Modélisation de l’oxydation des aciers inoxydables polycristallins par une approche en champs de phases couplée avec la mécanique

Austenitic stainless steels and nickel based alloys are widely used for their mechanical properties at high temperatures.Their durability can be increased by the addition of chromium resulting in the formation of a protective oxide layer such as chromia (Cr2O3).Nevertheless, it is established from vacuum mechanical tests that oxidation significantly decreases their fatigue life.In fact, oxide growth can be followed with the injection of defects such as vacancies, deleterious chemical elements and residual stresses, etc., into the metal.The resulting cracking micromechanisms are therefore governed by complex interactions between the environment and the metal surface, implying the chemical composition and the microstructure of the metal.To date, materials life prediction is a necessity for the nuclear industry due to safety and economic issues.The enhancement of the model dimensionality allow to explicitly account for multi-physics interactions between oxide and metallic phases under mechanical loads.The thesis is in line with it and relies on the development of a phase field model coupled with mechanics that heavily relies on the principles of continuum thermodynamics.The effective behaviour of the interface is obtained by homogenisation methods allowing the mixture of separate behaviours, as it is the case on a ductile metallic substrate and its fragile oxide.Oxide growth residual stresses and mechanical load induced stresses can be relaxed by viscoplasticity, which is isotropic and anisotropic respectively for the oxide and the substrate.Full field finite element simulations are performed to study both generalised and intergranular oxidation under mechanical loads.The simulations highlight the possibility of triggering breakaway oxidation by the generation of tensile stresses in the fragile oxide, which can be localised along oxide intrusions at grain boundaries.Les aciers austénitiques et alliages à base de nickel sont des matériaux de choix pour leurs propriétés mécaniques à haute température. L'enrichissement en chrome améliore leur durabilité de part la formation d'une couche d'oxyde protectrice à l'exemple de la chromine (Cr2O3).Il est néanmoins établi, par des essais mécaniques sous vide, que l'oxydation réduit de manière notable leur durée de vie en fatigue.En effet, la croissance d'oxyde peut être accompagnée d'une introduction de défauts tels que l'injection de lacunes, d'éléments délétères comme l'hydrogène mais également de contraintes résiduelles, etc., dans le métal.Les micromécanismes de fissuration sont ainsi régis par des interactions complexes entre l'environnement et la surface du métal, faisant intervenir composition chimique et microstructure.Aujourd'hui, les enjeux de sécurité et de compétitivité font de la prévision de la durée de vie de ces alliages une nécessité pour l'industrie nucléaire.L'augmentation de la dimension des modèles permet de prendre en compte de manière explicite les interactions multiphysiques du couple oxyde/métal sous l'action d'un chargement mécanique.La thèse s'inscrit dans cette démarche et propose une formulation d'un modèle de champ de phases couplé avec la mécanique et fondé sur les principes de la thermodynamique des milieux continus.Le comportement effectif de l'interface est présentement obtenu via des méthodes d'homogénéisation permettant de combiner des comportements mécaniques dissemblables, à l'image d'un substrat ductile et de son oxyde fragile.Les contraintes induites par la formation d'oxyde et également par le chargement mécanique peuvent être relaxées viscoplastiquement, de manière isotrope et anisotrope, respectivement dans l'oxyde et dans le substrat.Des simulations par éléments finis de l'oxydation généralisée ainsi que de l'oxydation intergranulaire sous chargement mécanique sont effectuées.Ces dernières mettent en évidence la possibilité d'un phénomène d'oxydation catastrophique par la génération de contraintes de tensions dans l'oxyde fragile, lesquelles peuvent être localisées le long des intrusions d'oxyde dans les joints de grains

Modelling stress-diffusion controlled phase transformations : application to stress corrosion cracking

National audienceStress Corrosion Cracking (SCC) represents a significant cause of failure in pressurised water reactors and many efforts have been made to address this problem [1]. It involves the combined action of the environment, mechanical stresses and material properties on the damage of engineering components. Current SCC models developed to predict crack growth behaviour or SCC susceptibility criteria do not fully incorporate the complex multiphysical processes that occur during oxidation at the scale of the microstructure. The aim of the work is to formulate a multi-physics modelling framework based on continuum thermodynamics able to describe the growth of an oxide film on a polycrystalline material using the phase field method

Big Ramsey degrees and infinite languages

This paper investigates big Ramsey degrees of unrestricted relational structures in (possibly) infinite languages. While significant progress has been made in studying big Ramsey degrees, many classes of structures with finite small Ramsey degrees still lack an understanding of their big Ramsey degrees. We show that if there are only finitely many relations of every arity greater than one, then unrestricted relational structures have finite big Ramsey degrees, and give some evidence that this is tight. This is the first time that finiteness of big Ramsey degrees has been established for an infinite-language random structure. Our results represent an important step towards a better understanding of big Ramsey degrees for structures with relations of arity greater than two.Comment: 21 pages. An updated version strengthening the statement of the positive results and fixing a mistake in the earlier version of the negative result which now needs an extra assumptio

Instability of the rhodium magnetic moment as origin of the metamagnetic phase transition in alpha-FeRh

Based on ab initio total energy calculations we show that two magnetic states of rhodium atoms together with competing ferromagnetic and antiferromagnetic exchange interactions are responsible for a temperature induced metamagnetic phase transition, which experimentally is observed for stoichiometric alpha-FeRh. A first-principle spin-based model allows to reproduce this first-order metamagnetic transition by means of Monte Carlo simulations. Further inclusion of spacial variation of exchange parameters leads to a realistic description of the experimental magneto-volume effects in alpha-FeRh.Comment: 10 pages, 13 figures, accepted for publication in Phys. Rev.