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L’acier Ferrium® M54® présente une composition chimique optimisée, basée sur 40 ans d’évolution et de développement des aciers à durcissement secondaire à précipitation de carbures M2C. Le compromis de propriétés Rm/KIC/KISCC obtenu par la nuance M54® permet d’envisager son utilisation dans les trains d’atterrissage d’avions gros porteurs. Cependant, les premiers essais mécaniques, réalisés par l’utilisateur pour la montée en maturité de la nuance, ont montré une variabilité des propriétés mécaniques suivant le traitement thermique appliqué. Ce travail de thèse s’applique donc à décrire les évolutions microstructurales au cours du traitement thermique de la nuance M54® et les impacts sur les propriétés mécaniques en se concentrant notamment sur le traitement par le froid. Les différentes conditions de mise en solution et de revenu testées ont montré une certaine stabilité de la précipitation au revenu et des propriétés mécaniques qui en découlent. La précipitation a été caractérisée à différentes échelles afin de la comparer avec celle issue des nuances de la même famille. En revanche, selon les conditions de traitement par le froid réalisées, la limite d’élasticité varie de manière significative sans qu’aucun des paramètres liés à la précipitation ne soient modifiés. Le taux d’austénite est en revanche un paramètre déterminant pour la limite d’élasticité et est très sensible aux conditions de traitement par le froid : temps et température entre la trempe à l’huile et le traitement cryogénique et température de traitement cryogénique. Un traitement thermique amélioré a ainsi été proposé pour obtenir un taux d’austénite réduit et constant et limiter ainsi les variations de limite d’élasticité. / Ferrium® M54® steel presents an optimized composition, based on 40 years of research and development on secondary hardening steels. This alloy exhibits an excellent Rm/KIC/KISCC balance that allows considering its use in landing gears applications of wide-body aircrafts in the future. However, initial mechanical tests performed by the end-user have shown variability in mechanical properties depending on the applied heat treatment. The main goal of this work is to describe the microstructural evolutions of the alloy M54® during heat treatment and their impact on the resulting mechanical properties with a specific focus on the effect of the cryogenic treatment.The different austenitizing and tempering conditions investigated have shown a stability of the tempering precipitation and mechanical properties. This precipitation has been characterized at different scales and compared with other grades of the same family. On the other hand, depending on cryogenic treatment conditions, a significant variation of the mechanical properties and in particular of the yield strength is observed without any modification in the precipitation distribution and volume fraction or size. Austenite content is critical for the yield strength and is very sensitive to the cryogenic treatment conditions: time and temperature before cryogenic treatment and temperature of cryogenic treatment. An improved heat treatment to obtain reduced and constant austenite content is proposed.

Ce manuscrit de thèse présente les résultats de travaux concernant les effets des transitoires thermiques sur la précipitation des phases intermétalliques dans le superalliage base nickel Inconel 718® et les conséquences de cette précipitation sur les propriétés mécaniques de cet alliage. Ces travaux se concentrent sur une approche couplée capable de décrire l’évolution de la précipitation de la phase γ'' et ses conséquences sur la limite d’élasticité de l’Inconel 718®. L’état de précipitation est modélisé grâce à l’implémentation des équations de la théorie classique de germination et de croissance, modifiées afin de prendre en compte la géométrie en plaquette des précipités γ''. Ce modèle de précipitation est ensuite validé par des données expérimentales issues de la littérature mais également d’observations au microscope électronique en transmission et d’essais de diffusion de neutrons aux petits angles. La distribution de taille de la phase γ'' ainsi simulée est utilisée comme donnée d’entrée d’un modèle de prédiction de la limite d’élasticité précipitation-dépendant. Ces limites d’élasticité simulées sont enfin comparées à des résultats expérimentaux issus d’essais de traction. Une maquette de soudage a ensuite été réalisée afin de tester le modèle de prévision de limite d’élasticité pour des transitoires thermiques, dans un cas industriel. / This thesis manuscript presents the results of work concerning the effects of thermal transients on the precipitation of intermetallic phases in the nickel-based superalloy Inconel 718® and the consequences of this precipitation on the mechanical properties of this alloy. This work focuses on a coupled approach capable of describing the evolution of the precipitation of the γ'' phase and its consequences on the yield strength of the Inconel 718®. The precipitation state is modeled thanks to the implementation of the equations of the classical germination and growth theory, modified to take into account the platelet geometry of the γ'' precipitates. This precipitation model is then validated by experimental data from the literature but also from transmission electron microscope observations and small angle neutron scattering tests. The size distribution of the γ'' phase thus simulated is used as the input data of a model for predicting the precipitation-dependent elasticity limit. These simulated elasticity limits are finally compared with experimental results from tensile tests. A welding model was then made to test the elastic yield prediction model for thermal transients in an industrial case.

19 Dec 2004. / Published through the Information Bridge: DOE Scientific and Technical Information. "IS-T 1983" Fei Tang. 12/19/2004. Report is also available in paper and microfiche from NTIS.

Les alliages conducteurs à haute tenue mécanique sont utilisés dans de nombreux domaines tels que le transport, l’énergie et l’industrie électronique. La demande en ces matériaux est croissante. Les alliages Cu-Be constituent à l’heure actuelle les matériaux conducteurs les plus résistants mais posent un problème de toxicité et de coût. Pour proposer une alternative à ces derniers, nous avons étudié des alliages Cu-Mg hypo-eutectiques. Deux voies de synthèse ont été utilisées afin d’obtenir des composites endogènes possédant une combinaison originale de propriétés structurelles et fonctionnelles : le refroidissement conventionnel et la solidification rapide par melt-spinning. Les microstructures obtenues ont en commun la présence d’agrégats eutectiques et diffèrent principalement par la dimension des paramètres métallurgiques tels que la taille de grains et la distance interlamellaire. Les systèmes présentent des forces motrices de transformation de phases importantes conduisant à une précipitation au cours d’un maintien thermique. L’investigation des propriétés mécaniques et électriques des alliages Cu-Mg montre que ces dernières égalent celles des alliages Cu-Be. En complément de l’approche expérimentale, nous avons adapté deux modèles : un modèle thermocinétique pour décrire l’évolution temporelle de la microstructure qui a lieu à haute température et un modèle mécanique permettant de rationaliser l'influence des paramètres microstructuraux sur les propriétés mécaniques et électriques. / Copper-based high strength conductive alloys are used in a myriad of applications such as transport, energy and electronic industries. The demand for these materials is booming. The strongest conductive alloy is based on the Cu-Be system which has the disadvantage of toxicity and cost. In an attempt to find an alternative to these, we have developed hypoeutectic Cu-Mg alloys. Two synthesis routes were applied to produce endogenous compounds with a unique combination of structural and functional properties: the conventional cooling and the rapid solidification by melt-spinning. The obtained microstructures share the presence of eutectic aggregates and differ mainly by the size of the metallurgical parameters such as grain size and interlamellar distance. The two systems provide high driving forces for phase transformations leading to precipitation during aging. The investigation of the mechanical and electrical properties of Cu-Mg alloys shows that they are equal to those of the Cu-Be alloys. In addition to experimental approach, we have adapted two models: a thermo-kinetic one to describe time evolution of the microstructure which takes place at high temperature and a mechanical one to rationalize the influence of microstructural parameters on mechanical and electrical properties.

Les propriétés magnétiques et mécaniques des alliages Fe-49%Co-2%V et Fe-27%Co, utilisés respectivement dans les générateurs et les transformateurs électriques de l’aéronef, ont été étudiées en fonction des traitements thermiques. Concernant le Fe-49%Co-2%V, les résultats expérimentaux ont montré que les propriétés magnétiques et mécaniques dépendent de la distance entre les interfaces qui séparent les cristallites présents dans la microstructure et ceci quel que soit le taux de recristallisation. Le champ coercitif dépend linéairement de l’inverse de la distance entre ces interfaces et la limite d’élasticité suit une loi de Hall et Petch modifiée pour tenir compte des sous-joints. De plus, une relation linéaire entre la contrainte de friction de la loi de Hall et Petch avec le taux d’ordre à grande distance mesuré par diffraction de neutrons a été établie. Finalement, en s’appuyant sur l’expérience, une loi polynomiale de degré deux reliant les pertes magnétiques et la limite d’élasticité a été proposée. Concernant l’alliage Fe-27%Co, un suivi de l’évolution de la composante de Goss {011}<100> pendant le processus de fabrication a été réalisé. Cette composante est recherchée car favorable pour les propriétés magnétiques. Il a été trouvé que l’origine de cette texture est intimement liée au cisaillement développé lors du laminage à chaud. De plus, le développement de la texture {111}<112> de déformation à froid a été étudié. Le système de glissement {110}<111>, majoritairement activé, a été mis en évidence via un couplage d’analyses EBSD et de simulations VPSC. Au cours de la déformation plastique, les composantes de Goss et Cube tournée ({001}<110>) tournent afin de positionner leur plan {110} pour le glissement facile. Ces rotations, respectivement de 35° et 55° autour de la direction transverse, permettent de former la composante {111}<112>. / Magnetic and mechanical properties of Fe-49%Co-2%V and Fe-27%Co alloys, used in electric generators and transformers of the aircraft, have been investigated as a function of heat treatments. As regards to Fe-49%Co-2%V alloy, experimental results have shown that the magnetic and mechanical properties depend on the boundary spacing, which separate the microstructure crystallites, whatever the recrystallized fraction. The coercivity depends linearly on the inverse of the boundary spacing and the yield strength follows a modified Hall Petch relationship by taking into account the sub-grain size. Moreover, a linear relationship between the frictional stress and the long range order degree measured by neutron diffraction has been established. Finally, based on the experiment, a square polynomial degree relationship between the magnetic losses and the yield stress has been proposed. As respect to the Fe-27%Co alloy, the evolution of the {011}<100> Goss component during the manufacturing process has been investigated. This sought component is favorable for the magnetic properties of the sheet. It has been found that the origin of this texture is related to its development by shearing during hot rolling. Moreover, the development of the {111}<112> cold deformation texture was studied. It has been established by coupling EBSD analysis and simulation based on VPSC model that the {110}<111> system is the predominant active slip system during the plastic deformation. During the deformation, the Goss and the rotated Cube ({001}<110>) components, rotate of 35° and 55° respectively with regard to the transverse direction. That happens in order to place the active slip plane {110} in the favorable position for gliding to develop the {111}<112> component.

In the transportation industry, weight reduction is essential in order to reduce fuel consumption. A solution towards lighter structures is to improve the mechanical properties while keeping a sufficient ductility for the forming operations. The aim of the thesis was to investigate the enhancement of the mechanical properties of metallic alloys by the refinement of the grain size while playing with other microstructural features in order to maintain the strain hardening, and thus the ductility, as high as possible. Various fine grained dual phase steels were produced by severe plastic deformation followed by thermal treatment. Nano-indentation and tensile tests have been performed to measure the change of flow properties associated to the grain refinement. A new methodology, based on performing nano-indentation tests at different depth inside each phases of the steels, has been proposed in order to separate the different hardening contributions affecting the behaviour of the material. In order to gain a better understanding of the link between the fine grained microstructure and the flow properties, three models were developed introducing successively a richer and richer description of the microstructure. The first model allows interpreting the nano-indentation data at different depths. The second model predicts the flow curve of dual phase steels by considering the accumulation of dislocations on the grain boundaries with the associated back stress and the saturation of this accumulation of excess dislocations, while introducing the second phase particles through a homogenization scheme. Finally, the third model promotes a new explanation of the Hall-Petch law and the interaction of the grain size and the texture for pure copper using a multigrain crystal plasticity model incorporating grain boundary effects. Guidelines are given to optimize the microstructures towards an improvement of the structural properties and formability.

For the automotive sector, plastics play the most important role when designing interior and exterior parts for cars. Currently, most parts are made from petroleum-based plastics but alternatives are needed to replace environmentally harmful materials while providing the appropriate mechanical performance and preferably reduce the cost for the final product. The objective of this work was to explore the use of soy flakes as natural filler in a composite with polypropylene and to investigate the mechanical properties, water absorption and thermal behaviour. For a better understanding of the filler, the soy flakes were characterized extensively with analytical and microscopic methods. Two types of soy fillers were investigated, soy flakes, provided by Bunge Inc., with a 48 wt-% protein content and an industrial soy based filler with 44 wt-% protein content and provided by Ford. The size of the soy flakes after milling was mainly between 50 and 200 µm and below 50 µm for the industrial filler. The aspect ratio for all filler was below 5. The soy flakes were used after milling and subjected to two pre-treatment methods: (1) one hour in a 50 °C pH 9 water solution in a 1 : 9 solid-liquid ratio; (2) one hour in a 50 °C pH 9 1M NaCl solution in a 1 : 9 solid-liquid ratio. A control filler, without pre-treatment was considered. The soy flakes were also compared to an industrial soy based filler provided by Ford (soy flour (Ford)). The thermogravimetric analysis showed an onset of degradation at 170 °C for the treated filler (ISH2O and ISNaCl) and 160 °C for the untreated filler. The biocomposites formulation consisted of 30 wt-% filler, and polypropylene with/without 0.35 wt-% anti-oxidant Irganox 1010 and with/without the addition of MA-PP as coupling agent. All biocomposites were compounded in a mini-extruder, pressed into bars by injection moulding and tested subsequently. The mechanical properties of the biocomposites are promising. An increase of the E-modulus was observed when compared to pure polypropylene. The addition of MA-PP as coupling agent increased the yield strength of the biocomposites. When pure polypropylene and the biocomposites were compared no difference could be seen for their yield strength. The thermal behaviour deduced from differential scanning calorimetry, revealed a similar behaviour for the biocomposites and the pure polypropylene. Only the samples treated in the presence of NaCl and without a coupling agent, appear to have a slightly higher degree of crystallinity. The melt flow index was slightly increased for the biocomposites containing soy flakes pre-treated with NaCl and decreased for biocomposites containing the soy flour. The water absorption behaviour of the biocomposites was quite similar at the beginning with a slightly lower absorption for the materials with coupling agent. After three months, all samples except the ones treated with water showed a weight loss that can be due to the leaching of the water soluble components in the untreated filler and the NaCl treated filler. In conclusion, soy flakes represent an attractive filler when used in a polypropylene matrix if an aqueous alkaline pre-treatment is performed. The aqueous alkaline extraction also leads to the recovery of the proteins that can be used in food products while the remaining insoluble material is used for the biocomposites, avoiding the competition with the use of soy for food products...

For the automotive sector, plastics play the most important role when designing interior and exterior parts for cars. Currently, most parts are made from petroleum-based plastics but alternatives are needed to replace environmentally harmful materials while providing the appropriate mechanical performance and preferably reduce the cost for the final product. The objective of this work was to explore the use of soy flakes as natural filler in a composite with polypropylene and to investigate the mechanical properties, water absorption and thermal behaviour. For a better understanding of the filler, the soy flakes were characterized extensively with analytical and microscopic methods. Two types of soy fillers were investigated, soy flakes, provided by Bunge Inc., with a 48 wt-% protein content and an industrial soy based filler with 44 wt-% protein content and provided by Ford. The size of the soy flakes after milling was mainly between 50 and 200 µm and below 50 µm for the industrial filler. The aspect ratio for all filler was below 5. The soy flakes were used after milling and subjected to two pre-treatment methods: (1) one hour in a 50 °C pH 9 water solution in a 1 : 9 solid-liquid ratio; (2) one hour in a 50 °C pH 9 1M NaCl solution in a 1 : 9 solid-liquid ratio. A control filler, without pre-treatment was considered. The soy flakes were also compared to an industrial soy based filler provided by Ford (soy flour (Ford)). The thermogravimetric analysis showed an onset of degradation at 170 °C for the treated filler (ISH2O and ISNaCl) and 160 °C for the untreated filler. The biocomposites formulation consisted of 30 wt-% filler, and polypropylene with/without 0.35 wt-% anti-oxidant Irganox 1010 and with/without the addition of MA-PP as coupling agent. All biocomposites were compounded in a mini-extruder, pressed into bars by injection moulding and tested subsequently. The mechanical properties of the biocomposites are promising. An increase of the E-modulus was observed when compared to pure polypropylene. The addition of MA-PP as coupling agent increased the yield strength of the biocomposites. When pure polypropylene and the biocomposites were compared no difference could be seen for their yield strength. The thermal behaviour deduced from differential scanning calorimetry, revealed a similar behaviour for the biocomposites and the pure polypropylene. Only the samples treated in the presence of NaCl and without a coupling agent, appear to have a slightly higher degree of crystallinity. The melt flow index was slightly increased for the biocomposites containing soy flakes pre-treated with NaCl and decreased for biocomposites containing the soy flour. The water absorption behaviour of the biocomposites was quite similar at the beginning with a slightly lower absorption for the materials with coupling agent. After three months, all samples except the ones treated with water showed a weight loss that can be due to the leaching of the water soluble components in the untreated filler and the NaCl treated filler. In conclusion, soy flakes represent an attractive filler when used in a polypropylene matrix if an aqueous alkaline pre-treatment is performed. The aqueous alkaline extraction also leads to the recovery of the proteins that can be used in food products while the remaining insoluble material is used for the biocomposites, avoiding the competition with the use of soy for food products...