One of the main sources of energy in this modern world is the gas turbine.  They reach extremely high working temperatures at peak capacity and hence require materials that can withstand the same.  Nickel-based superalloys have been widely used as working materials due to their various high-temperature properties, which help protect them from oxidation, corrosion,  and  creep.  This  high-temperature  strength  is  provided by the  gamma  prime (γ′)  phase  for  most  nickel-based  alloys.  This  study  deals  with understanding  the  effect of the mean γ′ phase size affected by different times and temperatures.  Different trial alloys have undergone different primary ageing cycles, followed by a subsequent secondary ageing  cycle.  Further,  the  mean  gamma  prime size  calculated  using  Image-Pro  coupled with  a  probability  density  function  for the mean  size  showed  significant  γ′  coarsening. Different temperatures and holding times also allowed for significant growth of secondary gamma prime structures.  These structures were found to impact the mean gamma prime size calculations significantly with their influence on the mechanical properties remaining unknown.  However, the mean gamma prime size remains similar to what was desired. / En av de viktigaste energikällorna i denna moderna värld är gasturbinen. De når extremt höga arbetstemperaturer vid toppkapacitet och kräver därför material som klarar av den höga temperaturen. Nickelbaserade superlegeringar har använts i stor utsträckning som material på grund av deras olika högtemperaturegenskaper, som hjälper till att skydda dem från oxidation, korrosion och krypning. Denna högtemperaturs hållfasthet uppnås från gamma prim-fasen (γ′) för de flesta nickelbaserade legeringar. Denna studie handlarom att förstå effekten av den genomsnittliga γ′-fasstorleken som påverkas av olika tider och temperaturer. Olika legeringar har genomgått olika primära åldringscykler, följt av en sekundär åldringscykel. Vidare visade den genomsnittliga gamma-prim storleken som beräknades med Image-Pro tillsammans med en sannolikhets densitetfunktion för medelstorleken av γ′ förgrovning. Olika temperaturer och hålltider möjliggjorde också betydande tillväxt av sekundära gamma-primstrukturer. Dessa strukturer visade sig påverka beräkningarna av den genomsnittliga gamma-primstorleken och egenskaperna av de mekaniska egenskaperna förblir okända. Den genomsnittliga gamma-primstorleken blir liknande det som var önskvärt.

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Increasing energy demand leads to two crucial problems for the whole society. One is the economic cost and the other is the pollution of the environment, especially CO2 emissions. Despite efforts to adopt renewable energy sources, fossil fuels will continue to dominate. The temperature and stress are planned to be raised to 700 °C and 35 MPa respectively in the advanced ultra-supercritical (AUSC) power plants to improve the operating efficiency. However, the life of the components is limited by the properties of the materials. The aim of this thesis is to investigate the high temperature properties of materials used for future power plants. This thesis contains two parts. The first part is about developing creep rupture models for austenitic stainless steels. Grain boundary sliding (GBS) models have been proposed that can predict experimental results. Creep cavities are assumed to be generated at intersection of subboundaries with subboundary corners or particles on a sliding grain boundary, the so called double ledge model. For the first time a quantitative prediction of cavity nucleation for different types of commercial austenitic stainless steels has been made. For growth of creep cavities a new model for the interaction between the shape change of cavities and creep deformation has been proposed. In this constrained growth model, the affected zone around the cavities has been calculated with the help of FEM simulation. The new growth model can reproduce experimental cavity growth behavior quantitatively for different kinds of austenitic stainless steels. Based on the cavity nucleation models and the new growth models, the brittle creep rupture of austenitic stainless steels has been determined. By combing the brittle creep rupture with the ductile creep rupture models, the creep rupture strength of austenitic stainless steels has been predicted quantitatively. The accuracy of the creep rupture prediction can be improved significantly with combination of the two models. The second part of the thesis is on the fatigue properties of austenitic stainless steels and nickel based superalloys. Firstly, creep, low cycle fatigue (LCF) and creep-fatigue tests have been conducted for a modified HR3C (25Cr20NiNbN) austenitic stainless steel. The modified HR3C shows good LCF properties, but lower creep and creep-fatigue properties which may due to the low ductility of the material. Secondly, LCF properties of a nickel based superalloy Haynes 282 have been studied. Tests have been performed for a large ingot. The LCF properties of the core and rim positions did not show evident differences. Better LCF properties were observed when compared with two other low γ’ volume fraction nickel based superalloys. Metallography study results demonstrated that the failure mode of the material was transgranular. Both the initiation and growth of the fatigue cracks were transgranular. / <p>QC 20160905</p>

A superliga policristalina à base de níquel MAR-M247 foi desenvolvida na década de 70. Sua ótima composição, estabilidade microestrutural, excelente resistência à fluência e à oxidação e boa resistência à corrosão a quente faz dessa superliga uma das mais utilizadas em rotores durante os últimos 20 anos. Os objetivos do presente trabalho são a produção e caracterização microestrutural e mecânica de rotores da superliga MARM247 (10% peso Co, 10W, 8.25Cr, 5.5Al, 3Ta, 1.5Hf, 1Ti, 0.7Mo, 0.15C, 0.05Zr, 0.015B, Ni balanço). Os rotores foram obtidos pela técnica de microfusão por cera perdida na empresa Açotécnica S.A. (Jandira/SP) em diferentes temperaturas de vazamento e isolamento do molde cerâmico. Os materiais foram analisados por microscopia ótica e eletrônica de varredura (MEV/EDS), análise térmica diferencial (DTA), macro/microdureza e ensaios de tração convencional a 25oC e a 760oC. Além disso, os materiais passaram por tratamentos térmicos de solubilização e envelhecimento em diferentes temperaturas e tempos a fim de avaliar as mudanças microestruturais. Com a ajuda do software de cálculo termodinâmico Thermocalc foram feitas simulações das relações de fases utilizando a composição média da superliga MAR-M247. Os resultados permitiram obter uma condição de processamento (temperatura de vazamento e condição de isolamento) com maior quantidade de peças sem defeitos. Além disso, os experimentos permitiram definir uma condição ideal para o tratamento térmico de solubilização da superliga MAR-M247. / The polycrystalline nickel-based superalloy MAR-M247 was developed in the 1970s. Its optimal alloy composition, microstructural stability, excellent resistance to creep and oxidation and good resistance to hot corrosion make the MAR-M247 superalloy one of the alloys widely used in turbine rotors during the last 20 years. The aims of the present work are the production and the microstructural and mechanical characterization of rotors made of MAR-M247 superalloy (10 wt. % Co, 10W, 8.25Cr, 5.5Al, 3Ta, 1.5Hf, 1Ti, 0.7Mo, 0.15C, 0.05Zr, 0.015B, Ni balance). The rotors were produced by using the lost-wax technique at different pouring temperatures and isolation of the shell mold at the Açotécnica S.A. company (Jandira/SP). The materials were analyzed by optical and scanning electron microscopy (SEM/EDS), differential thermal analysis (DTA), macro/microhardness and tensile tests at 25oC and 760oC. Besides, the materials were submitted to solution and ageing treatments at different temperatures and times in order to evaluate de microstructural changes. With the aid of the thermodynamic calculation software Thermocalc, simulations of phase relations were made using the average composition of the MAR-M247 superalloy. The results allowed to obtain a processing condition (pouring temperature and isolation condition) with larger amount of rotors without defects. Moreover, the experiments allowed to define an ideal solution heat treatment condition of MAR-M247 superalloy.

O uso do cálculo termodinâmico e consequentemente base de dados desenvolvidas para esse propósito é hoje uma das ferramentas fundamentais para o aprimoramento e desenvolvimento de superligas a base de níquel. A proposta deste trabalho foi avaliar a confiabilidade de uma base de dados comercial, específica para superligas a base de níquel, utilizada em softwares de cálculos termodinâmicos computacionais. Comparou-se os dados experimentais da superliga Inconel 713LC com informações obtidas através do calculo termodinâmico para a composição química específica da superliga estudada. As amostras da superliga comercial Inconel 713LC foram retiradas de barras da liga fundida a vácuo (VIM). Os tratamentos térmicos foram realizados sob atmosfera de argônio em cápsulas de quartzo nas temperaturas de 600?C, 900?C e 1250 ?C nos tempos de 10h, 24h, 50h, 100h, 250h, 500h, 1000h e 2000 horas, de acordo com a temperatura de tratamento utilizada, visando acompanhar o desenvolvimento microestrutural até alcançar a condição de equilíbrio termodinâmico. Previamente, a amostra no estado bruto de fusão e as amostras após os tratamentos de equilíbrio foram caracterizadas microestruturalmente via microscopia eletrônica de varredura (MEV) e a composição das fases medidas por microanálise de energia dispersiva (EDS). As temperaturas das transformações de fases foram medidas por analise térmica diferencial (DTA). Nas amostras também foram realizadas medidas de microdureza para acompanhar indiretamente a evolução microestrutural da fase gama prime. Para os cálculos termodinâmicos foram utilizados o software Thermo-Calc® e a base de dados TTNi8 da empresa Thermotech Ltd®. / The computational thermodynamic simulation and the databases designed for this purpose are today one the most important tools to develop new nickel based superalloys. The aim of this work was to check the reliability of a commercial database, designed specifically for Ni-based superalloys. The experimental phase diagram data of Inconel 713LC, such as, liquidus and solidus temperatures as well as chemical phase compositions were compared with thermodynamic information extracted from computational simulations. Samples of a commercial superalloy Inconel 713LC were extracted from an ingot produced by Vacuum Induction Melting (VIM) and heat treated under argon in quartz capsules at 600?C, 900 ?C and 1250?C for 10h, 24h, 50h, 100h, 250h, 500h, 1000h and 2000h, according to the treatment temperature used, aiming at reaching the thermodynamic equilibrium conditions. The as-cast and all heat-treated samples were characterized by Scanning Electron Microscopy (SEM) and the chemical compositions of the phases were measured via Energy Dispersive X-ray Spectroscopy (EDS). Temperatures of phase transformations were determined by Differential Thermal Analysis (DTA). Microhardness measurements were performed to indirectly follow the microstructural evolution of gamma prime. The Thermo-Calc® software and the TTNi8 database from Thermotech Ltd® were used for thermodynamic calculations.

This study develops a method to simulate a full-scale inertia friction weld with a sub-scale specimen and modifies a direct drive friction stir welder to perform the welding process. A torque meter is fabricated for the FSW machine to measure weld torque. Machine controls are modified to enable a force control during the IFW process. An equation is created to measure weld upset due to deflection of the FSW machine. Data obtained from a full-scale inertia friction weld are altered to account for the geometrical differences between the sub-scale and full-scale specimens. The IFW are simulated with the sub-scale specimen while controlling spindle RPM and matching weld power or weld RPM. The force used to perform friction welding is scaled to different values accounting for specimen size to determine the effects on output parameters including: HAZ, upset, RPM, torque, power and energy of the weld. Increasing force has positive effects to upset, torque, power and energy of the welds, while reducing the size of the HAZ.

Les générateurs de vapeur des réacteurs à eau pressurisée (REP) sont constitués d'alliage à base nickel. La surface d'échange associée représente environ 75 % de l'aire totale du circuit primaire. Sous l'effet de la corrosion généralisée, des cations nickel sont relâchés dans l'eau de ce circuit. Lorsque ces cations sont exposés au flux neutronique du coeur du réacteur, ils sont activés en éléments radioactifs (58Co). La circulation puis l'incorporation de ces produits de corrosion radioactifs sur les parois du circuit primaire posent alors problème lors des interventions humaines durant les opérations de maintenance.Le but de l'étude présentée ici est de créer par traitement thermomécanique une couche d'oxyde protectrice qui limite fortement le relâchement des cations nickel. L'influence de paramètres tels que la composition de l'atmosphère gazeuse lors du traitement thermique et l'état de surface initial de l'alliage considéré (laminage et rugosité de surface) a été étudiée. Les résultats présentés sont obtenus sur un alliage 690 oxydé à 700 °C sous deux atmosphères gazeuses de pressions partielles en oxygène (P(O2)) différentes.L'étude des cinétiques d'oxydation par analyse thermogravimétrique (ATG), met en avant une cinétique parabolique peu dépendante de la P(O2). Un laminage de 20 % n'affecte pas la cinétique d'oxydation alors qu'un laminage de 40 % et une rugosité de surface plus forte sont responsables d'une prise de masse plus importante. Un marquage à l'or permet de déterminer que la croissance de la couche est majoritairement cationique. L'aspect général de la surface des échantillons oxydés observé par microscopie électronique à balayage (MEB) est similaire quelle que soit la P(O2) : une couche d'oxyde continue d'aspect nanostructuré riche en chrome avec oxydation préférentielle du chrome le long des joints de grains de l'alliage sous-jacent. La spectroscopie de photoélectrons-X (XPS), la spectrométrie à décharge luminescente (SDL) et la diffraction des rayons-X (DRX) mettent en évidence la croissance d'une couche de chromine (Cr2O3) et la présence à l'interface gaz/oxyde de cristallites avec une cristallographie de type spinelle. Pour une faible P(O2) (de 1.10 -6 bar à 5.10 -6 bar) cette partie externe est riche en manganèse (MnCr2O4) alors que des cristallites riches en manganèse, chrome, nickel et en fer viennent s'intégrer dans cette configuration lors d'oxydations sous une P(O2) plus élevée (0,2 bar). L'ensemble des résultats obtenus permet de proposer des conditions de prétraitement sur l'alliage 690. Des échantillons ainsi traités sont testés en milieu primaire simulé et leur relâchement en nickel montre une nette baisse par rapport à des échantillons non traités thermiquement. / Steam generator tubes in pressurized water reactors (PWR) are made of nickel based alloy and represent nearly 75 % of the primary coolant loop surface. Due to generalized corrosion phenomena, nickel cations are released in the primary loop water. After neutron flux exposure in the reactor core, nickel cations can turn into radioactive products, such as 58Co. If carried through the circuit, these radioactive products precipitate and contaminate the loop, making maintenance more difficult.The present study goal is to form a very protective scale on the Alloy 690 surface by a pre-oxidation treatment performed at 700 °C. The impact of different parameters such as oxygen partial pressures of the gaz used during high temperature oxidation (low P(O2)=10 -6 bar or high P(O2)=0.2 bar) and the initial surface state of the sample (cold-worked or surface roughness) is studied. To minimize nickel cation release, the scale obtained must be continuous, homogeneous, rich in chromium and nickel free. The oxidation kinetics, studied using thermogravimetric analysis (TGA), show the existence of parabolic law independent of P(O2). The sample's surface aspect, observed using scanning electron microscopy (SEM), is similar for all P(O2). Top views show a preferential chromium oxidation along the grain boundaries and the formation of a chromium rich oxide scale, confirmed by X-ray photoelectron spectroscopy (XPS) and glow discharge optical emission spectrometry (GDOES); cross sectional views of the material exhibit holes beneath the oxide corresponding with the alloy's grain boundaries. The latter observation and an inert marking test with gold plots suggest a cationic growth of the oxide scale. X-ray diffraction (XRD) indicates the presence of chromia (Cr2O3) for low and high P(O2). For low P(O2), a Mn and Cr spinel is observed near the outer surface. For higher P(O2), a similar spinel layer is made up of Ni, Fe, Mn and Cr.Pretreatment conditions set to be applied on alloy 690 can be proposed thanks to the data collected. Samples, pretreated that way, are tested in simulated primary medium water and their nickel release is clearly cropping compared to non-thermaly-treated samples.

Tato práce obsahuje přehled superslitin na bázi niklu, jejich obrábění za vysokého tlaku chladicí kapaliny, týká se také metod optimalizace pro tento typ obrábění a následné analýzy opotřebení nástroje. Tato studie byla vytvořena ve spolupráci se společností Safran, experimentální část byla zrealizována ve výrobním závodě Evry-Corbeil ve Francii. Experimentální část práce se týká optimalizace řezných podmínek pro vnitřní hrubovací soustružení součástí pro letecký průmysl za vysokého tlaku chladicí kapaliny. Cíle práce byly následující: srovnání rozdílných vyměnitelných břitových destiček dodaných společností Safran, návrh hrubovacích strategií a určení optimálních podmínek pro každou z vybraných strategií. Byly uvažovány dvě rozdílné strategie: soustružení s konstantním posuvem a šířkou záběru ostří a tzv. „ramping“. Pro každou ze strategií byla provedena operace čelního a podélného soustružení. Výsledky byly porovnány na základě ekonomického a produktivního kritéria.

Les traitements de surface mécaniques sont appliqués dans la plupart des secteurs industriels comme procédé de finition afin de renforcer les propriétés des composants métalliques. Le grenaillage de précontrainte est probablement l’un des plus répandu. Ce procédé introduit des contraintes résiduelles de compression en générant un gradient de déformation plastique dans la profondeur de la pièce traitée. L’objectif de ce travail est de comprendre et prédire l’effet de la géométrie des composants sur la redistribution des contraintes résiduelles post-grenaillage. En effet, même lorsqu’elle est maîtrisée, l’opération de grenaillage peut générer un champ de contraintes résiduelles complexe qui dépend fortement de la géométrie de la pièce. Par suite, parmi les paramètres influents sur le comportement en fatigue des composants grenaillés, le paramètre géométrique peut donc avoir un rôle majeur. Puisque les approches conventionnelles de modélisation ne sont pas transposables aux géométries non planes, et ne sont pas conformes aux contraintes industrielles en termes de temps de calcul, une méthodologie basée sur la Méthode de Reconstruction des Eigenstrains est proposée. L’approche développée est construite à partir de relations analytiques pour des massifs plans traités de façon homogène. La principale contribution est la comparaison entre modélisation et expérimentation. Les données expérimentales sont obtenues à partir d’analyses de la microstructure et par diffraction des rayons X réalisées sur des échantillons d’un superalliage base nickel, pour plusieurs géométries complexes élémentaires (plaques minces, formes convexes et concaves). Par ailleurs cette étude vise à prendre en compte l’effet des contraintes résiduelles équilibrées sur la durée de vie en fatigue. A partir du critère de fatigue multiaxial de Crossland, la méthodologie complète est appliquée à des démonstrateurs industriels à géométrie complexe. / Most manufacturing industries perform mechanical surface treatments at the end of the manufacturing chain to reinforce relevant working parts. Shot peening is probably the most common of those processes. This treatment induces compressive residual stresses by generating in-depth plastic strains. The objective of this work is to understand and predict the effect of the geometry on the redistribution of residual stresses into shot peened mechanical parts. Indeed, even when properly controlled, shot peening treatment may induce a complex residual stress field depending on the geometry of the treated part. Hence, among the variables which affect the fatigue behaviour of shot peened components, the geometry could play a major role. Because the traditional approaches for the modelling of residual stresses are not convenient for complex non-flat geometries and not consistent with industrial constraints in terms of computing time, a methodology based on the Eigenstrains Reconstruction Method is proposed. The developed approach is built with analytical relationships for massive and plane geometries homogeneously treated. The main contribution lies in the capacity to provide a comparison between modelling and experiment. Experimental data are obtained by microstructural observation and by X-ray diffraction analyses, which are carried out on Ni-based superalloy samples with elementary complex geometries (thin sheets, convex and concave shapes). In addition, this study aims to take into account the effect of the rebalanced residual stresses for fatigue life prediction. Thus, using a Crossland criterion for high cycle fatigue regime, the complete methodology is applied on industrial demonstrator samples with complex geometry.

Les travaux présentés dans cette thèse sont réalisés dans le cadre du projet européen ACCENT et s’inscrivent dans la problématique de l’utilisation de la surveillance d’usinage dans la fabrication des pièces critiques de moteurs aéronautiques. L’objectif est de définir une technique de surveillance de l’usure de l’outil dans l’usinage de l’alésage central d’un disque haute pression réalisé en superalliage à base nickel. Trois grandes étapes peuvent être identifiées dans notre démarche : l’identification des modes de dégradation dans le perçage de l’Udimet® 720 LI, la définition de relations entre les différents types d’usure et les grandeurs physiques mesurées et finalement, la proposition de stratégies de surveillance basées sur les résultats précédents. Afin de répondre aux objectifs, un protocole expérimental a été défini. Les campagnes expérimentales nous ont permis de caractériser l’usure du foret et de définir une cartographie des modes de dégradation en fonction de conditions de coupe testées. Par ailleurs, l’impact des perturbations d’usinage (coupure de lubrification, modification du pourcentage d’émulsion du lubrifiant, défaut de position de l’outil par rapport à l’avant-trou, etc.) a été étudié. La deuxième partie consiste à identifier les relations entre les modes de dégradation et les signaux de surveillance enregistrés (efforts, couple, puissance, vibrations, etc.). Pour cela, nous avons privilégié deux pistes : une explication directe de l’évolution des signaux par des phénomènes physiques et une approche selon « la base de données », pour développer des modèles d’usures par des critères statistiques extraits sur les signaux. Cette analyse a aussi permis la détection de certaines perturbations d’usinage en utilisant les grandeurs physiques mesurées. Il s’agit notamment d’un défaut d’excentration de l’outil dans le porte-outil et d’un problème d’évacuation de copeaux. La troisième étape porte sur la sensibilité des signaux afin de détecter l’ensemble des phénomènes et la définition des stratégies de surveillance pouvant être mises en œuvre. Pour chaque relation définie, nous avons déterminé une probabilité de détection. Ensuite, nous avons utilisé les signaux en parallèle afin d’accroitre la robustesse de la détection. Les taux de probabilité ont été améliorés, en se basant sur l’hypothèse d’indépendance des signaux (sans calculer les biais existants). Enfin, l’association de l’approche phénoménologique à l’approche dite « statistique » a permis d’augmenter la robustesse de la surveillance. / The work presented in this thesis is a part of European project ACCENT and fits into the context of using process monitoring in the manufacture of critical parts for aircraft engines. The aim is to define a monitoring technique for the tool wear in the machining of the central bore of a high pressure disk made in nickel based superalloy. The major stages can be identified in our approach: identification of tool failure modes in drilling Udimet® 720 LI, defining the relationships between wear and measured physical quantities and finally, a proposal for monitoring strategies based on the previous results. In order to complete the objectives, an experimental protocol was defined. The experimental campaigns allowed us to characterize the drill wear and to define cartography of tool failure modes depending on cutting conditions tested. Moreover, the impact of machining disturbances (lubricant stop, change in percentage of lubricant emulsion, the position over the before hole, etc.) was studied. In the second part, the relations between the tool failure modes and monitoring signals recorded during the machining (forces, torque, power, vibrations, etc.) were identified. For this task, we focused on two directions: a straightforward explanation of signals evolution by physical phenomena and “black box” approach to develop wear models from a database obtained by statistical criteria extracts from the signals. This analysis also allowed the detection of certain machining disturbances using the physical quantities. These include an eccentricity of the tool in the tool holder and a chips evacuation problem. The third part concerns the signals sensitivity to detect the different phenomena and the definition of monitoring strategies which can be implemented. For each relation defined, a detection probability was determined. Further, we used signals in parallel to increase the robustness of detection. The probabilities were improved, with an assumption of independent signals (without calculating existing biases). Finally, the combination of the phenomenological approach to the so-called "statistical" has increased the robustness of the monitoring technique.