Compte tenu de l’évolution des températures d’entrée de turbines, le principal enjeu pour les métallurgistes est d’élaborer des alliages capables de présenter d’excellentes propriétés mécaniques à des températures de plus en plus élevées. Dans ce contexte, connaitre les relations entre les différents paramètres microstructuraux et les propriétés mécaniques à haute température (700°C et plus) est capital.La durabilité en fluage et en fatigue-temps de maintien ainsi que les processus d’endommagement de l’alliage AD730™, un nouveau superalliage pour disques de turbines, ont été analysés. Plusieurs paramètres ont été étudiés, qu’ils soient microstructuraux (joints de grains, taille de grains, taille et distribution des précités γ′), ou expérimentaux (température, environnement, contrainte appliquée, temps de maintien). L’utilisation de monograins de composition chimique identique à l’alliage de l’étude a permis de mettre en évidence le fait qu’une microstructure monogranulaire ne présente pas nécessairement de meilleures propriétés en fluage qu’une microstructure polycristalline. Ceci est attribué au rôle durcissant des joints de grains. Il a de plus été montré qu’à 700°C, la taille et la distribution des précipités γ′ est le paramètre microstructural pilotant les propriétés viscoplastiques à l’ordre 1.En fatigue avec temps de maintien, un comportement original a été observé pour les longs temps de maintien en fonction de la contrainte appliquée. Ce phénomène est attribué à un effet « type Bauschinger » apparaissant lors des phases de décharges. / In view of the turbine entry temperature evolution, the main challenge for metallurgists is to elaborate new alloys able to withstand higher temperatures while keeping great mechanical properties. Therefore, knowing the relationships between microstructural parameters and mechanical properties at high temperatures (700°C and more) is mandatory.The creep and dwell-fatigue durability as well as the damage mechanisms of AD730™, a new nickel base superalloy developed for turbine disks, have been analyzed. Several microstructural parameters were studied (grain boundaries, grain size, size and distribution of γ′ precipitates) as well as experimental parameters (temperature, environment, applied stress or dwell period). By using single crystalline specimens having the same chemical composition of the studied alloy, it has been shown unambiguously that single crystalline microstructures do not necessarily present better creep properties compared to polycristalline ones. This result is supposed to be caused by a grain boundary strengthening mechanism. Moreover, in creep at 700°C, it has been shown that the main viscoplasticity controlling parameters are the size and distribution of γ′ precipitates.An unexpected dwell-fatigue behavior has been observed for long hold times and in a specific applied stress window. This phenomena is attributed to a “Bauschinger type” effect, occurring during unloading phases.

Liquid organic chemical hydride is a promising candidate for hydrogen storage and transport. Methylcyclohexane (MCH) to toluene (TOL) cycle has been considered as one of the feasible hydrogen carrier systems, but selective dehydrogenation of MCH to TOL has only been achieved using the noble Pt-based catalysts. The aim of this study is to develop non-noble, cost-effective metal catalysts that can show excellent catalytic performance, mainly maintaining high TOL selectivity achievable by Pt based catalysts. Mono-metallic Ni based catalyst is a well-known dehydrogenation catalyst, but the major drawback with Ni is its hydrogenolysis activity to cleave C-C bonds, which leads to inferior selectivity towards dehydrogenation of MCH to TOL. This study elucidate addition of the second metal to Ni based catalyst to improve the TOL selectivity. Herein, ubiquitous bi-metallic nanoparticles catalysts were investigated including (Ni–M, M: Ag, Zn, Sn or In) based catalysts. Among the catalysts investigated, the high TOL selectivity (> 99%) at low conversions was achieved effectively using the supported NiZn catalyst under flow of excess H2. In this work, a combined study of experimental and computational approaches was conducted to determine the main role of Zn over Ni based catalyst in promoting the TOL selectivity. A kinetic study using mono- and bimetallic Ni based catalysts was conducted to elucidate reaction mechanism and site requirement for MCH dehydrogenation reaction. The impact of different reaction conditions (feed compositions, temperature, space velocity and stability) and catalyst properties were evaluated. This study elucidates a distinctive mechanism of MCH dehydrogenation to TOL reaction over the Ni-based catalysts. Distinctive from Pt catalyst, a nearly positive half order with respect to H2 pressure was obtained for mono- and bi-metallic Ni based catalysts. This kinetic data was consistent with rate determining step as (somewhat paradoxically) hydrogenation of strongly chemisorbed intermediate originating from TOL. DFT calculation indicated that Zn metal prefers to occupy the step sites of Ni where unselective C–C bond breaking was considered to preferentially occur, explaining suppression of hydrogenolysis activity. Additionally, it confirmed that the H-deficient species at methyl position group (C6H5CH2) was stable on the surface, making its hydrogenation being rate determining step, consistent with positive order in H2 pressure on TOL formation rate. This may explain the conclusive role by H2 in facilitating desorption of the H-deficient surface species that was produced through further dehydrogenation of TOL.

Cette étude s’est focalisée sur la compréhension des mécanismes de déformation et d’endommagementd’un superalliage à base de Nickel (Udimet 720Li) utilisé pour la confection de disques de turbine hautepression soumis à des sollicitations monotones et cycliques à des températures inférieures à 500 °C.L’amorçage de fissures de fatigue à 20 °C et 465 °C a été systématiquement observé suivant des facettescristallographiques pour différents états métallurgiques. Une étude quantitative des traces de glissementplastique en surface a permis de montrer que cet endommagement est la conséquence d’une nontransmission de la déformation plastique entre grains voisins. Plus précisément, un mécanisme de rotationcristalline localisée au sein de micro-volumes à l’extrémité de bandes de glissement dans le grain voisin aété identifié à l’origine des fissures.Les configurations cristallographiques favorisant ce processus impliquent notamment les relations entresystèmes de glissement de part et d’autre du joint de grains (angle de TWIST), ou encore la présence d’uneprécipitation fine participant au confinement des bandes de glissement. / This study contributes to the understanding of the deformation and damage mechanisms in a Nickelbasedsuperalloy (the Udimet 720Li) used for the manufacturing of aeroengines high pressure turbinedisks submitted to monotonic and cyclic loadings at temperatures below 500 °C under service conditions.Fatigue crack initiation at 20 °C and 465 °C was systematically observed at large crystallographic facetswhatever the microstructure. An extensive quantitative study of the slip activity at the surface showed thatcrack initiation results from a difficult slip transmission between neighbouring grains. In more details, alocalized crystallographic rotation confined in micro-volumes detected at the tip of slip bands interactingwith the grain boundary were identified to trigger crack initiation.The crystallographic configurations favouring such a process are closely related to the TWIST anglebetween active slip systems on each side of the grain boundary and to a dense and fine precipitationconfining the plasticity.Mots-clés

Ce travail s’inscrit dans une problématique industrielle relative à la fabrication d’échangeurs de chaleur utilisés dans les systèmes de conditionnement d’air sur avion. Ces échangeurs sont fabriqués à partir de tôles minces embouties en Ni 201, en Alliage 600 ou en AISI 444 brasées avec des tôles intercalaires et des barres de fermeture en Alliage 625. Le métal d’apport utilisé est la BNi-8, composé principalement de nickel, de manganèse, de silicium et de cuivre. Des défauts de brasage peuvent apparaître lors de la fabrication des faisceaux occasionnant un dysfonctionnement de l’échangeur. L’opération de brasage à l’échelle d’un échangeur implique un ensemble de phénomènes physiques couplés, liés au comportement thermomécanique de la structure alvéolaire, à l’hétérogénéité des conditions de brasage et à la métallurgie locale. Afin d’améliorer la compréhension des phénomènes liés au brasage, plusieurs axes d’études ont été entrepris. Dans un premier temps, le comportement du métal d’apport a été caractérisé avec l’étude des températures de fusion en fonction de la composition chimique, de la microstructure et des propriétés mécaniques associées. En parallèle, les métaux de base ont également été étudiés afin de connaître l’évolution de leur microstructure et donc des propriétés mécaniques en fonction de la température mais aussi de l’état des contraintes résiduelles présent dans le feuillard à l’issue de leur mise en forme par emboutissage. Une étude sur les tôles emboutis a également permis d’appréhender le comportement de ces produits minces en compression à chaud. Pour finir, afin de se rapprocher des conditions réelles de brasage en milieu industriel, des études ont été menées sur le couplage métal d’apport/métal de base d’un point de vue métallurgique mais aussi mécanique. L’ensemble de ces études a pour objectif de mieux appréhender les phénomènes mis en jeu pendant le cycle de brasage et de proposer des améliorations pour le procédé (géométrie des intercalaires, cycles thermiques, matériaux utilisés, etc.). / This work is devoted to improving the manufacturing process of heat exchangers used in aerospace applications. Heat exchangers included in air conditioning systems for aircraft are produced by brazing stamped thin alloys sheets made of nickel-based alloys, Alloy 600 and Ni 201, or stainless steel, AISI 444. Separation metal sheets and locking bars of Alloy 625 are used to complete the system. The used brazing filler metal BNi-8 is mainly composed of nickel, manganese, silicon and copper. However, brazing defects appear during the manufacture of theheat exchangers can lead to non-integrity of the components. In order to improve the understanding of the phenomena related to brazing, several axes of investigation have been considered. Firstly, the behavior of the brazing filler metal was characterized by studying the melting temperatures as a function of the composition, the microstructure and the associated mechanical properties. In parallel, the base metals were also studied in order to know the evolution of the microstructure and therefore the mechanical properties as a function of the temperature but also of the state of stress present in the metal sheet induced by the stamping. Studying of stamped thin alloys sheets also allowed to understand the behavior of these thin products in hot compression. Finally, in order to get closer to the actual brazing conditions in industrial environment, studies of the coupling of brazing filler metal/base metal from a metallurgical and mechanical point of view have been carried out. All of these studies pursue aim to better understand the phenomena involved during the brazing cycle and to propose improvements for the brazing process (geometry of stamped thin alloys sheets, thermal cycles, used materials, etc.).

In light of global warming’s environmental implications, research is shifted towards potential processes that can utilize CO2 and reduce its emissions in the industrial sector. One of the promising processes is dry reforming of methane (DRM), which is capable of utilizing CO2 and producing valuable syngas (H2 and CO). The main challenge of DRM is the deactivation of catalysts under the reaction temperatures (above 700 °C) due to sintering of the active metal and coke formation. Ni-based catalysts are the most widely investigated catalysts in literature for DRM due to their cost efficiency and availability. This study is an extension of the work done by Saudi Basic Industries Corporation (SABIC) devoted to investigating Ni-Ce-In system for DRM reaction. Five catalysts were synthesized by dry impregnation method according to SABIC synthesis procedure (Ni/Al2O3, Ni-In/Al2O3, Ni/CeO2/Al2O3, Ni/In-CeO2/Al2O3, and Ni-In/CeO2/Al2O3). The metallic loading targets were 7.5 wt.%, 10 wt.%, and 0.8 wt.% for nickel, cerium, and indium, respectively. The addition of indium in combination with cerium resulted in the highest catalytic activity. Additionally, the co-impregnation of indium and cerium resulted in enhancing the catalytic activity more than subsequential impregnation (Ni/In-CeO2/Al2O3 compared to Ni-In/CeO2/Al2O3). The addition of cerium or indium separately with nickel did not seem to affect activity since Ni/Al2O3, Ni-In/Al2O3, and Ni/CeO2/Al2O3 exhibited similar conversion values. All catalysts were stable for more than two days under DRM conditions without deactivating. Therefore, deactivation behaviors of the catalysts were not covered in this study.

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This thesis work concerns about the importance of the cast surfaces, surface phenomenon such as the formation of the oscillation marks and exudation and related defects including cracks and segregation that happened during the continuous casting. All of the investigated materials were collected during the plant trials while an in-depth analysis on these materials was performed at the laboratory scale with certain explanations supported by the schematic and theoretical models. The work consists on different material classes such as steels and phosphor bronzes with a focus on the surface defects and their improvements. In order to facilitate the theoretical analysis which could be capable of explaining the suggested phenomenon in the thesis, a reduced model is developed which required lesser computational resources with lesser convergence problems. / <p>QC 20160527</p> / Oscilation mark formation during continous casting of steel

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The simulation and design of advanced materials for fatigue resistance requires an understanding of the response of their hierarchical microstructure attributes to imposed load, temperature, and environment over time. For Ni-base superalloy components used in aircraft jet turbine engines, different competing mechanisms (ex. surface vs. subsurface, crystallographic vs. inclusion crack formation, transgranular vs. intergranular propagation) are present depending on applied load, temperature, and environment. Typically, the life-limiting features causing failure in Ni-base superalloy components are near surface inclusions. Compressive surface residual stresses are often introduced in Ni-base superalloy components to help retard fatigue crack initiation and early growth at near surface inclusions and shift the fatigue crack initiation sites from surface to sub-surface locations, thereby increasing fatigue life. To model the effects of residual stresses, inclusions, and microstructure heterogeneity on fatigue crack driving force and fatigue scatter, a computational crystal plasticity framework is presented that imposes quasi-thermal eigenstrain to induce near surface residual stresses in polycrystalline Ni-base superalloy IN100 smooth specimens with and without nonmetallic inclusions. In addition, the effect of near surface inclusions in notched Ni-base superalloy components on MSC growth and fatigue life scatter was investigated in this work. A fatigue indicator parameter (FIP)-based microstructurally small crack (MSC) growth model incorporating crack tip/grain boundary effects was introduced and fit to experiments (in both laboratory air and vacuum) for the case of 1D crack growth and then computationally applied to 3D crack growth starting (1) from a focused ion beam (FIB) notch in a smooth specimen, (2) from a debonded inclusion located at different depths within notched components containing different notch root radii, and (3) from inclusions located at different depths relative to the surface in smooth specimens containing simulated shot peened induced residual stresses. Computational predictions in MSC growth rate scatter and distribution of fatigue life were in general accordance with experiments. The general approach presented in this Dissertation can be used to advance integrated computational materials engineering (ICME) by predicting variation of fatigue resistance and minimum life as a function of heat treatment/microstructure and surface treatments for a given alloy system and providing support for design of materials for enhanced fatigue resistance. In addition, this framework can reduce the number of experiments required to support modification of material to enhance fatigue resistance, which can lead to accelerated insertion (from design conception to production parts) of new or improved materials for specific design applications. Elements of the framework being advanced in this research can be applied to any engineering alloy.