Les superalliages à base nickel des turbines aéronautiques sont susceptibles de subir des phénomènes de corrosion et/ou d’oxydation à haute température par les environnements agressifs rencontrés en service. Aussi, des revêtements d’aluminure sont appliqués par dépôt chimique afin d’assurer la protection des pièces contre ces phénomènes. La dégradation progressive de ces revêtements mène à la nécessité de les enlever afin d’en appliquer des nouveaux. Les bains chimiques industriels pour enlever les revêtements et les oxydes sont très toxiques, polluants et plutôt empiriques. Ainsi, ce travail de thèse se proposait d’étudier une méthode alternative et originale, par voie électrochimique permettant de contrer les limitations des approches chimiques. La voie électrochimique par application d’un potentiel (mode potentiostatique) a été étudiée afin de procurer la sélectivité entre le substrat et le revêtement lors de la dissolution, ainsi que le contrôle in-situ du procédé à l’aide d’une cellule à 3 électrodes. La faisabilité de la méthode a d’abord été démontrée, puis différentes procédures (par cycles cathodique/anodique, en continu et, parfois, avec modification du potentiel imposé) ont été développées. Nous avons pu mettre en relation les états métallurgiques des systèmes revêtement/substrat avec leur comportement électrochimique et avons mis également en lumière que le taux de dissolution est principalement gouverné par la concentration d’aluminium dans le revêtement alors que lorsque le platine est incorporé à ce même revêtement, le taux de dissolution est homogène. De même, nous avons démontré par XPS et par MET que la teneur en chrome modifie de manière significative l’homogénéité du décapage lors des phases de polarisation cathodique par un mécanisme de passivation de la surface, qui bloque l’activité électrochimique. Cependant, la tenue en oxydation cyclique des revêtements décapés par voies chimique et électrochimique n’a pas pu véritablement être démontrée car les revêtements avaient une microstructure différente. Enfin, des essais sur pièces de turbine ont montré le haut degré de sélectivité de l’approche ici étudiée. / Nickel based superalloys of aeronautical turbines are subjected to high temperature oxidation and/or corrosion in service conditions. Thus, protective aluminide coatings are applied onto the parts by chemical vapor deposition. The degradation of the coatings with time requires them to be removed prior to recoating the parts. The chemical baths industrially employed are toxic, polluting and quite empirical. Therefore, this thesis aimed at studying an alternative and original electrochemical method to circumvent the drawbacks of the chemical approach. Fixed potentials (potentiostatic mode) were thus applied to provide selectivity between the coating and the substrate upon the dissolution process, as well as to ensure in-situ control through a 3-electrode cell. The feasibility of the method was first demonstrated, then different procedures (cathodic/anodic cycles; continuous anodic and sometimes with modification of the potential) were investigated. The correlations between the metallurgical phases of the coating/substrate systems were elucidated. It also appeared that dissolution is mainly governed by the concentration of aluminium in the coating whereas the incorporation of platinum to the coating brought about the homogeneous dissolution. In addition, XPS and MET confirmed the hypothesis by which the chromium content drastically change the stripping homogeneity upon the cathodic polarization step by passivation of the surface and the subsequent electrochemical blocking. However, the results on the cyclic oxidation behaviour of the coatings priorly stripped chemically or electrochemically were not conclusive enough as the microstructure of the original coatings was different. Finally, quite a few stripping trials were carried out onto real turbine parts that confirmed the high selectivity of the electrochemical approach studied.

This thesis is about laser powder bed fusion (L-PBF) of the nickel-based superalloy: Alloy 247LC. Alloy 247LC is used mainly in gas turbine blades and processing the blades with L-PBF confers performance advantage over the blades manufactured with conventional methods. This is mainly because L-PBF is more suitable, than conventional methods, for manufacturing the complex cooling holes in the blades. The research was motivated by the need for academia and industry to gain knowledge about the processability of the alloy using L-PBF. The knowledge is essential in order to eventually solve the problem of cracking which is a major problem when manufacturing the alloy. In addition, dense parts with low void content should be manufactured and the parts should meet the required performance. Thus, the thesis answered some of the important questions related to process parameter-microstructure-defect relationships. The thesis presented an introduction in chapter 1. A literature review was made in chapter 2 to 4. In chapter 2, the topic of additive manufacturing was introduced followed by an overview of laser powder bed fusion. Chapter 3 focused on superalloys. Here, a review was made from the broader perspective of superalloys but was eventually narrowed down to the characteristics of nickelbased superalloys and finally Alloy 247LC. Chapter 4 reviewed the main research on L-PBF of Alloy 247LC. The methodology applied in the thesis was discussed in chapter 5. The thesis applied statistical design of experiments to show the influence of process parameters on the defects and microstructure, so a detail description of the method was warranted. This was given at the beginning of chapter 5 and followed by the description of the L-PBF manufacturing and the characterization methods. The main results and discussions, in chapter 6, included a preliminary investigation on how the process parameters influenced the amount of discontinuity in single track samples. This was followed by the results and discussions on the investigation of voids, cracks and microhardness in cube samples (detail presentation was given in the attached paper B). Finally, the thesis presented results of the microstructure obtainable in L-PBF manufactured Alloy 247LC. The initial results of the microstructure investigation were presented in paper A.

<div>Additively manufactured (AM) metals have been increasingly fabricated for structural applications. However, a major hurdle preventing their extensive application is lack of understanding of their mechanical properties. To address this issue, the objective of this research is to develop a computational model to simulate the creep behavior of nickel alloy 718 manufactured using the laser powder bed fusion (L-PBF) additive manufacturing process. A finite element (FE) model with a subroutine is created for simulating the creep mechanism for 3D printed nickel alloy 718 components.</div><div><br></div><div>A continuum damage mechanics (CDM) approach is employed by implementing a user defined subroutine formulated to accurately capture the creep mechanisms. Using a calibration code, the material constants are determined. The secondary creep and damage constants are derived using the parameter fitting on the experimental data found in literature. The developed FE model is capable to predict the creep deformation, damage evolution, and creep-rupture life. Creep damage and rupture is simulated as defined by the CDM theory.</div><div>The predicted results from the CDM model compare well with experimental data, which are collected from literature for L-PBF manufactured nickel alloy 718 of creep deformation and creep rupture, at different levels of temperature and stress. </div><div><br></div><div>Using the multi-regime Liu-Murakami (L-M) and Kachanov-Rabotnov (K-R) isotropic creep damage formulation, creep deformation and rupture tests of both the secondary and tertiary creep behaviors are modeled.</div><div>A single element FE model is used to validate the model constants. The model shows good agreement with the traditionally wrought manufactured 316 stainless steel and nickel alloy 718 experimental data collected from the literature. Moreover, a full-scale axisymmetric FE model is used to simulate the creep test and the capacity of the model to predict necking, creep damage, and creep-rupture life for L-PBF manufactured nickel alloy 718. The model predictions are then compared to the experimental creep data, with satisfactory agreement.</div><div><br></div><div>In summary, the model developed in this work can reliably predict the creep behavior for 3D printed metals under uniaxial tensile and high temperature conditions.</div>

No description available.

Ce travail porte sur l'étude du comportement sous sollicitation de fretting sirnple à haute température de superalliages monocristallins à base de nickel MC2 et CMSX-4. Il trouve son application dans le domaine aéronautique, plus précisément au niveau de la turbine haute pression des moteurs d'hélicoptère. Celle-ci a pour fonction de convertir l'énergie cinétique des gaz brûlés en un couple qui entraîne en rotation le compresseur. Elle participe ainsi directement à l'entretien du cycle de combustion du moteur ce qui fait d'elle un organe clé. Située directement en aval de la chambre de combustion, la turbine haute pression (HP) est composée d'un disque central polycristallin et d'aubages monocristallins rapportés, liés au disque par une liaison dite pied de sapin. La rotation de la turbine et la température élevée des gaz de combustion va générer sur les aubes, le disque et l'attache qui les relie，des sollicitations thermomécaniques importantes. Du fait de la force centrifuge et du défilement des aubes devant les étages fixes des distributeurs, l'attache pied de sapin est la cible d 'oscillations dynamiques à l'origine de phénomènes d'endommagement par fretting. Ce fretting,, l'interface de contact entre l'aube et le disque fait l'objet d'une attention toute particulière，ce qui a motivé la conduite de ces travaux. Après une première phase et développement et de validation d'un banc d'essai innovant qui a nécessité la mise en place d'un dispositif de chauffage par induction, un soin tout particulier est apporté à la préparation des échantillons. La nature monocristalline du matériau constitutif des aubes nécessite en effet de respecter précisément l'orientation de la microstructure par rapport aux axes de sollicitation et la surface de contact. Pour cela une mesure systématique de la désorientation des axes primaires et secondaires des barreaux bruts est réalisée. La désorientation relevée est ensuite compensée au cas par cas lors du prélèvement par électroérosion des échantillons. Le lot d'échantillons obtenu peut dès lors être considéré comme homogène en terme d'orientation, malgré le fait que les barreaux bruts présentent des désorientations différentes. [...] / The aim of this work is to study the behavior of MC2 and CMSX-4 nickel based superalloys when subjected to fretting load at high temperature. Since it drives the compressor shaft, the high pressure turbine is a key part of the helicopter engine. 1n order to increase the global reliability and efficiency of the engine, single crystal nickel based superalloys are employed for turbine blades while disk parts are made of polycrystalline materials. Each turbine blade is attached to the central disk via a special link called fir tree root. Due to high temperature and dynamic oscillations, the contact zone between blades and disk is subjected to high thermomechanical stresses. 1t causes fretting phenomena that can lead to wear and cracking damage. This work focuses on both the partial slip and gross slip regime in order to study the damage process of single crystal MC2 and CMSX-4 materials. To perform the mechanical tests, an innovative fretting device is designed to fit the specifications. The use of an induction heat system allows an accurate control of high temperatures. To be consistent with the real flying parts,，the specific orientation of the microstructure of the material with respect to the contact loading direction is taken into account. The microstructure misalignement of raw material bars is measured and compensated as the specimens are machined for extraction. Consequently the obtained set of samples is considered to be of homogeneous microstructure orientation even if their source material contains deviations in orientation. Four material states are investigated: precision grinding conventional shot-peening ultrasonic shot-peening and nitriding process. The results show that for the partial slip régime, shot peening processes are very useful for turbine blade applications. As a matter of fact, the risk for crack nucleation and extension are reduced by the introduction of residual stresses beneath the surface in spite of the high temperature. 1nvestigating the gross slip regime results show that wear of material leads to the formation of a third body and then to the formation of a thin layer called "glaze layer)) with low friction coefficient. To describe the formation process of the "glaze layer)) halted trials are performed. The results allow the drafting of a scenario in which wear debris are stuck and sintered beneath the contact.

Les alliages à base de Nickel constituent les meilleurs matériaux actuellement développés pour répondre aux sollicitations sous hautes températures dans les domaines de l’aéronautique du nucléaire etc… L’objectif de ce travail de thèse est d’étudier leur comportement en température sous sollicitations de fretting usure. Dans cette étude, on s’intéresse à un contact René125/Waspaloy représentatif d’une application aéronautique. Dans un premier temps nous avons étudié l’effet de la température. On montre qu’à partir de 400°C, l’interface génère la formation d’une glaze layer lubrifiante qui réduit considérablement la cinétique de l’usure. En fixant la température à 700°C (température de l’application industrielle), nous avons étudié la stabilité de ces couches protectrices vis-à-vis de la pression de contact, de l’amplitude de glissement, de la fréquence et du nombre de cycle appliqués. Cette analyse montre une évolution bilinéaire de l’usure avec une usure initiale rapide liée à la formation de la « glaze layer » puis une usure additionnelle quasiment nulle dès que la « glaze layer » est formée. Ces travaux montrent que le volume d’usure associée à la formation de la « glaze layer » est fonction de la vitesse de glissement. Au dessus d’une vitesse seuil de glissement, la formation de la « glaze layer » protectrice devient plus difficile. Une courbe maîtresse est ainsi établie. Des analyses chimiques des interfaces associées à des essais interrompus ont permis d’établir le scénario de formation de ces glaze layers. Pour finir, une étude comparative des revêtements développés dans le cadre du projet INNOLUB a été menée de façon à sélectionner le revêtement offrant les meilleures propriétés tribologiques pour l’application étudiée. / Nickel based alloys are the most developed materials nowadays for applications at high temperature, as in aeronautics, nuclear…The aim of this study is to understand their behavior at high temperature under fretting wear solicitations. Thereby, we had focused on a tribosystem formed of Waspaloy/René 125, which represent the crankcase/blade contact of the low pressure Turbine. We started studying the temperature effect, it is been noticed that above T = 400°C, a lubricant tribofilm, called the Glaze Layer is generated at the interface of the contact, which enable an abrupt reduction in friction and wear rate. The temperature was than fixed at 700°C (service temperature), so the glaze layer stability was analyzed as a function of contact pressure, sliding amplitude, frequency and number of cycles imposed. This analysis shows a bilinear wear evolution, characterized by a fast initial wear related to the formation of the glaze layer, followed by almost no wear once the glaze layer is formed. This study showed that the wear rate related to the glaze layer formation is dependent of the sliding velocity. Above a sliding velocity threshold, the formation of a stabilized glaze layer is quite difficult. A Master curve is here established. Microscopic and spectroscopic investigations are conducted to analyze the interface based on interrupted tests of a very short duration. Leading to a precise description of the glaze layer formation mechanisms. At the end of this study, a comparative analysis of different coatings developed to improve these components behavior, in the framework of INNOLUB project was established, allowing choosing the coating offering the best tribological properties and lifetime.

The thesis is focused on clarifying fatigue damage mechanisms and fatigue-creep damage mechanisms of MAR-M 247 and IN 713LC polycrystalline Ni-based superalloys. This thesis begins with basic information about nickel-based superalloys and their microstructure, followed by a description of fatigue and creep mechanisms and their mutual interaction. The next part contains experimentally obtained results describing the behavior of MAR-M 247 and IN 713LC superalloys under various sets of conditions. Three testing temperatures - 800, 900 and 950 °C were used for the measurement of fatigue properties under symmetrical loading cycle, because in the temperature range 800 – 950 °C, the mechanism of fatigue crack propagation of both superalloys should change from the originally crystallographic at "lower" temperatures (800 °C) to non-crystallographic at "higher" temperatures (950 °C). In addition the effect of processing technology on fatigue properties was studied on the superalloy IN 713LC. High-frequency cyclic loading (about 120 Hz) with high mean stress at elevated temperatures was applied to induce fatigue-creep interaction. The combined fatigue-creep loading was performed on the IN 713LC superalloy at 800 °C and on the MAR-M 247 superalloy at 900 °C.

Pesquisadores e indústrias de todo o mundo estão firmemente comprometidos com o propósito de fazer o processo de usinagem ser precisamente veloz e produtivo. A forte concorrência mundial gerou a procura por processos de usinagem econômicos, com grande capacidade de produção de cavacos e que produzam peças com elevada qualidade. Dentre as novas tecnologias que começaram a ser empregadas, e deve tornar-se o caminho certo a ser trilhado na busca da competitividade em curto espaço de tempo, está a tecnologia de usinagem com altas velocidades (HSM de High Speed Machining). A tecnologia HSM surge como componente essencial na otimização dos esforços para manutenção e aumento da competitividade global das empresas. Durante os últimos anos a usinagem com alta velocidade tem ganhado grande importância, sendo dada uma maior atenção ao desenvolvimento e à disponibilização no mercado de máquinas-ferramentas com rotações muito elevadas (20.000 - 100.000 rpm). O processo de usinagem com alta velocidade está sendo usado não apenas para ligas de alumínio e cobre, mas também para materiais de difícil usinabilidade, como os aços temperados e superligas à base de níquel. Porém, quando se trata de materiais de difícil corte, têm-se observado poucas publicações, principalmente no processo de torneamento. Mas, antes que a tecnologia HSM possa ser empregada de uma forma econômica, todos os componentes envolvidos no processo de usinagem, incluindo a máquina, o eixo-árvore, a ferramenta e o pessoal, precisam estar afinados com as peculiaridades deste novo processo. No que diz respeito às máquinas-ferramenta, isto significa que elas têm que satisfazer requisitos particulares de segurança. As ferramentas, devido à otimização de suas geometrias, substratos e revestimentos, contribuem para o sucesso deste processo. O presente trabalho objetiva estudar o comportamento de diversas geometrias ) de insertos de cerâmica (Al2O3 + SiCw e Al2O3 + TIC) e PCBN com duas concentrações de CBN na forma padrão, assim como modificações na geometria das arestas de corte empregadas em torneamento com alta velocidade em superligas à base de níquel (Inconel 718 e Waspaloy). Os materiais foram tratados termicamente para dureza de 44 e 40 HRC respectivamente, e usinados sob condição de corte a seco e com utilização da técnica de mínima quantidade de lubrificante (minimal quantity lubricant - MQL) visando atender requisitos ambientais. As superligas à base de níquel são conhecidas como materiais de difícil usinabilidade devido à alta dureza, alta resistência mecânica em alta temperatura, afinidade para reagir com materiais da ferramenta e baixa condutividade térmica. A usinagem de superligas afeta negativamente a integridade da peça. Por essa razão, cuidados especiais devem ser tomados para assegurar a vida da ferramenta e a integridade superficial de componentes usinados por intermédio de controle dos principais parâmetros de usinagem. Experimentos foram realizados sob diversas condições de corte e geometrias de ferramentas para avaliação dos parâmetros: força de corte, temperatura, emissão acústica e integridade superficial (rugosidade superficial, tensão residual, microdureza e microestrutura) e mecanismos de desgaste. Mediante os resultados apresentados, recomenda-se à geometria de melhor desempenho nos parâmetros citados e confirma-se a eficiência da técnica MQL. Dentre as ferramentas e geometrias testadas, a que apresentou melhor desempenho foi a ferramenta cerâmica CC650 seguida da ferramenta cerâmica CC670 ambas com formato redondo e geometria 2 (chanfro em T de 0,15 x 15º com raio de aresta de 0,03 mm). / Researchers and industry personnel around the world are firmly committed to the purpose of doing the machining process dramatically faster and more precise. The tough global competition has generated a search for more economical machining processes, with high ability for chip removal and, in this way, producing high quality workpieces. Among the new technologies available nowadays, the high speed machining (HSM) is pointed out as the main solution to obtain competitiveness in a short period of time. The HSM technology appears as an essential component to optimize the efforts for maintaining, and increasing, the global competitiveness. During the last years, high speed machining technology has received great attention, specially the development and availability in the market of machine tools with high rotational speeds (20.000 - 100.000 rpm). The HSM has been used not only to machine aluminum and copper alloys, but also to difficult to machine rnaterials, such as hardened steels and nickel based superalIoys. However, for difficult to machine materiais, the literature is very incipient, specially concerning the turning process. However, before the HSM technology be used in an economic way, alI the components involved in the machining process, including the machine, the spindle, the tool and the operators, need to be tuned with the peculiarities of this new process. Concerning the tooling, they have to satisfy peculiar requirements of safety. Due to the optimization of their geometries, substrates and coatings, the cutting tools are contributing to the success of the process. The present work aims at the study of several insert geometries of ceramic tools (Al2O3 + SiCw and Al2O3 + TiC) and PCBN, with two concentrations of CBN, in the standard format and with modifications on the cutting edge geometry, working in the high speed turning of nickel based superaIloys (lnconel 718 and Waspaloy]. MateriaIs were heat treated to hardness of 44 and 40 HRC, respectively, and machined under dry cutting condition and also with minimal quantity of lubricant (MQL) to attend environmental requirements. The nickel based superalloys are known as difficult to cut materials due to their high hardness, high mechanical strength at high temperature, chemical affinity to tool materiaIs and lower thermal conductivity. The machining of superalloys affects negatively the integrity of the workpiece. For this reason, tool life and surface integrity of the machined component must be carefully analyzed throughout the control of the main machining parameters. Practical experiments were implemented using several cutting conditions and tool geometries to evaluate the following parameters: cutting force, temperature, acoustic emission and surface integrity (surface finishing, residual stress, microhardeness and microstructure) and wear mechanisms. Analyzing the results, the most suitable geometry for the mentioned parameters is recommended and the efficiency of the MQL technical is confirmed. Among all inserts and geometries tested, the CC650 ceramic tool presented better results, followed by the CC670 ceramic tool, both with round format and edge geometry number 2 (chamfer in T 0,15 x 15° with hone of 0,03 mm).

Pesquisadores e indústrias de todo o mundo estão firmemente comprometidos com o propósito de fazer o processo de usinagem ser precisamente veloz e produtivo. A forte concorrência mundial gerou a procura por processos de usinagem econômicos, com grande capacidade de produção de cavacos e que produzam peças com elevada qualidade. Dentre as novas tecnologias que começaram a ser empregadas, e deve tornar-se o caminho certo a ser trilhado na busca da competitividade em curto espaço de tempo, está a tecnologia de usinagem com altas velocidades (HSM de High Speed Machining). A tecnologia HSM surge como componente essencial na otimização dos esforços para manutenção e aumento da competitividade global das empresas. Durante os últimos anos a usinagem com alta velocidade tem ganhado grande importância, sendo dada uma maior atenção ao desenvolvimento e à disponibilização no mercado de máquinas-ferramentas com rotações muito elevadas (20.000 - 100.000 rpm). O processo de usinagem com alta velocidade está sendo usado não apenas para ligas de alumínio e cobre, mas também para materiais de difícil usinabilidade, como os aços temperados e superligas à base de níquel. Porém, quando se trata de materiais de difícil corte, têm-se observado poucas publicações, principalmente no processo de torneamento. Mas, antes que a tecnologia HSM possa ser empregada de uma forma econômica, todos os componentes envolvidos no processo de usinagem, incluindo a máquina, o eixo-árvore, a ferramenta e o pessoal, precisam estar afinados com as peculiaridades deste novo processo. No que diz respeito às máquinas-ferramenta, isto significa que elas têm que satisfazer requisitos particulares de segurança. As ferramentas, devido à otimização de suas geometrias, substratos e revestimentos, contribuem para o sucesso deste processo. O presente trabalho objetiva estudar o comportamento de diversas geometrias ) de insertos de cerâmica (Al2O3 + SiCw e Al2O3 + TIC) e PCBN com duas concentrações de CBN na forma padrão, assim como modificações na geometria das arestas de corte empregadas em torneamento com alta velocidade em superligas à base de níquel (Inconel 718 e Waspaloy). Os materiais foram tratados termicamente para dureza de 44 e 40 HRC respectivamente, e usinados sob condição de corte a seco e com utilização da técnica de mínima quantidade de lubrificante (minimal quantity lubricant - MQL) visando atender requisitos ambientais. As superligas à base de níquel são conhecidas como materiais de difícil usinabilidade devido à alta dureza, alta resistência mecânica em alta temperatura, afinidade para reagir com materiais da ferramenta e baixa condutividade térmica. A usinagem de superligas afeta negativamente a integridade da peça. Por essa razão, cuidados especiais devem ser tomados para assegurar a vida da ferramenta e a integridade superficial de componentes usinados por intermédio de controle dos principais parâmetros de usinagem. Experimentos foram realizados sob diversas condições de corte e geometrias de ferramentas para avaliação dos parâmetros: força de corte, temperatura, emissão acústica e integridade superficial (rugosidade superficial, tensão residual, microdureza e microestrutura) e mecanismos de desgaste. Mediante os resultados apresentados, recomenda-se à geometria de melhor desempenho nos parâmetros citados e confirma-se a eficiência da técnica MQL. Dentre as ferramentas e geometrias testadas, a que apresentou melhor desempenho foi a ferramenta cerâmica CC650 seguida da ferramenta cerâmica CC670 ambas com formato redondo e geometria 2 (chanfro em T de 0,15 x 15º com raio de aresta de 0,03 mm). / Researchers and industry personnel around the world are firmly committed to the purpose of doing the machining process dramatically faster and more precise. The tough global competition has generated a search for more economical machining processes, with high ability for chip removal and, in this way, producing high quality workpieces. Among the new technologies available nowadays, the high speed machining (HSM) is pointed out as the main solution to obtain competitiveness in a short period of time. The HSM technology appears as an essential component to optimize the efforts for maintaining, and increasing, the global competitiveness. During the last years, high speed machining technology has received great attention, specially the development and availability in the market of machine tools with high rotational speeds (20.000 - 100.000 rpm). The HSM has been used not only to machine aluminum and copper alloys, but also to difficult to machine rnaterials, such as hardened steels and nickel based superalIoys. However, for difficult to machine materiais, the literature is very incipient, specially concerning the turning process. However, before the HSM technology be used in an economic way, alI the components involved in the machining process, including the machine, the spindle, the tool and the operators, need to be tuned with the peculiarities of this new process. Concerning the tooling, they have to satisfy peculiar requirements of safety. Due to the optimization of their geometries, substrates and coatings, the cutting tools are contributing to the success of the process. The present work aims at the study of several insert geometries of ceramic tools (Al2O3 + SiCw and Al2O3 + TiC) and PCBN, with two concentrations of CBN, in the standard format and with modifications on the cutting edge geometry, working in the high speed turning of nickel based superaIloys (lnconel 718 and Waspaloy]. MateriaIs were heat treated to hardness of 44 and 40 HRC, respectively, and machined under dry cutting condition and also with minimal quantity of lubricant (MQL) to attend environmental requirements. The nickel based superalloys are known as difficult to cut materials due to their high hardness, high mechanical strength at high temperature, chemical affinity to tool materiaIs and lower thermal conductivity. The machining of superalloys affects negatively the integrity of the workpiece. For this reason, tool life and surface integrity of the machined component must be carefully analyzed throughout the control of the main machining parameters. Practical experiments were implemented using several cutting conditions and tool geometries to evaluate the following parameters: cutting force, temperature, acoustic emission and surface integrity (surface finishing, residual stress, microhardeness and microstructure) and wear mechanisms. Analyzing the results, the most suitable geometry for the mentioned parameters is recommended and the efficiency of the MQL technical is confirmed. Among all inserts and geometries tested, the CC650 ceramic tool presented better results, followed by the CC670 ceramic tool, both with round format and edge geometry number 2 (chamfer in T 0,15 x 15° with hone of 0,03 mm).

Superalloys offer high temperature strength, excellent creep, corrosion and oxidation resistances, microstructural stability and good fatigue life at elevated temperatures. The composition of the superalloys has been modified continuously to improve the properties. The addition of Pt improves oxidation resistance without compromising the mechanical properties of the superalloys. To further enhance the performance of the superalloy components, various coatings are applied on them. The-(NiPt)Al intermetallic compound bond coats, which are presently utilized, have certain drawbacks. Diffusion of Al from the bond coat to superalloy during service leads to accumulation of stress near the bond coat. The refractory elements present in superalloy precipitate as topological close packed (TCP) phases in the interdiffusion zone. Consequently, a Pt enriched γ(Ni) + γ’(Ni3Al) phase mixture has been proposed as a possible alternative since TCP phases do not form in the interdiffusion zone. In this thesis, diffusion studies are performed on several binary and ternary systems with the primary purpose of understanding the effect of Pt in Ni based superalloys and also in γ + γ’ phase mixture bond coats. Further, a detailed interdiffusion study is conducted in Mo- and W- based binary and ternary systems to understand the growth of the TCP phases. By performing bulk and multifoil diffusion couple experiments, different diffusion parameters like, inter, intrinsic, tracer, impurity diffusion coefficients and activation energy that are necessary to understand the diffusion mechanism are determined. Additionally using the nanoindentation technique on diffusion couples, variation of mechanical properties such as, hardness and modulus with composition is studied. First, interdiffusion in Ni-Pt, Co-Pt, Co-Ni, Ni-Fe and Co-Fe binary systems is examined. In Ni-Pt and Co-Pt, experimental results show that Pt is the slower diffusing species at all compositions. In both the systems, driving force is found to be the reason for higher values of intrinsic diffusion coefficients observed in the range of 40-60 at. % Pt. Contribution of vacancy wind effect on diffusion parameters is found to be negligible. It is found from the multifoil diffusion couple experiments that Ni is the faster diffusing species in the Co-Ni system. Bulk diffusion couple experiments are conducted in the Co-Ni-Pt and Co-Ni-Fe systems, by coupling binary alloys with the third element. Uphill diffusion is observed for Co and Ni in Pt rich corner of the Co-Ni-Pt system. Main and cross interdiffusion coefficients are calculated at the compositions where two diffusion profiles intersect. In both the systems, the main interdiffusion coefficients are positive over the whole composition range and the cross diffusion coefficients show both positive and negative values at different regions. Hardness measured by performing the nanoindentations on diffusion couples of both the systems, shows the higher values at intermediate compositions. The effect of Pt in and’ phases of Ni-Al system are examined by conducting interdiffusion experiments between Ni(xPt) alloys and (NixPt)40Al alloy of β phase, so that both and’ phases grow in the interdiffusion zone. The interdiffusion coefficients in Ni-Al binary system increases with the Al content in the -phase, and they do not vary significantly with composition in the ’ phase. The average effective interdiffusion coefficients of Ni and Al in the and ’ phases increase with the addition of Pt. Nanoindentation studies on diffusion couples show that the hardness of both and ’ phase increases with the addition of Pt. In the +’ phase mixture bond coats, effect of Pt on interdiffusion of major alloying elements of CMSX4 superalloys are discussed. A phase mixture of and ’ with increasing Pt content is coupled with CMSX4 superalloy. The addition of Pt to the +’ phase mixture increases the diffusion rate of Ni, while the diffusion rate of Al, decreases with the addition of 5% Pt, and increases with further addition of Pt. No significant change in the diffusion rates of Co or Cr is observed. The growth kinetics and diffusion in systems (both binary and ternary) with TCP phases are examined. Interdiffusion studies performed in Co-Mo system show significant volume change because of the growth of the phase. Intrinsic diffusion coefficient of Mo is found to be higher than that of Co. Diffusion studies conducted in Ni-Mo system show reasonably low activation energy in the phase, indicating the grain boundary controlled diffusion process. The Co-Ni-Mo and Co-Ni-W ternary phase diagrams are revisited and the phase boundary composition of the TCP phases are found to be different from those reported earlier. Following, the average effective interdiffusion coefficients are calculated and compared with the data calculated in the binary systems to examine the role of the third element. It is noticed that the average effective interdiffusion coefficients in the Co(Ni,Mo) and Co(Ni,W) solid solution increases with the addition of Ni. On the other hand, these diffusion coefficients decrease with the addition of Ni in thephase in both the systems. The role of the driving force for diffusion and possible change in defect concentrations on different sublattices are discussed.