The thesis work discuss about how the materials mechanical properties are influenced by the microstructure. The most common way of altering the microstructure of the material is by heat treatment.The mechanical properties that are of interest are strength, toughness, ductility, creep and fatigue. The material under consideration iswidely used superalloy In718. Two different sets of specimens areheat treated at different temperatures and influence of heat treatmenton the grain size is analyzed. In order to get better understanding ofthe grain size on mechanical properties, microstructural investigation was done using SEM. Efforts are made to understand the influence of different elements on the overall characteristic of the material. The tensile, creep and stress relaxation tests were conducted and the results were discussed. / Master Thesis

L’Inconel 738 est un superalliage base nickel utilisé dans les turbines de moteurs aéronautiques. Le soudage engendre souvent l’apparition de fissures, ce qui constitue un handicap pour ce matériau car on ne peut ni le réparer par rechargement, ni concevoir de pièces assemblées par soudage. L'objectif de la thèse est de définir les conditions de soudage de ce superalliage, de manière à exclure tout risque de fissuration à la suite d’opérations de rechargement, de réparation ou d'assemblage par soudage.Une étude expérimentale a tout d’abord permis de comprendre et de relier les mécanismes de fissuration aux paramètres opératoires du procédé (intensité, tension…). Puis un modèle de simulation numérique du soudage a été développé sur la base d’un couplage entre aspects thermiques, métallurgiques et mécaniques. La corrélation entre les résultats expérimentaux et numériques a permis de déterminer des critères de fissuration pour ce superalliage et d’optimiser les paramètres de soudage. / Abstract

The aim of this work is to characterize the microstructural and mechanical behaviors of Additive Manufactured (AM) Inconel 718 and investigate the feasibility of using this material to produce thermal management devices. This was done in two parts: a literature review of AM phenomena that effect heat transfer rates and impede or benefit production using these methods, and a study that characterized Laser Engineered Net Shaping (LENS) Inconel 718 while investigating and manipulating the thermal history. The literature review provides evidence that while there are still many unsolved issues, it’s quickly becoming possible to use AM to produce thermal management devices that will exceed current limitations. The study showed that although LENS Inconel 718 exhibits nonstandard material behaviors, evidence indicate that it’s possible to control these behaviors to influence desired results. Overall, it is believed that the use of AM in producing thermal management devices will be inevitable and beneficial.

Inconel 718 is a nickel-based superalloy with a series of superior properties, such as high strength, creep-resistance, and corrosion-resistance. Additive manufacturing (AM) is appealing to Inconel 718 because of its near-net-shape production capability to circumvent poor machinability. However, AM parts are prone to detrimental porosity, reducing their fatigue resistance. Thus, further understanding of AM fatigue behavior is required before widespread industrial use. The microstructural and fatigue properties of heat treated AM Inconel 718, produced using Laser Engineered Net Shaping (LENSTM), are evaluated at room and elevated temperatures. Fully reversed, strain-controlled fatigue tests were performed on cylindrical specimens at strain amplitudes of 0.001 to 0.01 mm/mm. Fracture surfaces were inspected using a scanning electron microscope (SEM). Heat treatment caused initial dendritic microstructure to mostly reorm into an equiaxed grain structure. AM specimens experienced reduced fatigue life in testing as compared to wrought material due to inclusions or pores near the surface

Inconel X-750 is a Ni-rich super-alloy with high strength and creep resistance. In CANDU reactors, it is used as tight fitting fuel channel annulus spacers (in the form of a spring). Unlike other reactor designs, the CANDU reactor has a high thermal neutron flux spectra, which, combined with the higher thermal neutron cross section of 58Ni results in an enhancement of the radiation damage and the internal production of helium and hydrogen. In recent years, it has been observed these spacers are losing ductility and strength following irradiation. The mechanical property evolution of these components is dependent on the irradiation temperature and dose. The primary degradation mechanism remains unclear, and thus provides the focus of this investigation. Inconel X-750 irradiated to these extreme conditions have never been examined prior to this research. The microstructural characterization included in this dissertation include: fractography, microstructural evolution and mechanical property evolution. The discussion of the microstructural evolution is focused on the characterization of helium bubbles. The bubbles form homogeneously in the matrix and aligned along sinks such as dislocations, grain boundaries and precipitates. Electron energy loss spectroscopy (EELS) has been used to probe individual nano-sized bubbles to provide insights into the helium density, helium-to-vacancy ratio, and pressures through the use of a hard sphere equation of state (HSEOS). In addition to understanding the influence of irradiation on helium bubbles, the evolution of secondary strengthening precipitates, gamma prime, are of interest as these precipitates play an important role on the strength and creep resistance of the unirradiated material. The stability of these precipitates with irradiation is thus an important factor to consider with respect to the microstructural degradation. The microstructure is linked to the mechanical properties via microhardness testing on adjacent material. A major contribution to this field is an approach to utilize a focused ion beam (FIB) and transmission electron microscopy (TEM) to perform high ii resolution failure analysis of an intergranular fracture surface. Although this technique is not altogether revolutionary, the application of this approach towards post irradiation examination of heavily irradiated Inconel X-750 is unique. This approach provides direct evidence of likely degradation mechanisms, and provides insights for future post irradiation failure analysis for other applicable nuclear components. Working with ex-service material creates some complications with respect to known and unknown variables making it difficult to assess all factors responsible for material degradation. To compliment the program, a controlled proton irradiation program has been performed to gain additional insights into in the effects of irradiation on the microstructure and mechanical property evolution of Inconel X-750, due to the inability to perform controlled experiments with in-service reactor components. In addition to providing a detailed analysis of a CANDU component’s degradation, this study provides comprehensive information on irradiation damage processes applicable to other reactor core components. / Thesis / Doctor of Philosophy (PhD)

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No description available.

Cette étude traite de la prise en compte du gradient de contrainte et du frottement pour calculer l’amorçage de fissures en fatigue. On s’intéresse notamment au dimensionnement d’une manille en Inconel 718 munie de bagues en Inconel 718montées avec interférence. Pour cela, on réalise des essais de fretting et de fatigue pour étudier les conditions d’amorçage de fissures en présence de gradient de contrainte et de frottement. Les résultats expérimentaux mettent en évidence l’effet du gradient des contraintes sur l’amorçage de fissures et justifient la prise en compte du gradient pour le dimensionnement en fatigue des structures. A travers ces essais balayant une gamme de gradient de contrainte étendue, on confirme que la présence d’un gradient de contrainte retarde l’amorçage de fissures. Les avantages et les inconvénients de plusieurs approches de fatigue multiaxiale intégrant le gradient de contrainte sont étudiés. De cette analyse, une proposition est faite permettant de décrire au mieux les résultats expérimentaux de fatigue et de fretting. Cette approche s’appuie sur l’utilisation d’une fonction de poids dépendant du gradient de la pression hydrostatique. Cette approche est similaire à des approches mises en oeuvre sur d’autres problématiques de fatigue ce qui permet de dégager quelques points de convergence pour traiter le problème du gradient de contrainte en fatigue. Un essai de fatigue sur une manille munie d’une bague montée avec interférence est réalisé. L’effet du gradient de contrainte sur l’amorçage de fissures dans ce système est trop faible pour que l’utilisation d’une approche intégrant l’effet de gradient soit pertinente. Cette observation est cohérente avec les résultats expérimentaux précédents obtenus dans cette étude. L’inconel 718 apparaît donc comme un matériau peu sensible à l’effet de gradient de contrainte. Une approche de fatigue multiaxiale de type Crossland permet une description raisonnable de la durée de vie du système. Une analyse numérique montre que la tenue en fatigue du système est peu sensible au coefficient de frottement alors que l’interférence a un impact beaucoup plus important. / This study focuses on the stress gradient effect and the friction to estimate the crack initiation condition in fatigue. The industrial application is a lug in Inconel 718 with shrink fitted rings also in Inconel 718.Fretting tests and fatigue tests are performed to measure crack initiation conditions with friction and stress gradient. The experimental results confirm the beneficial effect of the stress gradients on the fatigue crack initiation. These results justify the consideration of stress gradients to design structures against fatigue. Through these experiments which sweep over a large range of stress gradients, it is confirmed that stress gradient delays the crack initiation. The benefits and the drawbacks of several multiaxial fatigue approaches dealing with stress gradients are studied. Then, a proposal is done to describe accurately the experimental results of fatigue and of fretting. The proposal is based on a weight function and on the use of the gradient of hydrostatic pressure. This approach is very similar to some approaches already applied to other fatigue issues. This enables to highlight some similarities to deal with the effect of stress gradient in fatigue. Fatigue tests are finally performed on a lug with shrink fitted rings. The stress gradient effect on this application is too low to make relevant the use of a fatigue approach dealing with stress gradient. These experimental results are consistent with the previous results of fatigue and fretting. Therefore, the Inconel 718 appears as a low sensitive material to stress gradient. A multiaxial fatigue approach such the Crossland criterion enables to describe correctly the fatigue strength of this system. The numerical analysis shows that the fatigue strength of the system is not very sensitive to the coefficient of friction whereas the interference impacts significantly on the fatigue strength of the system.

Cette étude financée par SNECMA et le CNES a pour objectif de mieux comprendre et caractériser le résultats de la solidification de l'inconel 718. Cet objectif est motivé par le besoin grandissant de fiabilité sur les pièces brutes de fonderies utilisées dans l'assemblage des moteurs de fusée. En effet, lors de la solidification, la ségrégation chimique provoque la formation de phase fragile. C'est notamment le cas des phases de Laves. Ainsi, un four de fonderie sous vide et un moule instrumenté ont été mise au point afin de couler au laboratoire des plaques de géométrie variable. Le but étant d'obtenir plusieurs échantillons obtenu sous différente conditions de solidifications. Une méthode d'analyse quantitative au MEB a par la suite été mise au point pour analyser les 4 coulées exploitables. Elle a permis de quantifier la quantité d'intermétalliques en fonction de la vitesse de refroidissement. Mais aussi leur répartition vis à vis des joints de grains d'une part et l'impact de la désorientation entre chaque grain d'autre part. La ségrégation chimique a aussi pu être quantifiée selon ces mêmes paramètres.De travaux numériques ont été entrepris en parallèles avec tout d'abord la simulation macroscopique des coulées avec le logiciel Thercast. Ces simulations ont permis de connaître les conditions de solidification de l'ensemble des échantillons coulés et ont fourni des données d'entré pour les autres aspects numériques. Par exemple, avec un modèle élément fini et automate cellulaire, la formation de la structure granulaire a pu être modélisée. Enfin, la simulation de la microségrégation a permis de valider notre compréhension des phénomènes prédominant lors de la solidification de l'inconel 718. Ces simulations numériques apportent à la fois un éclairage sur les résultats expérimentaux mais aussi démontrent leurs possibilités dans l'optique de mettre en place un modèle global de solidification recouvrant ces trois échelles pour des pièces industrielles / The metallic alloy “Inconel 718” is frequently found in aerospace and aeronautic components. In this study, it is used in the case of the main engine turbopump of the Ariane 5 launcher. This case is obtained by investment casting. Thereby, a great focus is turned on as cast defect like detrimental interdendritic phase. For example, the Laves phase, about 1%, has poor mechanicals characteristics. Several samples was cast under vacuum at MINES ParisTech Cemef. Various superheat and cooling rate are tested. After specific preparation, the microstructure was observed by scanning electron microscopy (SEM). On a surface representative of solidification phenomenon, the NbC phase, the Laves phase and the Delta phase were quantified and chemical segregation measurements were performed. The results give a tendency of the detrimental phases to form at the grain boundaries. This phenomenon is stronger as the cooling speed is lower. Furthermore, the grains boundaries disorientations as an impact. This experimental information is used to model the sample solidification at three different scales. At the macroscopic scale with Thercast software, at an intermediate scale whit the simulation of the grain structure. These simulations were performed whit a model developed at Mines Paristech, CEMEF. The last scale is the simulation of the microsegregation. This model takes into account thermodynamic equilibrium, cooling rate and microstructure geometries. This numerical work improved the comprehension of the solidification phenomenon. A global solidification simulation of industrial piece is the aim of Cnes (French space agency) and Snecma who support this work.

A nacionalização de componentes para a indústria nuclear é de grande importância para o Brasil não só pela diminuição de custo como também pela absorção de tecnologia incorporada no desenvolvimento dos materiais e processos envolvidos. A grade espaçadora fabricada em tiras de Inconel 718 e soldada por feixe de elétrons é um desses componentes que não são fabricados no Brasil e constituem papel fundamental na estrutura do Elemento Combustível. Na realização deste projeto foram empregadas tiras de Inconel 718, fornecidas pelas Indústrias Nucleares do Brasil S.A. - INB. Inicialmente as tiras no estado anterior à soldagem foram caracterizadas através de análise química, ensaios de tração e avaliação microestrutural por microscopia óptica e eletrônica. Após montagem das tiras em arranjos adequados para execução dos ensaios, os corpos de prova foram soldados por feixe de elétrons em vácuo (<= 5?10-4 mbar) variando-se os principais parâmetros envolvidos no processo, como a corrente do feixe, corrente de focalização e tempo de soldagem. As juntas soldadas foram avaliadas quanto à geometria do cordão obtido e mecanicamente através de ensaio de dureza e de cisalhamento, objetivando determinar a força máxima anterior à ruptura. Este último realizou-se em uma máquina de tração convencional usando corpo de prova adaptado. Também foram realizadas avaliações por microscopia óptica e eletrônica de varredura (MEV) acoplada a um espectrômetro de massa por energia dispersiva (EDS) para evidenciar as principais características microestruturais do cordão de solda, como a presença e morfologia das fases constituintes e a extensão e zonas de transição presentes. Posteriormente à definição dos parâmetros de soldagem a serem empregados, foram realizados tratamentos térmicos de solubilização e envelhecimento para diferentes ciclos térmicos. As amostras resultantes dos diferentes ciclos térmicos também foram caracterizadas através dos procedimentos descritos anteriormente, com o objetivo de se definir quais condições aplicadas no tratamento térmico resultaram em propriedades mais adequadas ao produto. A partir dos resultados obtidos, tornou-se possível correlacionar a influência da soldagem e do tratamento térmico com a evolução da microestrutura e das propriedades mecânicas avaliadas. Tais resultados auxiliarão na definição de parâmetros de processo a serem empregados pelas Indústrias Nucleares do Brasil S.A. na fabricação do componente citado. / The nationalization of componets for the nuclear industry is very important to Brazil, not only for the cost reduction, but also for the technological gain when materials and process are developed. The spacer grid fabricated with Inconel 718 and welded by electron beam is one of this components which is not fabricated in Brazil and it is a very important constituent of the fuel assembly structure. On the development of this project it were employed Inconel 718 straps, supplied by Indústrias Nucleares do Brasil S.A. - INB. Initially, before the welding phase, the straps were characterized by chemical analysis, tensile tests and microstructural evaluation using optical and electron microscopy. After the assembly of the straps in an adequate arrangement for execution of the tests, the test specimen were welded by electron beam in a vacuum (<= 5?10-4 mbar) and the main process parameters were ranged, like the beam current, focusing current and the welding time. The welded joints were evaluated according with the obtained weld filet geometry and it were also mechanically evaluated by the hardness and shear tests, aiming to determine the maximum force before collapse. The shear test was performed on a conventional tensile test machine using an adaptative test specimen. It were done, also, evaluations by optical and scanning electron microscopy (SEM) connected with Energy Dispersive Spectrometry (EDS), to evidence the main microstructural characteristics of the weld filet, like the presence and morfology of the constituent phases, and the presence of transition zones. After the definition of these weld parameters to be employed, it were done annealing and aging thermal treatments for differents thermal cycles. The resultings samples of the differents thermal cycles were also characterized by the procedures described earlier, with the purpose to define which condition used for the thermal treatment resulted in more suitable properties for the product. With the obtained results, it become possible to correlate the influence of welding and the thermal treatment with the microstructural evolution and the evaluated mechanical properties. Such results will help Indústrias Nucleares do Brasil S.A. - INB to define the employed parameters in the manufacture process of this component.