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Multiple studies on the microstructures of advanced high-strength steels are presented here that seek to add to the already substantial body of knowledge on martensite in steel. These studies seek to gain additional insight into the role that the martensite transformation has on the observed mechanical properties of modern steels. Crystallographic Reconstruction of Parent Austenite Twin Boundaries in a Lath Martensitic Steel The study of post-transformation microstructures and their properties can be greatly enhanced by studying their dependence on the grain boundary content of parent microstructures. Recent work has extended the crystallographic reconstruction of parent austenite in steels to include the reconstruction of special boundaries, such as annealing twins. These reconstructions present unique challenges, as twinned austenite grains share a subset of possible daughter variant orientations. This gives rise to regions of ambiguity in a reconstruction. A technique for the reconstruction of twin boundaries is presented here that is capable of reconstructing 60 degree twins, even in the case where twin regions are comprised entirely of variants that are common between the twin and the parent. This technique is demonstrated in the reconstruction of lath martensitic steels. The reconstruction method utilizes a delayed decision-making approach, where a chosen orientation relationship is used to define all possible groupings of daughter grains into possible parents before divisive decisions are made. These overlapping, inclusive groupings (called clusters) are compared to each other individually using their calculated parent austenite orientations and the topographical nature of the overlapping region. These comparisons are used to uncover possible locations of twin boundaries present in the parent austenite. This technique can be applied to future studies on the dependence of post-transformation microstructures on the special grain boundary content of parent microstructures. Coupling Kinetic Monte Carlo and Implicit Finite Element Methods for Predicting the Strain Path Sensitivity of the Mechanically Induced Martensite Transformation The kinetic Monte Carlo method is coupled with a finite-element solver to simulate the nucleation of martensite inside the retained austenite regions of a TRIP (transformation induced plasticity) assisted steel. Nucleation kinetics are expressed as a function of load path and kinematic coupling between retained austenite regions. The model for martensite nucleation incorporates known elements of the kinetics and crystallography of martensite. The dependence of martensite transformation on load path is simulated and compared to published experimental results. The differences in transformation rates of retained austenite are shown to depend on load path through the Magee effect. The effects of average nearest neighbor distance between austenite grains is shown to affect the rate at which martensite nucleates differently depending on load path. Ductility and Strain Localization of Advanced High-Strength Steel in the Presence of a Sheared Edge The localization of strain in the microstructures of DP 980 and TBF 980 is quantified and compared. Of particular interest is the difference in final elongation observed for both materials in the presence of a sheared edge. Scanning electron micrographs of etched microstructures near the sheared edge are gathered for both materials at varying amounts of macroscopic strain. These micrographs are used to generate strain maps using digital image correlation. A two point statistical measure for strain localization is developed that utilizes strain map data to quantify the degree to which strain localizes around the hard phase of both materials. The DP steel exhibits higher strain localization around the martensite phase. Reasons for differences in strain localization and shear banding between the two materials are suggested, and the role played by the mechanically induced martensite transformation is speculated.

Cette thèse est dédiée à la caractérisation des vents descendants de vallée en terrain complexe d'orographie modérée à moyenne latitude, dans le contexte de la réglementation des rejets atmosphériques de Cadarache. Cadarache est un des centres de recherche du "Commissariat à l'énergie atomique et aux énergies alternatives" (CEA), installé dans une petite vallée (CV) confluente à la vallée de la Durance (DV). Ces deux vallées se distinguent par leur taille, et sont le siège d'écoulements aux caractéristiques différentes en stratification stable. Un forçage synoptique faible associé à un ciel dégagé sont dans la région des conditions fréquentes qui favorisent la stabilité atmosphérique et consécutivement la mauvaise dispersion des polluants, faisant de cette situation un sujet d'intérêt majeur. La campagne de mesure KASCADE (KAtabatic winds and Stability over CAdarache for Dispersion of Effluents) constitue le volet expérimental de l'étude. Réalisée pendant l'hiver 2013 elle a couvert 3 mois d'observation continue et complétée de 23 périodes d'observation intensive (POI). L'analyse montre que les écoulements descendant les vallées de Cadarache (CDV) et de la Durance (DDV) dominent pendant toute la période d'étude. La stabilité s'installant dès le coucher du soleil, le courant CDV s'épaissit progressivement. Le profil de vent en forme de jet présente son maximum à environ 30 m où il atteint 2 à 3 m s-1. Il se maintient toute la nuit et disparaît avec l'inversion de stabilité. Comme la station météorologique du centre manque de capteur de vent dans la CV même, une méthode a été développée pour diagnostiquer le CDV en exploitant l'instrumentation actuelle. Ainsi, si la prévision de ce vent n'est pas à la portée du modèle méso-échelle WRF avec une résolution kilométrique, cette méthode le permet en combinant une descente d'échelle dynamique et statistique. Le vent DDV est identifié comme un vent qui suit l'axe de la vallée, fortement corrélé à la stabilité à l'échelle régionale car il n'apparaît que la nuit lorsque le forçage synoptique est faible. Ce vent n'arrive à Cadarache que 6 à 9 heures après le coucher du soleil avec une grande variabilité. D'un autre côté, il est à son maximum au lever du soleil avant que les processus convectifs ne démarrent, et présente un jet autour de 200 m avec des vitesses de 4 à 8 m s-1 et dont la hauteur est corrélée à la profondeur de la vallée. Dans les simulations avec WRF, malgré des défauts, la DV étant bien résolue avec une maille de 1 km, l'occurrence de ce vent est assez bien simulée. Par ailleurs l'examen de ses caractéristiques spatiales montre qu'il s'agit soit d'un écoulement de drainage, soit d'un écoulement canalisé forcé. Bien qu'on ne dispose pas de données suffisantes pour élucider le mécanisme dominant de déclenchement du vent DDV, les deux précédemment identifiés sont de bons candidats. / Stable stratification can be one of the most penalizing condition concerning pollutants in the atmospheric boundary layer. Over complex terrain under these conditions, the relief may modify the flow. Therefore the knowledge of down-valley wind characteristics influencing the wind field at Cadarache and its close surroundings is crucial for safety regulation in the context of sanitary impact of the site. Cadarache is a CEA research centre and located in the Prealps of southeast France. It is embedded in a small valley, the Cadarache Valley (CV), which is one of the tributaries of the larger Durance Valley (DV). The two valleys are distinct in size and therefore react differently to stable conditions, and are investigated by means of observations (field experiment KASCADE : KAtabatic winds and Stability over CADarache for Dispersion of Effluents) and simulations (the Weather Research and Forecasting (WRF) model). To investigate the valley wind behaviour, the KASCADE campaign has been designed and conducted in the winter of 2013, covering a 3-month period and 23 intensive observation periods (IOP). It resulted in a well-documented campaign, from which the analysis shows that the Cadarache and Durance down-valley (CDV and DDV respectively) winds are both dominant flows during the period of investigation. The CDV wind is a thermally driven flow, with regular wind speeds up to 2 - 3 m s-1 up to 50 m agl. It persists throughout the night and disappears in the early morning with the stability. The current observational network of Cadarache lacks means of measurement for inside CDV wind. This work shows that it can be nowcasted from available meteorological tower observations. Due to the CV small scale, currently a wind forecast on kilometer resolution is out of reach, but the methodology developed here can be used to forecast the wind through a combination of dynamical and statistical downscaling. The DDV wind has been recognized as down-valley oriented, and strongly related to stability at a regional scale, as it exists only after sunset when synoptic forcing is very weak. DDV wind arrival at Cadarache is mostly observed 6 to 9 hours after sunset, but however dominantly present around sunrise, when convectively driven processes are not yet established. Jets are observed mostly at around 200 m agl with wind speeds between 4 and 8 m s-1. Despite some (general) deficiencies of the WRF model, the DDV wind is simulated close to reality thanks to the 1-km resolution allowing a correct representation of the Durance valley orography. The ensemble of 23 simulated IOPs allowed further to characterize the flow in a spatial sense and to recognize drainage and flow channelling as most important candidates for the flow mechanism.