Ni-based superalloys remain state-of-the-art materials for use in the high-temperature, corrosive environments experienced by turbine blades in gas turbine engines used for propulsion and energy generation. Increasing the operating temperatures of turbine engines can yield increased engine efficiencies. However, appreciably higher operational temperatures can exceed the capabilities of Ni-based superalloys. Consequently, interest exists to develop high-melting refractory complex concentrated alloys (RCCAs) with the potential to surpass the hightemperature property limitations of Ni-based alloys. RCCAs are multi-principal element alloys, often comprising 5 or more elements in equal or near equal amounts. Conventional solidificationbased processing methods (e.g., arc melting) of RCCAs tend to yield coarse-grained samples with a large degree of microsegregation, often requiring long subsequent homogenization annealing times. Additionally, the large differences in melting temperatures of component elements can further complicate solidification-based fabrication of RCCAs. <div>Herein, the feasibility of a new chemical synthesis, powder metallurgy route for generating fine-grained, homogenous RCCAs is demonstrated. This is achieved by first employing the Pechini method, which is a well-developed process for generating fine-grained, oxide powder mixtures. The fine oxide powder mixture is then reduced at a low temperature (600°C-770 ºC) to yield fine-grained metal alloy powder. Hot pressing of the metallic powder is then used to achieve dense, fine-grained metallic alloys. While this process is demonstrated for generating fine-grained, high-melting MoW and MoWCr alloys, this method can be readily extended to generate other finegrained RCCA compositions, including those unachievable by solidification-based processing methods.</div>

In this project, we have assessed the structural tolerance of advanced refractory alloys to simulated nuclear fusion reactor environments, by using intense proton beams to mimic fusion neutron damage and analysing the proton damaged structures using in-situ/ex-situ transmission electron microscopy and nano-hardness measurements. Refractory metals such as tungsten or tantalum, and their binary alloy combinations, are considered as promising structural materials to withstand the unprecedented high heat loads and fast neutron/helium fluxes expected in future magnetically-confined fusion reactors. Tungsten is currently the frontrunner for the production of plasma-facing components for fusion reactors. The attractiveness of tungsten as structural material lies in its high resistance to plasma-induced sputtering, erosion and radiation-induced void swelling, together with its thermal conductivity and high-temperature strength. Unfortunately, the brittle nature of tungsten hampers the manufacture of reactor components and can also lead to catastrophic failure during reactor operations. We have focused on two potential routes to enhance the ductility of tungsten-containing materials, namely alloying tungsten with controlled amounts of tantalum, and using alternatively tantalum-based alloys containing specific tungsten additions, either as a full-thickness structural facing material or as a coating of first wall reactor components. The aim was to investigate the formation and evolution of radiation-induced damaged structures in these material solutions and the impact of those structures on the hardness of the material. The main results of this work are: (1) the addition of 5wt%Ta to W leads to saturation in the number density and average dimensions of the radiation-induced a/2 dislocation loops formed at 350C, whereas in W the loop length increases progressively and evolves into dislocation strings, and later into hydrogen bubbles and surface blisters, (2) the recovery behaviour of proton irradiated W5wt.%Ta alloy is characterized by dislocation loop growth at 600-900C, whereas voids form at 1000C by either vacancy absorption or loop collapse, (3) the presence of radiation-induced a loops at 590C in Ta hinders the formation and ordering of voids observed with increasing damage levels at 345C, (4) the addition of 5-10wt.%W to Ta delays the evolution of a/2 dislocation loops with increasing damage levels, and therefore the appearance of random voids. These results expand the composition palette available for the safe selection of refractory alloys for plasma facing components with enhanced, or at least predictable, tolerance to the heat-radiation flux combinations expected in future nuclear fusion plants.

L'amélioration du rendement des turboréacteurs requiert un accroissement de leur température de service. Le développement de nouveaux alliages, issus du système Nb-Si, permet d'envisager des températures de fonctionnement de 200°C supérieures à celles offertes par les superalliages base nickel utilisés actuellement. La première partie de ce manuscrit rappelle les principaux résultats scientifiques ayant menés à la sélection des alliages composites à base de siliciures de niobium (Nbss-Nb5Si3). La microstructure de ces alliages associe une matrice ductile de niobium pouvant solubiliser de nombreux éléments d'addition à une dispersion de siliciures durs et fragiles conférant aux alliages leurs bonnes propriétés en fluage et une meilleure résistance à l'oxydation à haute température. Malheureusement, ces alliages sont caractérisés par une récession rapide du métal associée au développement d'oxydes non protecteurs. L'oxygène réagit rapidement avec le substrat, se dissout dans la solution solide de niobium et y diffuse rapidement. L'effet des éléments Al, Si et Ti a été étudié en considérant à la fois les modifications microstructurales et les propriétés en oxydation lors de ces additions. Bien que ces optimisations de compositions conduisent à une amélioration significative de la résistance à l'oxydation des alliages Nbss-Nb5Si3, certaines nuances souffrent d'une résistance à l'oxydation catastrophique vers 800°C. L'ajout graduel d'étain au sein des alliages permet de modifier foncièrement la microstructure, notamment en initiant le développement d'une phase de type A15-Nb3Sn. A 800°C, l'étain supprime la dissolution de l'oxygène au sein de Nbss responsable du comportement en oxydation catastrophique rencontré par les nuances sans étain. Malgré ces progrès, la résistance à l'oxydation de ces alliages reste insuffisante et le développement de revêtements protecteurs contre l'oxydation a été nécessaire. Dans ce sens deux familles de siliciures Nb3X3CrSi6 et Nb4X4Si7 (X = Fe, Co ou Ni) ont été sélectionnées et leur stabilité thermodynamique ainsi que leur comportement en oxydation ont été évaluées. Ces phases se sont avérées capables de résister à l'oxydation à des températures d'exposition allant jusqu'à 1300°C. Le mécanisme d'oxydation de chacun de ces siliciures a été déterminé. Finalement, le dépôt de ces siliciures à la surface des alliages Nbss-Nb5Si3 via le procédé de pack cémentation s'est révélé possible. Les alliages revêtus par les siliciures choisis présentent des durées de vie pouvant aller jusqu'à 3000 cycles d'oxydation d'une heure à 1100°C / The improvement of the efficiency of turbine engine can be achieved by increasing the working temperature. The development of new alloys based on Nb-Si system allows a jump of 200°C of the operating temperature in comparison to that offered by current nickel based alloys. The first part of this manuscript focuses on the evolutions which have led to the development of niobium silicide in situ composites (Nbss-Nb5Si3). The microstructure of these alloys consists in a ductile niobium matrix where number of alloying elements can solubilise and of strengthening niobium silicides which are intended to provide creep and oxidation resistance at high temperature. Unfortunately, these alloys exhibit a poor oxidation resistance characterised by a high metal recession rate and the formation of non-protective oxide scale. Thus, oxygen can easily react with the substrate, dissolve in Nbss and diffuse quickly through this phase. The effect of Al, Si and Ti additions on both microstructure and oxidation resistance were investigated. Although, these composition optimisations lead to a significant enhance of oxidation resistance, some compositions still suffers from catastrophic oxidation behaviour around 800°C. In these alloys tin additions involve high microstructural changes, especially by initiating the formation of A15- Nb3Sn phase. At 800°C, Sn additions suppress oxygen dissolution in Nbss responsible of the catastrophic oxidation behaviour of these alloys. Nevertheless, the oxidation resistance of these alloys remains too low for the foreseen applications and protective coatings are required. Thermodynamic stability and oxidation resistance of two silicide families (Nb3X3CrSi6 and Nb4X4Si7 (X = Fe, Co or Ni)) were investigated. These silicides have exhibited a high oxidation resistance up to 1300°C by the formation of a protective silica layer. Finally, these silicides were deposited on Nbss-Nb5Si3 substrate by using the pack cementation process. Some coated alloys have then exhibited lifetime going up to 3000 one hour cycle at 1100°C

L'assiette de fibrage utilisée pour la production de la laine de verre d'isolation subit à haute température de nombreuses contraintes (chimiques, mécaniques et thermiques). La ruine des assiettes peut être due à l'oxydation par les gaz chauds, à la corrosion par le verre fondu ou à la déformation par fluage. La première partie des travaux de thèse a porté sur l'amélioration des propriétés mécaniques des alliages pour le fibrage à 1200°C. Dans un premier temps, la teneur en chrome au coeur d'alliages à base de cobalt a été réduite afin d'augmenter la réfractarité en conservant la microstructure. Cependant, cette réduction a dégradé le comportement en oxydation de ces alliages. Afin de conserver un bon niveau en oxydation, un enrichissement en chrome de la surface a été réalisé à l'aide de la technique de pack-cémentation. Le comportement en fluage de ces alliages a été maintenu à un niveau correct. Dans un second temps, de nouveaux systèmes métallurgiques ont été explorés, basés sur le nickel et le fer, et ont été renforcés mécaniquement par des éléments lourds en solution solide ou par des précipités intermétalliques. La réfractarité et le comportement en oxydation se sont révélés intéressants mais la tenue en fluage a été décevante. La seconde partie de ces travaux a consisté à améliorer le comportement en oxydation de l'alliage utilisé lors du fibrage à 1000-1050°C. Deux voies ont été explorées : un enrichissement en chrome de la surface de l'alliage par pack-cémentation ou l'addition d'un élément réactif, l'yttrium. La seconde solution a apporté des résultats intéressants avec une amélioration significative du comportement en oxydation cyclique / The spinner used to product glass wool for thermal building insulation undergoes several stresses (chemical, mechanical and thermal) at high temperature. The lifetime of the spinner is limited by oxidation by hot gases, corrosion by molten glass or creep deformation. The first part of this thesis has concerned the improvement of the mechanical properties of the alloys for the fiberizing at 1200°C. First, the bulk chromium content of the usual cobalt-base alloys has been decreased to increase their refractoriness at constant microstructure. However, this reduction has degraded the oxidation behavior of these alloys. To keep a good oxidation behavior, a surface chromium enrichment of the Cr-impoverished alloys is achieved by pack-cementation technique. The creep behavior of these alloys is kept at a good level. Secondly, several new metallurgical systems were explored: alloys based on nickel and iron and reinforced by heavy elements in solid solution or by precipitates inter metallic particles. The refractoriness and the oxidation behavior appeared to be interesting but the creep deformation was disappointing. The second part of this work has concerned the improvement of the oxidation and corrosion properties of the alloys for the process at 1000-1050°C. Two ways have been explored: a chromium enrichment on the sub-surface or an addition of a reactive element, as yttrium. The second solution has given interesting results with a significant improvement of the oxidation behavior, and particularly in cyclic oxidation

<p>Ni-based superalloys continue to exert themselves as the industry standards in high stress and highly corrosive/oxidizing environments, such as are present in a gas turbine engine, due to their excellent high temperature strengths, thermal and microstructural stabilities, and oxidation and creep resistances. Gas turbine engines are essential components for energy generation and propulsion in the modern age. However, Ni-based superalloys are reaching their limits in the operating conditions of these engines due to their melting onset temperatures, which is approximately 1300 °C. Therefore, a new class of materials must be formulated to surpass the capabilities Ni-based superalloys, as increasing the operating temperature leads to increased efficiency and reductions in fuel consumption and greenhouse gas emissions. One of the proposed classes of materials is termed refractory complex concentrated alloys, or RCCAs, which consist of 4 or more refractory elements (in this study, selected from: Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, and W) in equimolar or near-equimolar proportions. So far, there have been highly promising results with these alloys, including far higher melting points than Ni-based superalloys and outstanding high-temperature strengths in non-oxidizing environments. However, improvements in room temperature ductility and high-temperature oxidation resistance are still needed for RCCAs. Also, given the millions of possible alloy compositions spanning various combinations and concentrations of refractory elements, more efficient methods than just serial experimental trials are needed for identifying RCCAs with desired properties. A coupled computational and experimental approach for exploring a wide range of alloy systems and compositions is crucial for accelerating the discovery of RCCAs that may be capable of replacing Ni-based superalloys. </p> <p>In this thesis, the CALPHAD method was utilized to generate basic thermodynamic properties of approximately 67,000 Al-bearing RCCAs. The alloys were then down-selected on the basis of certain criteria, including solidus temperature, volume percent BCC phase, and aluminum activity. Machine learning models with physics-based descriptors were used to select several BCC-based alloys for fabrication and characterization, and an active learning loop was employed to aid in rapid alloy discovery for high hardness and strength. This method resulted in rapid identification of 15 BCC-based, four component, Al-bearing RCCAs exhibiting room-temperature Vickers hardness from 1% to 35% above previously reported alloys. This work exemplifies the advantages of utilizing Integrated Computational Materials Engineering- and Materials Genome Initiative-driven approaches for the discovery and design of new materials with attractive properties.</p> <p> </p> <p><br></p>