<p>Thesis Abstract: Laser powder bed fusion (LPBF), a specialization within additive manufacturing, is a high precision metal powder processing technique that has gained immense attentions in the past decade. The layer-by-layer densification technique provides a unique set of abilities that permits the large-scale production of geometrically complicated structures with highly tunable microstructures. Alloy 718 (718) is one of the most studied materials within the LPBF field due to its extraordinary printability. Although it has a significant industrial and academic focus, there are consequential questions that still need to be addressed because of the immense LPBF design space.</p><p>Our works demonstrate the multiple pathways that an alloy system like 718 can be optimized for specific applications by altering the processing parameters or by the addition of oxide particles to create a fine dispersion for high temperature capabilities. Room temperature tensile testing revealed that the processing parameters directly controlled the mechanical properties, allowing tailoring of the tensile strength and elongation to the needs of specific applications. Similar experiments were conducted to exhibit the flexibility of LPBF by incorporating a wider, economic, bimodal powder size distribution that maintained similar mechanical properties. Additions of oxide particles enabled the findings of the reactive nature within this welding process, which ultimately led to a refined oxide dispersion strengthened (ODS) 718 matrix with superior mechanical properties up to 900$^\circ$C. This novel metal matrix ceramic was lastly showcased by producing a complex microlattice structure. Detailed in-situ tensile tests in combination with electron backscatter diffraction (EBSD) and finite element modeling revealed that crystallographic reorientation around bending nodes enhanced the global ductility of the material.</p>

Du fait de leurs très bonnes propriétés mécaniques jusqu’à des températures approchant les 700°C, les superalliages base nickel polycristallins sont utilisés pour la fabrication de disques de turbine (ou compresseur) de moteur d’avion. La voie conventionnelle pour l’élaboration et la mise en forme de ces alliages est la voie dite «coulé-forgé». Ainsi, une première série de forgeages, appelée conversion, est appliquée au lingot coulé afin d’homogénéiser et de raffiner la microstructure. Elle aboutit à un demi-produit appelé billette qui est ensuite forgée/matricée à son tour pour obtenir l’ébauche de la pièce finale. Pour les superalliages γ-γʹ avec de hautes teneurs en éléments d’alliage, il est courant que l’étape de conversion ne soit pas suffisamment efficace pour complètement homogénéiser la microstructure. C’est le cas pour l’alliage AD730TM récemment mis au point par la société Aubert&Duval, dont les billettes présentent des zones de grains équiaxes recristallisés et des plages restaurées caractéristiques. L’objectif de cette thèse est de comprendre comment les hétérogénéités de microstructure peuvent se résorber pendant les dernières étapes de forgeage menant à la microstructure finale. Après avoir caractérisé les hétérogénéités de microstructure présentes dans les billettes d’alliage AD730TM, des essais thermomécaniques simulant un procédé de forgeage ont été appliqués à la billette afin de suivre l’évolution des différentes microstructures locales. Les mécanismes d’évolutions statiques (pendant les traitements thermiques) et dynamiques (pendant la déformation) des zones équiaxes et des plages restaurées ont été caractérisés par microscopie électronique à balayage, EBSD et EDS. Une attention toute particulière est portée à l’influence des précipités γʹ sur ces évolutions. Notamment, une interaction front de recristallisation-précipités jusque-là très peu reportée dans la littérature et générant des précipités γʹ en quasi relation de macle ou de quasi même orientation que la matrice a été étudiée en détail. / Due to their excellent mechanical properties at temperatures up to 700°C, polycrystalline nickel-based superalloys are widely used in aero-engine turbine (or compressor) disk manufacturing. These alloys are usually processed following the conventional “cast-and-wrought” route. During this route, the cast ingot goes through a first series of forging operations which is named “conversion”. The goals of the conversion are to homogenize and refine the microstructure. It leads to a semi-finished product called billet. Then, the billet is forged again to obtain a draft of the final part. Yet, for the γ-γʹ nickel-based superalloys with high contents in alloying elements, it is common that the conversion process does not succeed in fully homogenizing the microstructure. Such is the case of the alloy AD730TM which has been recently developed by the Aubert&Duval Company. Indeed, AD730TM billets show both recrystallized equiaxed areas and characteristic recovered areas. The objective of the current PhD thesis is to understand how such microstructural heterogeneities can disappear during the last forging operations which lead to the final microstructure. First, the microstructural heterogeneities found in AD730TM billets have been characterized. Then, thermomechanical tests which aimed at simulating a forging process have been performed on billet samples in order to follow the evolutions of each local microstructure. The static (during thermal treatments) and dynamic (during deformation) evolutions of both equiaxed and recovered areas have been characterized using scanning electron microscopy, EBSD and EDS. A special attention has been paid to the influence of the γʹ precipitates on those evolutions. In particular, a specific interaction between a recrystallization front and γʹ precipitates have been studied in detail. This interaction, which has been weakly reported in literature so far, produces γʹ precipitates with either an imperfect twin orientation relationship to the matrix or an imperfect cube-cube orientation relationship to the matrix.

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

Les assiettes de fibrage utilisées pour la production de fibres de verre subissent d’importantes contraintes mécaniques, chimiques et thermiques. Par conséquent, cette pièce se dégrade et doit être périodiquement remplacée. Le but de ces travaux est d’augmenter la durée de vie des assiettes de fibrage en améliorant les propriétés mécaniques et chimiques des superalliages qui les constituent. Plus précisément, l’objectif de cette thèse est, dans un premier temps, d’améliorer les alliages utilisés pour le fibrage à 1000°C, puis dans un second temps, de trouver un superalliage pouvant fibrer à 1125°C. Ces travaux ont été menés sur deux familles de superalliages : base nickel et base cobalt. Les propriétés chimiques ont été caractérisées par des essais thermogravimétriques et les propriétés mécaniques par des essais de fluage flexion 3 points. Pour l’alliage base Ni, l’influence des éléments minoritaires subis, Mn et Si, a été étudiée. Les propriétés mécaniques de cet alliage ont fait, ensuite, l’objet d’essais d’amélioration par l’addition de titane, zirconium, tantale, hafnium et de niobium. L’addition d’aluminium a été aussi expérimentée mais ayant pour but d’améliorer les propriétés chimiques de l’alliage. Enfin, des études ont été aussi menées sur l’addition de métaux nobles (Pd et Ru) et de terres rares (Y, La et Ce). Concernant les superalliages base cobalt, l’étude s’est portée sur un alliage base Co qui a antérieurement montré de bonnes propriétés générales pour une utilisation à 1125°C et non à 1000°C. Des variantes avec une teneur abaissée en tantale, sans hafnium et sans tungstène ont été étudiées. Pour clore ces travaux, l’augmentation de la teneur en nickel et un traitement thermique ont été étudiés afin d’améliorer la résistance à l’oxydation de ce second alliage / The fiberizing spinners used to produce the glass fibers undergo sizable mechanical, chemical and thermal solicitations. Therefore, this piece is degraded and must be periodically replaced. The purpose of these works is to increase the lifetime of the fiberizing spinners by improving the mechanical and chemical properties of the superalloys that constitute them. More precisely, the objective of this thesis is, in the first time, to improve the alloys used to fiberize at 1000°C, and in a second time, to find a superalloy that can fiberize at 1125°C. These works are focused on two families of superalloys: nickel-based and cobalt-based. The chemical properties are characterized by the thermogravimetry tests and the mechanical properties by 3 points flexural creep tests. For the Ni-based alloy, the influences of unwanted minority elements, Mn and Si, were studied. The mechanical properties of this alloy are subject to improvement tests by the addition titanium, zirconium, tantalum, hafnium and niobium. The aluminium addition was also experimented but with the aim of improving the chemical properties of the alloy. Finally, the studies were also driven on the addition of noble metals (Pd and Ru) and of rare earths (Y, La and Ce). Concerning the cobalt-based superalloys, the study focused on an alloy which has previously shown good general properties to be used at 1125°C and no at 1000°C. The versius with a lowered content in tantalum, without tungsten and without hafnium were studied. To close this work, the increase of nickel and a heat treatment were studied in order to improve the oxidation resistance of this second alloy

Hydrogen embrittlement in ferrous and non-ferrous alloys is, and has been for over a century, a prominent issue within many sectors of industry. Despite this, the mechanisms by which hydrogen embrittlement occurs and the suitable means for its prevention are yet to be fully established. As hydrogen fuel becomes a prominent feature in modern concepts of a sustainable global energy infrastructure and nuclear power enters its renaissance, with commercially viable fusion plants on the horizon, hydrogen embrittlement is becoming an ever more pertinent issue. This has led to a considerable demand for novel alloys resistant to hydrogen embrittlement, notably within the bearings industry, where the commonly conflicting properties of high strength and hydrogen embrittlement resistance are required. This work investigates the mechanisms through which hydrogen embrittlement and irradiation damage occur in steels and nickel-based alloys respectively, with novel alloys designed for improved resistance. Through the engineering of secondary phases, optimised for helium and/or hydrogen trapping capacity, the novel alloys present the benefits of such trapping species with respect to embrittlement resistance. Such species have been studied in depth with respect to their interactions with hydrogen, establishing a novel mechanism of hydrogen embrittlement - the hydrogen enhanced dissolution and shearability of precipitates, leading to enhanced localised plasticity.

Cette étude porte sur la sensibilité à la fragilisation par l'hydrogène (FPH) d'un alliage base nickel, l’Inconel 718 issu d’un procédé de fabrication additive (FA), sous sollicitation cyclique. Cematériau est utilisé pour la réalisation de certains composants des ensembles propulsifs d’Ariane qui sont fabriqués par Ariane Group. Dans ce domaine, certaines pièces sont sollicitées sous « ambiancehydrogène ». Ainsi, le risque de fragilisation de ces pièces est une problématique de premier ordre.Pour cela, nous avons caractérisé l’état métallurgique d’un Inconel 718 FA à différentes échelles structurales afin d’observer un possible impact du procédé de mise en œuvre sur la microstructure, puis d’envisager sa conséquence sur le comportement mécanique et la sensibilité à la FPH de l’alliage. Notons que nous avons conduit une étude plus réduite sur un alliage forgé afind’avoir des éléments de comparaison. Afin de comprendre les mécanismes de fragilisation par hydrogène de l’Inconel 718 FA, il est nécessaire de connaitre l’état et la mobilité de l’hydrogène dans le matériau. Des analyses deperméation électrochimique (PE) et gazeuse (PG), ainsi que de spectroscopie de désorption thermique (TDS) apportent des éléments de compréhension des mécanismes de piégeage et de diffusion de l’hydrogène. Ils permettent de discuter l’implication des joints de grains dans les mécanismes dediffusion ainsi que l'effet des hétérogénéités microstructurales sur les mécanismes de piégeage dans l’Inconel 718 FA.D'autre part, l’influence de l’hydrogène sur le comportement mécanique a été questionnée en traction et en fatigue oligocyclique en terme de comportement élasto-viscoplastique, de processus d’initiation, de propagation de fissure en fatigue et de ténacité. Dans ce cadre nous présentons, dans unpremier temps, l’étude du comportement en traction et sa sensibilité à l’hydrogène pour l’Inconel 718 FA. Nous questionnons lors de cette partie l’effet de la vitesse de sollicitation sur les mécanismes defragilisation ainsi que sur les différentes interactions hydrogène/matériau. Des essais de traction interrompue ont été réalisés afin de questionner l’effet de l’hydrogène sur le comportement viscoplastique et notamment questionner les interactions hydrogène/plasticité.Dans un deuxième temps, le comportement en fatigue de l’Inconel 718 FA en présence d’hydrogène gazeux est interrogé à l’aide d’essais de fatigue pour un rapport de charge, R de 0,1 sous une pression d’hydrogène de 300 bar. Il est clairement mis en évidence une réduction du nombre decycles à rupture en présence d’hydrogène ainsi qu’un changement de mode de rupture. L’impact de l’hydrogène a été évalué sur les étapes d’initiation et de propagation de fissures ainsi que sur la ténacité du matériau en fonction de la métallurgie de l’alliage.L'ensemble des résultats obtenus permettent une discussion de la sensibilité du matériau à la FPH, tenant compte en particulier des interactions hydrogène/plasticité. / This study focuses on the hydrogen embrittlement sensitivity (HE) under cyclic loading of a nickel based alloy, Inconel 718, manufactured by the additive manufacturing process (AM). This material is used in some components of Ariane cryogenic engines that are manufactured by ArianeGroup. Some of these components are solicited under "hydrogen atmosphere", and the risk of embrittlement is a major problem.The metallurgical states of the Inconel 718 AM alloy have been characterized at different structural scales in order to observe a possible impact of the manufacturing process on the microstructure and discuss the possible consequences on the mechanical behaviour of the alloy underhydrogen pressure. Moreover, a forged alloy has also been studied for comparison.To discuss the mechanisms of hydrogen embrittlement in the material, it is first necessary to study the hydrogen behaviour in the material. Electrochemical and gaseous permeation analyses as well as TDS were performed to provide insights into the mechanisms of hydrogen diffusion. Fromthese results, the involvement of grain boundaries in the diffusion mechanisms and the effect of microstructural heterogeneities on the trapping mechanisms in this material have been discussed.On the other hand, the influence of hydrogen on the mechanical behaviour has been investigated under monotonic and cyclic loading in terms of elasto-viscoplastic behaviour, crack initiation process, fatigue crack propagation and toughness. In a first part, the tensile behaviour of the Inconel 718 AMunder hydrogen pressure has been considered. The effects of the loading rate on the mechanisms of embrittlement have been addressed. Moreover, interrupted tensile tests have been carried out to identify the effect of hydrogen on the viscoplastic behaviour and allowing to discuss hydrogen /plasticity interactions.The fatigue behaviour of Inconel 718 AM in the presence of hydrogen gas was investigated by cyclic tests for a 0.1 load ratio (R) under a 300 bar H2 at room temperature. It has been shown that hydrogen leads to a clear decrease of the number of cycles to rupture and to a change in failure mode.The impact of hydrogen has been evaluated on the fatigue crack initiation and propagation stages as well as on the toughness of the material.Finally, all the obtained results allowed a discussion of the hydrogen embrittlement sensitivity of the material, taking into account the hydrogen / plasticity interactions.

Electrochromic properties of sputter-deposited nickel-based oxide films have been studied with a two-fold goal. From a practical point of view, the optical switching performance has been improved by optimizing the deposition conditions and film stoichiometry with respect to oxygen and hydrogen, and further by adding Mg, Al, Si, Zr, Nb or Ta to the films. From a theoretical point of view, details of the coloration mechanism have been studied by means of electrochemical intercalation (CV, GITT), optical measurements (UV, VIS, NIR and MIR), RBS, XRD, XPS and EXAFS. Optimization of deposition conditions has been illustrated by the example of films made by sputtering of a non-magnetic Ni(93)V(7) % wt. target in an atmosphere of Ar/O2/H2. The optimized films exhibit transmittance modulation between 20% and 75 % at 18 mC/cm2 charge intercalation. The remaining problem with nickel oxide and nickel vanadium oxide films is their residual yellow-brown color tint in the bleached state, which disappears as the short-wavelength transmittance increases upon addition of Mg, Al, Zr or Ta. Optimization of deposition conditions by co-sputtering from two targets and the film composition for mixed oxide films has been illustrated by the example of nickel aluminium oxide. The mechanisms of coloration upon electrochemical charge insertion and ozone exposure have been investigated. In the beginning of the electrochemical cycling, first, a reconstruction and crystallization is observed with the outer most part of the grain surface being transformed from oxygen rich nickel oxide into nickel oxy-hydroxide and hydroxide by transfer of H+ and OH- groups. After the charge capacity has been stabilized, only a transfer of H+ occurs with two reversible reactions involved: the first one from Ni(OH)2 to NiOOH and the second one from NiO and Ni(OH)2 to Ni2O3. Ozone coloration is described by a similar reaction scheme. The ozone molecule is split on the surface and dehydrogenates Ni(OH)2 into NiOOH. Further dehydrogenation produces Ni2O3 as in the electrochemical coloration.

The reduction of manufacturing costs and the facilitation of direct-oxidation of hydrocarbon fuels have been identified as means of promoting the commercialization of the solid oxide fuel cell, a technology that offers both environmental and fuel conservation benefits compared to conventional energy conversion technologies. This research was conducted with the aim of realizing the production of direct-oxidation anodes using atmospheric plasma spraying, which has been identified as a fabrication technique that has the potential to reduce the manufacturing costs of solid oxide fuel cells. This thesis details the rationale behind the selection of the anode compositions (BaO-Ni-YSZ, CeO2-Ni-YSZ, and Sn-Ni-YSZ) and the specifics of the specialized fabrication strategy (SPS-SPPS) that was devised with the aim of realizing microstructures similar to those where the secondary phases (BaO, CeO2, and Sn) coat the surfaces of the primary Ni and YSZ phases. Results of XRD, SEM and EDS analyses are presented.

The reduction of manufacturing costs and the facilitation of direct-oxidation of hydrocarbon fuels have been identified as means of promoting the commercialization of the solid oxide fuel cell, a technology that offers both environmental and fuel conservation benefits compared to conventional energy conversion technologies. This research was conducted with the aim of realizing the production of direct-oxidation anodes using atmospheric plasma spraying, which has been identified as a fabrication technique that has the potential to reduce the manufacturing costs of solid oxide fuel cells. This thesis details the rationale behind the selection of the anode compositions (BaO-Ni-YSZ, CeO2-Ni-YSZ, and Sn-Ni-YSZ) and the specifics of the specialized fabrication strategy (SPS-SPPS) that was devised with the aim of realizing microstructures similar to those where the secondary phases (BaO, CeO2, and Sn) coat the surfaces of the primary Ni and YSZ phases. Results of XRD, SEM and EDS analyses are presented.

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.