Analyse des interactions entre les processus d'oxydation et les mécanismes de déformation conduisant à un mode de propagation des fissures soit transgranulaire ductile, soit intergranulaire fragile, le dernier mode étant associé à une propagation beaucoup plus rapide des fissures. Proposition de différents modèles phénoménologiques permettant de relier le mode d'oxydation au mode de déformation, ainsi que le mode d'oxydation au mode de propagation des fissures.

The aim of this master thesis project was to statistically correlate various powder characteristics to the quality of additively manufactured parts. An additional goal of this project was to find a potential second source supplier of powder for GKN Aerospace Sweden in Trollhättan. Five Inconel® alloy 718 powders from four individual powder suppliers have been analyzed in this project regarding powder characteristics such as: morphology, porosity, size distribution, flowability and bulk properties. One powder out of the five, Powder C, is currently used in production at GKN and functions as a reference. The five powders were additively manufactured by the process of laser metal deposition according to a pre-programmed model utilized at GKN Aerospace Sweden in Trollhättan. Five plates were produced per powder and each cut to obtain three area sections to analyze, giving a total of fifteen area sections per powder. The quality of deposited parts was assessed by means of their porosity content, powder efficiency, geometry and microstructure. The final step was to statistically evaluate the results through the analysis methods of Analysis of Variance (ANOVA) and simple linear regression with the software Minitab. The method of ANOVA found a statistical significant difference between the five powders regarding their experimental results. This made it possible to compare the five powders against each other. Statistical correlations by simple linear regression analysis were found between various powder characteristics and quality of deposited part. This led to the conclusion that GKN should consider additions to current powder material specification by powder characteristics such as: particle morphology, powder porosity and flowability measurements by a rheometer. One powder was found to have the potential of becoming a second source supplier to GKN, namely Powder A. Powder A had overall good powder properties such as smooth and spherical particles, high particle density at 99,94% and good flowability. The deposited parts with Powder A also showed the lowest amount of pores compared to Powder C, a total of 78 in all five plates, and sufficient powder efficiency at 81,6%.

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.

Microstructure controls many physical properties of a material such as strength, ductility, 1density, conductivity, which, in turn, determine the application of these materials. This thesis work focuses on studying microstructural features (such as grain size, shape, defects, orientation gradients) and mechanical properties (such as hardness and yield strength) of multilayered materials that have undergone different loading and/or operating conditions. Two materials that are studied in detail are 18 nm Cu-Nb nanolaminates and 3D printed Inconel 718. Copper-Niobium (Cu-Nb) nanolaminate is a highly stable, high strength, nuclear irradiation resistant composite, which is destabilized with application of high pressure torsion (HPT). This work focuses on understanding the deformation and failure behavior of Cu-Nb using a novel orientation mapping technique in transmission electron microscopy in (TEM) called Automated Crystal Orientation Mapping (ACOM) and Digistar (ASTARTM) or Precession Electron Diffraction (PED). A new theory is postulated to explain strengthening mechanisms at the nanoscale using a data analytics approach. In-situ TEM compression and tensile testing is performed to image dislocation movement with the application of strain. This experiment was performed by Dr. Lakshmi Narayan Ramasubramanian at Xi’an Jiaotong University in China. Another major aspect of this research focuses on the design, fabrication, and microstructural characterization of 3D printed Inconel 718 heat exchangers. Various heat exchanger designs, machine resolution, printing techniques such as build orientation, power, and velocity of the laser beam are explored. Microstructural and mechanical properties of printed parts (before and after heat treatment) are then analyzed to check consistency in grain size, shape, porosity, hardness in relation to build height, scan parameters, and design. Various tools have been utilized such as scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), x-ray computed microtomography (at Advanced Photon Source at Argonne National Lab), hardness and micro-pillar compression testing for this study.

Superligas à base de níquel vêm sendo extensivamente utilizadas em diversas aplicações nas indústrias devido ao excelente comportamento mecânico e anticorrosivo. No entanto, essas ligas possuem certas particularidades que levam à necessidade de desenvolvimento de técnicas de inspeção e caracterização metalúrgica, como forma de garantir a integridade estrutural dos componentes fabricados com essas ligas. Neste trabalho, a técnica de correntes parasitas foi aplicada à superligas à base de níquel com duas propostas distintas: 1) Em um primeiro momento foi realizado o estudo da evolução microestrutural da liga Inconel 718 durante o processo de envelhecimento através da combinação do ensaio por correntes parasitas, análise de difração de raios-X, análise metalográfica, medidas de dureza e tamanho de grão. As medidas foram realizadas em amostras submetidas a diferentes ciclos de tratamentos térmicos variando entre 620-1035°C. Os resultados mostraram que as diferentes microestruturas do Inconel 718 têm efeitos distintos na condutividade elétrica quando medidos através da técnica de correntes parasitas. A influência da microestrutura na condutividade pode ser mostrada sendo devido à competição de dois efeitos sobre o espalhamento de elétrons: a purificação da matriz e a morfologia, distribuição e tamanho dos precipitados. A combinação dos valores de dureza e condutividade elétrica provou ser uma forma rápida e prática de determinar o nível de envelhecimento da liga; 2) Em um segundo momento foi desenvolvido um processo de otimização de sensores através de modelagem por elementos finitos (MEF). Através de uma metodologia de otimização, os parâmetros de construção e operação de um sensor foram otimizados para inspeção de defeitos superficiais e subsuperficiais esperados em materiais cladeados com Inconel 625. O sensor com a geometria ótima foi construído e testado a fim de verificar a eficiência do processo de otimização. Uma ótima correlação entre os resultados numéricos e experimentais foi encontrada e o sensor ótimo se mostrou eficiente na inspeção de pequenos defeitos superficiais e subsuperficiais na liga Inconel 625 quando operado nas frequências apropriadas. / Nickel-based superalloys have been extensively used in various industries due to its unique mechanical and corrosion behavior. However, these alloys show particular characteristics which lead to the need for specific inspection and metallurgical characterization techniques in order to ensure the structural integrity of components manufactured from these alloys. In this work , the eddy current technique was applied to nickel-base superalloys with two aims: 1 ) Firstly, the microstructural evolution of Inconel 718 during aging processes has been studied through a combination of eddy current testing, X-ray diffraction analysis, metallography, hardness and grain size measurements. Measurements were carried out in samples subjected to different heat treatment cycles between 620-1035°C. Results show that different microstructures of Inconel 718 have a distinguishable effect on electrical conductivity when this is measured through an appropriately sensitive technique (i.e. eddy current testing). The influence of microstructure on conductivity could be shown to be due to the competition between two effects on the scattering of electrons: matrix purification and precipitate size, distribution and morphology. A combination of hardness values and electrical properties proved to be a fast and practical way of determining the stage of aging of the alloy; 2) An optimization method of eddy current sensor design was developed through finite element modeling (FEM). Through a methodology of optimization, the construction and operation parameters of the sensor were optimized for inspection of superficial and subsuperficial defect, commonly found in weld overlay Inconel 625 claddings. A prototype of this sensor with the optimum geometry was built and tested on blocks identical to those considered in the models in order to verify the efficiency of the optimization process. A very good agreement between numerical and experimental results was found. Moreover, the optimal sensor was efficient to detect small surface and subsurface defects in Inconel 625 when operated at appropriate frequencies.

In this master thesis, work done in the TURBO POWER project High temperature fatigue crack propagation in nickel-based superalloys during spring 2012 will be presented. The overall objective of this project is to develop and evaluate tools for designing against fatigue in gas turbine applications, with special focus on the crack propagation in the nickel-based superalloy Inconel 718. Experiments have been performed to study the effect of initial overloads, and it has been shown that even for small initial overloads a significant reduction of the crack growth rate is received. Furthermore, FE simulations have been carried out in order to describe the local stress state in front of the crack tip since it is believed to control, at least partly the diffusion of oxygen into the crack tip and thus also the hold time crack growth behaviour of the material. Finally, an evaluation method for the stresses is presented, where the results are averaged over an identifiable process/damaged zone in front of the crack tip.

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.

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 objective of this work is to study crack path deflection under proportional and non-proportional mixed-mode fatigue and predict crack branching direction based on linear elastic fracture mechanics (LEFM) driving forces. Under proportional in-phase mixed Mode I / Mode II loading conditions, crack growth direction has previously been observed in some materials to shift from tensile-dominated Mode I to shear-dominated Mode II or mixed-mode crack growth at higher proportions of initial Mode II loading, but non-proportional loads are not well-characterized. An LEFM approach is desired in order to implement the model in crack growth software such as the boundary element-based fracture analysis package FRANC3D. A novel specimen configuration has been designed and analyzed for generation of wide ranges of mixed-mode loading conditions in a single test. This specimen and a more conventional thin-walled tubular specimen have been used to test polycrystalline nickel-base superalloy Inconel 718 under proportional in-phase and 3 kinds of non-proportional fatigue loading. Stress intensity factors for the various configurations have been analyzed with FRANC3D. Modal transition from Mode I (tensile) to Mode II (shear) crack branching has been observed in several load cases. Qualitative microscopy of fracture surfaces was used to characterize the difference between crack branch modes. An LEFM approach based on an effective stress intensity factor range, which incorporates the maximum value and range of each appropriate stress intensity (Mode I or Mode II), has been used to successfully predict the crack deflection angles, and in some cases to quantify modal transition, within each load case considered. Variability between load cases and specimen configurations points to the limitations of LEFM in providing a general predictor of crack path behavior across all types of non-proportional mixed mode loading.