC) among all prior reported catalysts containing Ni alone for hydrous hydrazine decomposition. This superior performance of the Ni/CeO₂ catalyst is attributed to small Ni particle size, large pore size and moderate defect concentration.</div><div>As the next step, SCS technique was used to develop more efficient and cost-effective catalysts for hydrous hydrazine decomposition. In the third part (Chapter 4), noble-metal-free NiCu/CeO₂ catalysts were synthesized and investigated. The characterization results indicated that the addition of Cu to Ni/CeO₂ exhibits a synergistic effect to generate significant amounts of defects in the CeO₂ structure which promotes catalytic activity. The 13 wt% Ni_0.5Cu_0.5/CeO₂ catalysts showed 100% H₂ selectivity and 5.4-fold higher activity (112 h^-1 at 50 ºC) as compared to the 13 wt% Ni/CeO₂ (20.7 h^-1). This performance is also superior to that of most reported non-noble metal catalysts and is even comparable to several noble metal-based catalysts. In the fourth part (Chapter 5), low Pt loading NiPt/CeO₂ catalysts were studied. The modified SCS technique was developed and applied to prepare NiPt/CeO₂ catalysts, that overcomes the typical problem of conventional SCS which leads to deficiency of Pt at catalyst surface due to the diffusion of Pt into bulk CeO₂. The Ni_0.6Pt_0.4/CeO₂ catalysts with 1 wt% Pt loading exhibited high activity (1017 h^-1 at 50 ºC) along with 100% H₂ selectivity owing to the optimum composition of NiPt alloy, high metal dispersion and a large amount of CeO₂ defects. Its activity is higher than most of the reported NiPt-based catalysts which typically contain high Pt loading (3.6-42 wt%).</div><div>Next, the intrinsic kinetics of hydrous hydrazine decomposition over the NiPt/CeO₂ catalysts, which are necessary for efficient design and optimization of the hydrous hydrazine-based hydrogen generator system, were investigated (Chapter 6). From the experimental data obtained at different reaction temperatures, the intrinsic kinetic model based on the Langmuir-Hinshelwood mechanism was established. The developed model</div><div>provides good predictions with the experimental data, especially over a wide range of initial reactant concentration, describing well the variation of reaction order from low to</div><div>high reactant concentration.</div><div>Finally, the conclusions of the dissertation and recommendations for future work are summarized in Chapter 7.</div>