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La compréhension des effets du grenaillage ultrasonore sur l’intégrité de surface des composants métalliques représente un enjeu industriel important. Dans le cadre de cette thèse, une modélisation physique multi-échelle de la plasticité cristalline à la DEM (Discrete Element Model) a été développée. Le grenaillage ultrasonore consiste à mettre en mouvement des billes à l’intérieur d’une enceinte par vibration de la sonotrode. Les impacts répétés sur le matériau entraînent un écrouissage en surface, l’établissement de contraintes résiduelles de compression et la formation d’une couche nanostructurée. L’objectif de cette thèse est d’obtenir une meilleure compréhension des mécanismes conduisant à ces modifications sur un alliage à base nickel. Le mouvement des billes obtenu par DEM est relié aux modifications de la microstructure sous impacts via un modèle éléments finis utilisant une loi de plasticité cristalline. Après validation de chaque étape par des mesures expérimentales, le modèle a permis d’étudier l’effet de la quantité de billes utilisées dans le procédé. Ainsi, un nombre croissant de billes induit une augmentation d’impacts en biais de faible vitesse permettant de concentrer les contraintes résiduelles de compression en extrême surface. De plus, les simulations multi-impacts utilisant une loi de plasticité cristalline ont montré que ces impacts en biais engendraient une densité totale de dislocations et un niveau de désorientations élevées pouvant expliquer la fragmentation des grains et la nanostructuration de la surface. Enfin, ces modifications de la microstructure, visibles jusqu’à 300 µm de profondeur, sont en accord avec les profils de dureté obtenus par nano-indentation et les profils de désorientation issus de l’analyse EBSD (Electron BackScatter Diffraction). / Ultrasonic shot peening is widely used to improve mechanical properties of metallic components. Mastering the effects of this surface treatment is a major industrial issue. A physical multi-scale modelling based on crystal plasticity and DEM (Discrete Element Model) was developed in this PhD thesis. This process is performed in a closed chamber where spherical balls are moved by sonotrode vibration. Thousands of impacts induce hardening, residual compressive stress and microstructure modification leading to a nanostructured layer. The aim of this work was to improve our understanding of the mechanisms occurring during this process on nickel-based alloys. Ball motion was computed by DEM and linked to microstructure modifications induced by impacts through a crystal plasticity finite element model. Experimental analyses were performed in order to validate each step of the multi-scale modelling. Then the model was applied to investigate ball quantity effects on peened surface modification. Increasing the number of balls created a larger quantity of low speed oblique impacts which concentrated the residual compressive stress near the surface. Furthermore multi-impacts performed with a crystal plasticity law showed oblique impacts enhanced dislocation storage and disorientations within grains, which could explain the nanostructuration of the peened surface. Moreover, the numerical microstructure modification, observed up to 300 µm in depth, was in agreement with hardness profiles obtained by nanoindentation and disorientation profiles measured by EBSD (Electron BackScatter Diffraction) analysis.

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.

Cette thèse contribue au remplacement du revêtement de Stellite 6 utilisé aux zones de frottement dans le circuit primaire du réacteur à neutron rapide. Elle comprend trois parties : 1) Une étude bibliographique afin de présider au choix des matériaux de remplacement et du procédé de dépôt. 2) Une étude paramétrique en vue d’obtenir des dépôts sains (bonne adhésion avec le substrat, peu de porosité, absence de fissure, dilution faible). 3) Une étude du comportement tribologique des dépôts réalisée pour deux valeurs de température. Ces essais tribologique ont été réalisés sous atmosphère inerte afin d’évaluer la résistance à l’usure des matériaux choisis sans l’influence d’une éventuelle couche d’oxydation. De l’étude bibliographique il ressort les choix suivants mis en oeuvre dans notre étude : * le procédé projection laser qui présente des avantages tels que :- Bonne adhésion (métallurgique)- Vitesse de refroidissement élevée- Taux de dilution faible- Large plage paramétrique *deux alliages base nickel : le Colmonoy-52 et le Tribaloy-700. Ces alliages présentent un bon comportement tribologique à sec et sont déposables par laser.Pour la partie obtention d’un dépôt sain, tout d’abord nous avons caractérisé la poudre métallique. Ensuite, une recherche paramétrique a été conduite afin de disposer d’un jeu de paramètres qui permette d’obtenir un dépôt sain de Stellite 6 (référence), de Colmonoy-52 et de Tribaloy-700. A cette occasion, les relations entre trois paramètres principaux du procédé de projection laser (puissance du faisceau laser, vitesse de balayage de la surface, débit de poudre) ont été investigués en relation avec la microstructure et la composition chimique finale du dépôt.Lors de l’étude tribologique, un tribomètre de type pion-disque a été utilisé et les essais de frottement ont été réalisés sous atmosphère d’argon, à température ambiante et à 200°C. Les mécanismes d’usure ont été identifiés pour les 3 matériaux et leur contre pièce (bille en AISI 316L). Les volumes usés ont quant à eux été analysés en fonction de la charge appliquée et l’effet de la température. / This thesis contributes to the replacement of the coating of Stellite 6 which is used in friction areas for the primary circuit of the reactor fast neutron.It contains three parts:1) A literature review for selecting the materials and the deposition process2) A parametric study to get healthy deposits (good adhesion with the substrate, little porosity, no cracks, low dilution)3) A study wear behavior of deposits obtained, at high temperature (200°C) under an atmosphere inert gas, to determine the wear resistance of materials selected without the influence of an eventual oxidation layer.From the literature review, it appears the following choices implemented in our study : * the method laser cladding with advantages such as: - Good adhesion (metallurgical) - High cooling speed - Low dilution rate - Wide parametric range * two nickel-based alloys: Colmonoy-52 and Tribaloy-700. These alloys have good dry wear behavior and could be deposited by the laser. In the manufacturing part of the healthy deposit, firstly, we characterized the metal powder. Then, a parametric study was performed to look for a good parametric range that makes us getting a healthy deposit of Stellite 6 (reference) of Colmonoy-52 and Tribaloy-700. In this case, relationships among three main process parameters laser cladding (laser beam power, surface scanning speed, rate of powder) with the microstructure and chemical composition of the deposit are studied. In study the wear behavior, a pin-on-disc type of tribological was used and tests were carried out in argon at room temperature and 200°C. We investigated the wear mechanism of the best deposition of Stellite 6, Colmonoy-52 and Tribaloy-700. The wear resistance of these materials were thourghly compared.

The aim of this project was to investigate the potential of fused-slurry coatings for improving the oxidation resistance of wrought alloys. Slurry-aluminised coatings were deposited on Alloy 800H (Fe-33Ni-20Cr), Alloy HCM12A (Fe-12Cr-2W), Alloy 214 (Ni-16Cr-4Al-3Fe), Fe-27Cr-4Al and Fe-14Cr-4Al alloys. The slurry contained a cellulose-based binder in an aqueous carrier and spherical aluminium powder, with a particle size below 20 microns. The slurries were applied with a paint-brush, dried in air and heat treated in either hydrogen or argon at temperatures between 700 and 1150C. The slurries were characterised by thermogravimetry, differential scanning calorimetry and viscometry. The coatings were characterised by optical microscopy, scanning electron microscopy, energy-dispersive X-ray analysis, X-ray diffraction and Vickers hardness measurements. The oxidation resistance of selected slurry-coated specimens was assessed in air at 1000 and 1100C in tests lasting up to 1000 hours.Slurry-aluminising was found to be a simple, effective way of forming protective coatings that were similar in composition and microstructure to chemical vapour deposits. However, it was difficult to control the amount of slurry applied to the substrate and produce coatings of uniform thickness.The coatings on Alloy HCM12A and the Fe-Cr-Al alloys contained cracks in the brittle FeAl phase due to tensile stresses arising from differences in the thermal expansion coefficients of the substrates and the coatings. Rapid interdiffusion between the coatings and the ferritic substrates resulted in the appearance of Kirkendall voids.Coatings on Alloy 214 required a two-stage heat treatment to convert the brittle δ-Ni2Al3 to β-NiAl. Cracking along the coating/substrate interface was prevented by limiting the coating thickness to a maximum of 250 microns. During oxidation at 1100C, the β-NiAl was converted to γ'-Ni3Al. After 1000 h, the centre of the coating consisted chiefly of γ'-Ni3Al and bands of austenite (γ-Ni) were present at the inner and outer edges of the coating. The aluminium content at the coated surface was higher than the original aluminium content of the alloy, the protective alumina scale was improved and the oxidation life of the substrate was extended. An additional life of 1250 h at 1100C is estimated from a slurry coating before the aluminium content returns to that of the original alloy (4%), providing a potential improvement in oxidation resistance.Microstructural changes such as grain growth, sensitisation and formation of aluminium nitride particles near the coating/substrate interface, were detected in the alloy substrates after forming the slurry coatings. However, these microstructural changes did not detract from the good performance of the coatings during oxidation tests at 1100C.The work in this study has demonstrated a low-cost method of coating high-temperature alloys providing coatings with microstructures, densities and modes of degradation similar to those obtained by other coating methods. The coatings are potentially applicable to a wide range of high-temperature substrates.

This study focused on molten salt purification processes to effectively reduce or eliminate the corrosive contaminants without altering the salt's chemistry and properties. The impurity-driven corrosion behavior of HAYNES® 230® alloy in the molten KCl-MgCl2-NaCl salt was studied at 800 ºC for 100 hours with different salt purity conditions. The H230 alloy exhibited better corrosion resistance in the salt with lower concentration of impurities. Furthermore, it was also found that the contaminants along with salt's own vaporization at high temperatures severely corroded even the non-wetted surface of the alloy. The presence of Mg in its metal form in the salt resulted in an even higher mass-loss possibly due to Mg-Ni interaction. The study also investigated the corrosion characteristics of several nickel and ferrous-based alloys in the molten KCl-MgCl2-NaCl salt. The average mass-loss was in the increasing order of C276 < SS316L < 709-RBB* < IN718 < H230 < 709-RBB < 709-4B2. The corrosion process was driven by the outward diffusion of chromium. However, other factors such as the microstructure of the alloy i.e. its manufacturing, refining, and heat-treatment processes have also shown to influence the corrosion process. Lowering the Cr content and introducing W and Mo in the alloy increased its resistance to corrosion but their non-uniform distribution in the alloy restricted its usefulness. To slow-down the corrosion process, and enhance the material properties, selected alloys were boronized and tested for their compatibility in the molten KCl-MgCl2-NaCl salt. The borided alloys exhibited better resistance to molten salt attack, where the boride layer in the exposed alloy was still intact, non-porous, and strongly adhered to the substrate. The alloys also did not show any compensation in their properties (hardness). It was also found that the boride layer always composed of an outermost silicide composite layer, which is also the weakest and undesired layer as it easily cracks, breaks, or depletes under mechanical and thermal stresses. Various different grades of "virgin" nuclear graphites were also tested in the molten KF-UF4-NaF salt to assist in the selection of tolerable or impermeable graphites for the MSR operational purposes. It was found that molten salt wettability with graphite was poor but it still infiltrated at higher pressure. Additionally, the infiltration also depended on the pore-size and porosity of the graphite. The graphite also showed severe degradation or disintegration of its structure because of induced stresses. / Doctor of Philosophy / Molten salts are considered as potential fuel and coolant candidates in MSRs because of their desirable thermophysical properties and heat-transfer capabilities. However, they pose grave challenges in material selection due to their corrosive nature, which is attributed to the impurities and their concentration (mostly moisture and oxygen-based) in the salt. This study focused on purifying the salt to reduce these contaminants without compromising its composition and properties. The influence of purification processes on the corrosion behavior of HAYNES® 230® alloy was studied in the molten chloride salt with different purity conditions. Various nickel and ferrous-based alloys were also studied for their compatibility in the molten chloride salt. This will assist in expediting the material selection process for various molten salt applications. It was observed that several factors such as alloy composition, its microstructure, impurities in the salt attribute to molten salt corrosion. It was also quite evident that corrosion in molten salts is inevitable and hence, the focus was shifted on slowing down this process by providing protective barriers in the form of coatings (i.e. boronization). The borided (coated) alloys not only improved the corrosion resistance but also enhanced and retained their properties like hardness after exposure to molten salts. Since these studies were conducted under static conditions, a more detailed investigation is needed for the selected alloys by subjecting them to extreme flow-conditions and for longer a duration of time. To achieve this objective, a forced circulation molten salt loop was designed and fabricated to conduct flow corrosion studies for alloys in molten chloride salt. Graphite is another critical component of the MSR where it is used as a moderator or reflector. Generally, molten salts exhibit poor wettability with graphite, but they can still infiltrate (graphites) at higher applied pressures, and result in the degradation or disintegration of graphite's structure, and eventually its failure in the reactor. This study provides infiltration data, and understanding of the degradation of various 'virgin' nuclear graphite grades by the molten fluoride salt. This should assist in the selection of tolerable or impermeable graphite grades for MSR operational purposes.

La prévision de la durée de vie des composants des centrales REP, en particulier les générateurs de vapeur, est cruciale. De nombreux travaux ont été menés pour comprendre les mécanismes de détérioration des composants par corrosion sous contrainte. Ces mécanismes restent mal connus. La couche d'oxyde doit jouer un rôle prépondérant. Une méthodologie, associant les techniques ToF-SIMS in situ et XPS, a été développée pour déterminer les cinétiques d'oxydation et comprendre les mécanismes de croissance des couches d'oxyde. La cinétique, la composition et la stratification des couches d'oxyde formées dans l'eau à haute température sur l'alliage 600 ont été comparées à des couches d'oxyde modèles formées in situ dans les bâtis XPS et ToF-SIMS en milieu gazeux à 300°C sous faible pression d'oxygène. On montre ainsi que l'oxydation en milieu gazeux à 300°C est un bon modèle pour simuler l'oxydation en milieu primaire. Une méthodologie originale a ensuite été mise au point à l'aide du ToF-SIMS pour déterminer les mécanismes de transport des espèces. Des échantillons préalablement oxydés en phase gazeuse ou dans l'eau à haute température ont été, dans une deuxième étape, réoxydés in situ dans le bâti ToF-SIMS à la même température, sous basse pression d'oxygène isotopique 18. Il apparait que la croissance de l'oxyde est localisée à l'interface métal/oxyde du fait de la diffusion des anions O2-, des phénomènes d'échange isotopique se produisant simultanément à l'oxydation. Le coefficient de diffusion de l'oxygène a pu être déterminé à l'aide d'un modèle approprié développé au cours de ce travail. / The lifetime prediction of pressurized water reactor (PWR) components, especially the steam generators (SG), is of high interest. Several works have been dedicated to the understanding of the stress corrosion cracking (SCC) mechanisms. However these mechanisms are still relatively poorly understood. The oxide layer should play an important role. A methodology, combining in situ ToF-SIMS and XPS techniques, has been developed to determine the oxidation kinetics and to understand the oxide layer growth mechanisms. Kinetics, composition and stratification of the oxide layers, formed in high temperature water on Alloy 600, were compared to model oxide layers formed in situ at 300°C under a low oxygen pressure. It shows that thermal oxidation under low oxygen pressure is a suitable model to simulate the oxidation in high temperature water. Finally, an original methodology based on ToF-SIMS has been developed to determine the transport mechanisms governing the oxide growth. Samples previously oxidized under a low oxygen pressure or in high temperature water, are, in a second step, re-oxidized in situ in the ToF-SIMS chamber at the same temperature under low oxygen isotope 18 pressure. It appears that the oxide growth is located at the metal/oxide interface due to the diffusion of O2- anions, isotopic exchange phenomena occuring simultaneously with oxidation. The oxygen diffusion coefficient was determined using an appropriate model developed in this work.

This diploma thesis is focusing on possibilities of influencing casting’s structure of nickel superalloy Inconel 713LC by force effects caused by vibration and rotation during crystallization. In this research were cast three test castings of turbine wheels with different solidification conditions that have affected the size and distribution of the grains. The aim was to compare the different microstructure and macrostructure of castings and then compare the dependence of grain refinement on mechanical properties. Changes in the structure were analyzed by means of light and electron microscopy methods after the fracture of test bars.

This project was ordered by Sandvik Materials Technology and was performed by a group of students at Uppsala university. The purpose of the project was to study precipitation behavior and structure stability in six advanced heat resistant alloys. Each sample were subjected to a creep rupture test in 600 or 700°C depending on the alloy type. Two parts of each alloy where examined; one part which had been affected by creep and another part which was unaffected by creep. A literature study was performed first to gain knowledge of the scientific theory utilized in this project, namely creep, precipitation hardening, and about the different materials which were analyzed. Preliminary results for the phase composition of the materials were obtained from a Thermo-Calc (TC) simulation. The SEM-images showed nothing noteworthy for any sample due to the roughness of the sample surfaces. The EDS-analysis showed chromium depletion in the centers of the aged samples of HT9 and Sanicro® 75X. Other minority phases such as Cr23C6 in Sanicro®70, P-phase and a titanium nitride phase in sanicro® 60X, VB in Esshete 1250 and Sigma-phase in 4C54 were identified using EDSmapping. It was found that when using XRD to analyze the phase compositions of small samples it is impractical to have the samples cast in bakelite beforehand. The XRD-results obtained in this project showed that more than 90% of the XRD diffractogram for every sample was graphite, which made the identification of minority phases impossible. The quality of the LOM-images varied greatly between samples, for 4C54 grain sizes were measured in all images, for Esshete 1250 grain sizes were measured for the crept sample, and for Sanicro® 60X measurements could only be taken from one image. Most of the sample preparation was insufficient to achieve the test results necessary for complete microstructural analysis and phase analysis of the samples. The mistakes in the practical steps of the project were noted and improvements for these mistakes are presented in the conclusion.