Les culasses de moteur diesel PSA sont réalisées en alliages d'aluminium de fonderie durcis par précipitation. La culasse atteint une température stable lors de l'utilisation du véhicule mais la succession de démarrages et d'arrêts engendre un phénomène de fatigue thermique au niveau du pontet inter-soupapes. L'exposition à des températures élevées modifie la microstructure de précipitation provoquant une diminution des propriétés mécaniques et donc un vieillissement de l'alliage. Il est donc nécessaire de prendre en compte le vieillissement thermique dans le dimensionnement. Pour y parvenir, nous avons d'abord analysé la microstructure de précipitation (nature des phases précipitées, morphologie et taille) en Microscopie Electronique en Transmission (MET) et le comportement cyclique viscoplastique pour différents vieillissement isothermes. Des essais thermomécaniques anisothermes, simulant le chargement en service, ont été réalisés afin d'analyser les relations entre la déformation plastique et la précipitation. Les paramètres géométriques de la précipitation ont été déterminés en MET pour différents nombres de cales thermomécaniques et ont été comparés à ceux de la précipitation résultant de l'exposition de l'alliage au seul cycle thermique pendant le même nombre de cycles.La microdureté étant un bon indicateur du vieillissement des alliages d'aluminium durcis par précipitation, des modèles d'évolution de microdureté ont été développés afin de simuler sa décroissance dans la structure lors d'un essai représentatif des conditions en service. Ces simulations permettent d'améliorer l'estimation du vieillissement de ces alliages lors de l'utilisation de la culasse. / Cylinder heads of automotive engines are produced by casting of precipitation strengthened Al-Si alloys.The cylinder head quickly reaches a steady state temperature but the succession of starts and stops generates thermomechanical fatigue in the area between the valve seats. At service temperature, the precipitation microstructures evolve from their state after precipitation hardening heat treatment to an aged state, which results in the reduction of their mechanical properties. The increase of thermomechanical stresses in new engines requires taking thermal aging into account for sizing. To that end, the precipitation microstructures (precipitates crystal structures, composition, morphology, and distribution) have been analyzed by Transmission Electron Microscopy (TEM) for various aging conditions and the corresponding cyclic least-viscoplastic behavior has been determined. The effect of plastic deformation on the kinetics of precipitate growth has been studied throughout loadings representative of engine operation. The morphological characteristics of the precipitation microstructure resulting from the thermomechanical aging were determined by TEM for different numbers of cycles and compared with those resulting from the mere thermal aging in order to assess the effect of plastic deformation on the kinetics of precipitate growth. As micro hardness is a good indicator of the aging of precipitation strengthened aluminium alloys, micro hardness evolution models have been developed to calculate its decrease in the cylinder head during a test simulating in service conditions. The simulations give us a better understanding of the in-service aging of these alloys.

Cette thèse s'inscrit dans un programme de développement technologique de la Société LEONI. Ce programme de recherche a pour principal objectif de disposer, à l'issue des trois années de l'étude, d'un panel de connaissances scientifiques le plus pertinent possible sur la durabilité des alliages d'aluminium dans des environnements caractéristiques d'un faisceau électrique automobile. Il s'agit, à terme, d'intégrer, à l'échelle industrielle, l'aluminium ou l'un de ses alliages dans les câblages électriques automobiles. Après une première étape de sélection des matériaux, l'alliage retenu est un alliage d'aluminium 6101. La fabrication des faisceaux électriques automobiles nécessite l'utilisation de brins de petits diamètres qui sont obtenus, à partir d'une ébauche, par un procédé de mise en forme associant plusieurs étapes de tréfilage et de revenu. Chacune de ces étapes conditionne les propriétés de l'alliage. Les travaux réalisés dans le cadre de cette thèse ont eu pour objectif d'analyser et de quantifier l'influence des différentes étapes de ce procédé de mise en forme sur la durabilité de l'alliage 6101. Compte tenu de l'environnement en service des faisceaux électriques, deux types de sollicitation ont été considérés ; ces sollicitations sont associées à un endommagement en corrosion d'une part et à un endommagement en fatigue-corrosion d'autre part. La première partie de ces travaux est donc consacrée à une étude du comportement en corrosion en milieu contenant des ions chlorures de l'alliage d'aluminium AA 6101 à l'état métallurgique T4, correspondant à l'état microstructural du matériau d'ébauche. L'influence des traitements thermomécaniques liés au procédé de mise en forme des fils fins sur la microstructure, les propriétés mécaniques et le comportement en corrosion de l'alliage AA 6101 fait l'objet de la seconde partie des travaux. Enfin, l'influence de contraintes mécaniques cycliques sur le comportement en corrosion de l'alliage AA 6101 en milieu NaCl 0,5M a été étudiée. L'ensemble de ces résultats met en évidence un couplage environnementmicrostructure-état de contraintes avec un fort impact des traitements thermo-mécaniques sur les microstructures et donc sur les propriétés mécaniques et le comportement en corrosion du matériau / This work is in the framework of a technological development program with LEONI. The aim of this program research is to bring technical skills and scientific knowledge, the most pertinent as possible on the durability of aluminum alloys in automotive harness environment. The purpose is to integrate aluminum alloys in automotive wiring systems. After a first step of materials selection, the AA 6101 aluminum alloy has been selected. Automotive wiring harnesses need small diameters of strands which are obtained by a forming process combining several cold-drawing steps and aging heat treatments. Each step of this forming process influences the alloy properties. The aim of this work is to analyze and quantify the influence of each step of the forming process on the durability of AA 6101 aluminum alloy. Considering the automotive harness environment, two kinds of damages have been studied: corrosion damage in one hand and fatigue-corrosion damages in the other hand. The first part of this work was dedicated to the study of the corrosion behavior of AA 6101 aluminum alloy at T4 metallurgical state in chloride media. The influence of thermo-mechanical treatments, due to the process forming of the strands on the microstructure, the mechanical properties and the corrosion behavior of the AA 6101 has been studied in the second part. Finally, the influence of cyclic mechanical stresses on the corrosion behavior, i.e. fatigue-corrosion phenomena, of the AA 6101 in 0.5M NaCl has been considered. The whole results highlight a coupling between environment-microstructure-stress states with a severe influence of the thermo-mechanical treatments on the microstructures, the mechanical properties and on the corrosion behavior

La tenue au fluage des aubes de turbine haute pression est une problématique de premier ordre vis-à-visde la certification d'un turbomoteur d’hélicoptère. Les excellentes propriétés mécaniques à hautes températuresdes superalliages monocristallins base nickel en font les matériaux les plus utilisés pour la fabrication de cesaubes. Pour ces composants, les exigences réglementaires de certification imposent la réalisation d’essaisanisothermes, plus sévères que les conditions de fonctionnement en service, basés sur le mixage de différentsrégimes de fonctionnement d’un hélicoptère.L’enjeu de cette thèse est de mieux prédire le comportement mécanique et la durée de vie de cesmatériaux lors d’essais de certification des moteurs d’hélicoptères présentant des endommagementsprépondérants de type fluage grâce à l’établissement d’un modèle de comportement et d’endommagement. Cemodèle doit intégrer les effets transitoires de comportement mécanique et les effets d’anisotropie ; il doit êtreprédictif en termes de durée de vie et doit être apte à modéliser de manière satisfaisante les allongementsrencontrés lors de chargements complexes.Le premier objectif a été d’étudier l’impact de l’anisotropie cristalline sur les propriétés en fluageisotherme, puis son impact en conditions de fluage anisotherme à haute température.Le second objectif fut la formulation d’une modélisation mécanique du comportement en fluage sous trajets dechargements complexes à l’aide du modèle POLYSTAR, modèle de plasticité cristalline couplécomportement/endommagement et enrichi de nouvelles variables internes représentant explicitement lesévolutions rapides de microstructure. / The creep of high pressure turbine blades is a critical issue for the certification of a helicopter turboshaftengine. Due to their excellent mechanical properties at high temperatures, nickel-based single crystalsuperalloys are widely used for the manufacturing of these blades. For these components, certificationrequirements include non-isothermal conditions based on the mixing of different operating conditions (cruise,take-off …), conditions much more damaging than standard isothermal conditions used conventionally inlaboratories.The aim of this thesis is to get a better prediction of the mechanical behavior and creep life of thesematerials during certification procedure of helicopter turboshaft engines under which creep is the main lifelimiting factor. For this, a new constitutive modeling approach has been used for the creep behavior and damageevolution. This model includes the impact of microstructure evolutions and the impact of the crystal anisotropyon the mechanical behavior and creep life.The first objective was to study the impact of crystalline anisotropy on the isothermal creep propertiesand its impact in terms of non-isothermal creep at high temperatures.The second objective was to propose a mechanical modeling of the creep behavior under complex loads pathsusing POLYSTAR model, a crystal plasticity model including a coupling between creep behavior and creepdamage and new internal variables explicitly representing rapid changes in the precipitation state.

Les matériaux cimentaires tels que le béton, le ciment et le plâtre sont largement utilisés dans la construction, les matières premières dont ils sont faits étant abondantes sur Terre. Cette tendance ne devrait pas changer dans les prochaines décennies. Mais ces matériaux subissent l'impact du fluage. Le fluage des matériaux cimentaires est une problématique complexe. D'une part, dans les matériaux cimentaires, le fluage est souvent couplé avec d'autres phénomènes tels que le séchage, l'hydratation et la fissuration, et peut être influencé par différents paramètres comme la température, le niveau de contrainte, la teneur en eau et la formulation. D'autre part, la mesure du fluage par un test macroscopique traditionnelle du fluage requiert du temps (il est recommandé de réaliser l'essai de fluage du béton sur plusieurs mois afin de donner une caractérisation fiable du fluage à long terme) et s'avère fastidieuse, puisque les paramètres expérimentaux doivent être bien contrôlés sur de longues périodes de temps. Cette thèse étudie la micro indentation à l'échelle de la pâte de ciment ou du plâtre pour évaluer les propriétés de fluage propre à long terme des matériaux cimentaires, en comparant les fonctions de fluage obtenues par des tests de micro indentation de quelques minutes avec celles obtenues par des expériences macroscopiques de fluage réalisées pendant de longues années. Pour la pâte de ciment, la comparaison a été faite à l'échelle du béton à l'aide d'une certaine homogénéisation. L'étude a validé le fait que un test de micro indentation de quelques minutes peut fournir une mesure des propriétés à long terme de matériaux cimentaires. Une fois validée la technique d'indentation, nous avons étudié l'effet de la microstructure (c'est-à-dire la distribution des phases) et celui de l'eau sur le fluage propre à long terme des matériaux cimentaires. L'effet de la microstructure a été étudiée sur des matériaux tels que des pâtes de C3S et de C2S ainsi que sur des compacts de C-S-H synthétique, de portlandite (CH) et leurs mélanges préparés par compression de poudres. Une attention particulière a été consacrée à créer des compacts avec de grandes fractions volumiques de phase cristalline. Pour tous les échantillons examinés, nous avons identifié le bon modèle micromécanique qui permette de prédire les résultats. Le choix du modèle micromécanique concorde avec les observations microstructurales. L'effet de l'humidité relative a été étudié par le conditionnement et l'indentation de certains de ces matériaux (par exemple la pâte de C3S, de compact de C-S-H et de compact de CH) dans différentes humidités relatives allant de 11% à 94%. L'humidité relative a eu un effet significatif sur le fluage : pour tous les matériaux testés, une plus grande humidité a conduit à un fluage plus important. Le compact de portlandite fut le plus sensible à l'humidité relative, sans doute parce que le fluage se produit au niveau des interfaces entre les cristaux de portlandite. Pour la pâte de C3S, une relation simple a été identifiée entre les propriétés de fluage à long terme et la teneur en eau. Enfin, nous avons proposé des modèles micromécaniques qui permettent la prédiction des propriétés de fluage à long terme de matériaux cimentaires avec une large gamme de fraction volumique de phase cristalline et sur une gamme d'humidités relatives étendue / Cementitious materials such as concrete, cement and gypsum are widely used in construction, as the raw materials of which they are made are abundant on Earth. Such trend is unlikely to change in the coming decades. But these materials suffer from creep. The creep of cementitious materials is a complex issue. On one hand, in cementitious materials creep is often coupled with other phenomena such as drying, hydration and cracking, and can be influenced by various parameters such as temperature, level of stress, water content and mix design. On the other hand, measuring creep by traditional macroscopic creep testing is time-consuming (creep test on concrete is recommended to be carried out over several months in order to provide a reliable characterization of long-term creep) and tedious, since experimental parameters need to be well controlled over extensive periods of time. This thesis studied microindentation at the scale of cement paste or gypsum plaster for the assessment of long-term basic creep properties of cementitious materials, by comparing creep functions obtained by minutes-long microindentation testing with those obtained with macroscopic creep experiments which lasted up to years. For cement paste, the comparison was made at the scale of concrete with the aid of upscaling tools. The study validated that minutes-long microindentation testing can provide a measurement of the long-term creep properties of cementitious materials. With the validated indentation technique, we studied the effect of microstructure (i.e., the distribution and the spatial organization of phases) and of water on long-term basic creep of cementitious materials. The effect of microstructure was studied on materials such as C3S pastes and C2S pastes as well as on compacts of synthetic C-S-H, portlandite (CH) and their mixtures prepared by compaction of powders. For all samples considered, we identified the right micromechanical model that allows predicting the results. The choice of micromechanical model was consistent with microstructural observations. The effect of relative humidity was studied by conditioning and testing some of those materials (i.e., C3S paste, compact of C-S-H, and compact of CH) in various relative humidities ranging from 11% to 94%. Relative humidity had a significant effect on creep: for all materials tested, a greater humidity led to a greater creep. The compact of portlandite was the most sensitive to relative humidity, probably because creep occurs at interfaces between portlandite crystals. For C3S paste, a linear relation was identified between long-term creep properties and water content at relative humidities ranging from 11% to 75%.Finally, we proposed micromechanical models that allow predicting long-term basic creep properties of cementitious materials with a wide range of volume fraction of crystalline phase and over a wide range of relative humidities

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.).

No description available.

Microstructure determines fracture toughness of materials through the activation of different fracture mechanisms. To tailor the fracture toughness through microstructure design, it is important to establish relations between microstructure and fracture toughness. To this end, systematic characterization of microstructures, explicit tracking of crack propagation process and realistic representation of deformation and fracture at different length scales are required. A cohesive finite element method (CFEM) based multiscale framework is proposed for analyzing the effect of microstructural heterogeneity, phase morphology, texture, constituent behavior and interfacial bonding strength on fracture toughness. The approach uses the J-integral to calculate the initiation/propagation fracture toughness, allowing explicit representation of realistic microstructures and fundamental fracture mechanisms. Both brittle and ductile materials can be analyzed using this framework. For two-phase Al₂O₃/TiB₂ ceramics, the propagation fracture toughness is improved through fine microstructure size scale, rounded reinforcement morphology and appropriately balanced interphase bonding strength and compliance. These microstructure characteristics can promote interface debonding and discourage particle cracking induced catastrophic failure. Based on the CFEM results, a semi-empirical model is developed to establish a quantitative relation between the propagation toughness and statistical measures of microstructure, fracture mechanisms, constituent and interfacial properties. The analytical model provides deeper insights into the fracture process as it quantitatively predicts the proportion of each fracture mechanism in the heterogeneous microstructure. Based on the study on brittle materials, the semi-analytical model is extended to ductile materials such as AZ31 Mg alloy and Ti-6Al-4V alloy. The fracture resistance in these materials not only depends on the crack surfaces formed during the failure process, but also largely determined by the bulk plastic energy dissipation. The CFEM simulation permits surface energy release rate to be quantified through explicit tracking of crack propagation in the microstructure. The plastic energy dissipation rate is evaluated as the difference between the predicted J value and the surface energy release rate. This method allows competition between material deformation and fracture as well as competition between transgranular and intergranular fracture to be quantified. The methodology developed in this thesis is potentially useful for both the selection of materials and tailoring of microstructure to improve fracture resistance.

Finite element simulation of metal cutting processes offers a cost-effective method to optimize the cutting conditions and to select the right tool material and geometry. A key input to such simulations is a constitutive model that describes material behavior during severe plastic deformation. However, the vast majority of material models used in prior work are phenomenological in nature and are usually obtained by fitting a non-physically based mathematical equation to the macro-scale stress-strain response of the material. Moreover, the deformation range covered by the stress-strain response used in the model calibration process usually falls short of the ranges typically observed in metal cutting. This thesis seeks to develop a unified material model that explicitly incorporates microstructure evolution into the constitutive law to describe the macro-scale plastic deformation response of the material valid over the range of strains, strain rates and temperatures experienced in machining. The proposed unified model is based on the underlying physics of interactions of mobile dislocations with different short and long range barriers and accounts for various physical mechanisms such as dynamic recovery and dynamic recrystallization. In addition, the inclusion of microstructure evolution into the constitutive model enables the prediction of microstructure in the chip and the machined surface. In this study, the unified material model is calibrated and validated in the severe plastic deformation regime characteristic of metal machining and is then implemented in finite element simulations to evaluate its ability to predict continuous and segmented chip formation in machining of pure metals such as OHFC copper and commercially-pure titanium (CP-Ti). Due to the physical basis of the proposed unified material model, the continuous chip formation observed in orthogonal cutting of OFHC copper is shown to be successfully predicted by the finite element model utilizing a version of the unified material model that explicitly accounts for microstructure evolution as well as dislocation drag as a plausible deformation mechanism applicable at the high strain rates common in metal cutting operations. The segmented or shear localized chip formation in orthogonal cutting of CP-Ti is also shown to be successfully simulated by the unified model after incorporating the inverse Hall-Petch effect arising from the ultrafine grain structure within the shear band. For both metals, the model is experimentally validated using flow stress data as well as machining data including cutting and thrust forces and relevant chip morphology parameters. Machining simulations carried out using the unified material model also yield useful insights into the microstructure evolution during the machining process, which is shown to be consistent with the available experimental data and the known physical understanding of severe plastic deformation behavior of the metals.

published_or_final_version / abstract / Mechanical Engineering / Doctoral / Doctor of Philosophy

The relationship between some mechanical properties (tensile, creep and thermal fatigue behaviour) and microstructure has been investigated for lead sheet containing <0.06 wt % Cu (the eutectic composition), made either by a conventional milling (rolling) process, or by continuous casting (Direct Method). Milled lead sheet exhibits a recrystallised microstructure, the grain size of which decreases with increasing copper content; copper is present as particles of ≃1-5 μm long, formed from the initial needles or plates of the cast billet by spheroidisation, or by particle break-up during rolling. A comparison of materials produced by several manufacturers indicates that the copper distribution and final grain size are dependent on the thermo-mechanical history of the sheet. Direct Method (DM) sheet exhibits a cast cellular structure within grains which usually extend through the full thickness of the cast sheet; copper is present as a fine dispersion (particles ≃0.5 μm diameter) at cell boundaries. The copper distribution in both materials is stable to prolonged heating (100 hours at 200<SUP>o</SUP>C), but some grain growth occurred in the milled sheet. It was not possible to obtain a fully dispersed eutectic microstructure at the eutectic composition; primary lead dendrites (or cells) were always present. This is thought to be due to the difficulty of nucleating a copper particle at the very low copper concentrations used in this work. The tensile behaviour of specimens was investigated at various strain rates and temperatures. DM sheet exhibits an increase in UTS both with increasing copper content at each strain rate, and with increasing strain rate for each copper content; no systematic variation of strain with copper content was observed. The UTS of milled lead sheet (at ambient temperature and slow strain rates) was a maximum at 0.02 - 0.03 % copper. A steady increase in UTS with increasing copper content was obtained at higher strain rates (2.67 min<SUP>-1</SUP> and above) and low temperatures (≃200 K) and indicated that a time and temperature dependent softening process is active at ambient temperature and slower strain rates (up to 1.33 min^-1) which is thought to be grain boundary sliding, although no evidence for this has been detected in the recrystallised microstructure. Values of the work-hardening coefficient (n) and the strain-rate sensitivity (m) were determined for milled and DM sheet, and were found in all cases to be high. DM sheet exhibits good creep resistance, which increases with increasing copper content, owing to the large grains and stable grain boundaries at ≃90' to the direction of stress. Milled lead sheet was less creep resistant, exhibiting maximum creep resistance at 0.03% copper; this is thought to be due to competing processes of strengthening by copper (which pins grain boundaries to some extent) and grain boundary sliding, which increases with the corresponding decrease in grain size. Thermal fatigue tests have been conducted externally, using lead flashing lengths fixed to an outside wall, and in the laboratory using specially developed apparatus. The development of the apparatus, specimen shape and test cycle length is described. Cracking is usually multiple and intergranular; examination of the fracture surface indicates a combination of intergranular fatigue, creep processes (cavitation) and regions of ductile failure. The use of acetate replicas to trace crack growth has shown the migration of grain boundaries to directions of high stress in milled lead, and in 0.01% copper DM sheet. Grain boundary migration was not observed in the more stable DM microstructures of higher copper content.