Les propriétés mécaniques des alliages de fer sont largement conditionnées par leur microstructure et la population de défauts locaux qu’elle contient. Comme il s'agit d'un moment de forte interaction entre ces deux éléments, l'étude des transformations α-γ et γ-α du fer mérite une attention particulière. Outre le passage d'une structure cristalline cubique centrée à une structure cubique faces centrées – donc de compacités différentes –,elles sont associées à une réduction des paramètres de maille. Le changement de volume correspondant est responsable de déformations mécaniques locales autour des sites de germination. La technique de corrélation d'images numériques(CIN) s'est montrée fiable pour ce qui est de calculer des champs cinématiques à l'échelle de quelques grains. Dans ce travail, elle est adaptée à la capture de localisations de la déformation pendant les transformations allotropiques. Un banc expérimental est conçu pour obtenir des images haute résolution avec un contrôle fin des sollicitations thermiques. Des essais de validation sont d'abord effectués sur du fer industriel. Puis des échantillons de fer haute pureté sont ensuite soumis à des cycles de transformation α-γ-α et les champs de déformation correspondant sont calculés par CIN. Associés à l'acquisition des orientations initiales et finales, ils sont utilisés pour valider les mécanismes de transformation proposés dans la littérature. En parallèle, un modèle, écrit en petites déformations, est construit en incorporant des composants liés à la transformation dans une fonctionnelle dont les conditions de stationnarité sont équivalentes au problème thermomécanique à résoudre. Les incréments des variables internes, incluant glissements plastiques et fractions volumique transformées, sont obtenus en minimisant cette fonctionnelle. / Mechanical properties of iron-based alloys are largely conditioned by their microstructure and the population of local defects inside this microstructure. As it is a moment of massive interplay between these two elements, the study of the α-γ and γ-α transformations in iron is of particular interest. Besides a change from a body-centered cubic to a face-centered cubic crystal structure – and thus a change in compacity –, they lead to a lattice parameter reduction. The corresponding change in volume is responsible for local mechanical deformations around transformation sites. Digital Image Correlation (DIC) technique has been proven reliable to compute kinematic fields at the scale of a few grains. In the present work, an adaptation of this technique to the observation of strain localizations induced by the formation of a new phase is proposed. A home-made device is designed to obtain high resolution images and to control heating and cooling. Tests are first conducted on industrial iron to assess the viability of the procedure. High-purity iron samples are then submitted to α-γ-α transformation cycles and the associated strain fields are computed. In combination with the acquisition of initial and final orientations they are used to validate transformation mechanisms proposed in the literature. In parallel, a model, written under the small strain format, is built by incorporating transformation related components into a power functional whose stationarity conditions are equivalent to the thermomechanical problem. In accordance with variational principles, the evolution of internal variables,including plastic slip increments and fraction of the material locally transformed, are computed through the minimization of the functional.

Diffusion plays a significant role in most materials systems by controlling microstructural development. Consequently, the overall properties of a material can be largely dependent upon diffusion. This study investigated the interdiffusion behavior of three binary systems, namely, Mo-Zr, Fe-Mo, and Fe-Zr. The main interest in these particular metals is for application in nuclear fuel assemblies. Nuclear fuel plates generally consist of two main components which are the fuel and the cladding. Due to diffusional interactions that can occur between these two components, a third is sometimes added between the fuel and cladding to serve as a diffusion barrier layer. Fe, Mo, and Zr can act as either cladding or barrier layer constituents and both Mo and Zr also serve as alloying additions in uranium based metallic fuels. Therefore, a fundamental understanding of the diffusional interactions in these systems is critical in predicting the performance and lifetime of these fuels. In order to study this diffusion behavior, a series of solid-to-solid diffusion couples were assembled between Fe, Mo, and Zr. These couples were then diffusion annealed isothermally for various predetermined times over a range of temperatures, including some both above and below the allotropic transformation temperatures for Fe and Zr. Following the diffusion anneal, the couples were water quenched, cross-sectioned, and prepared for microstructural and compositional characterization. A combination of scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and electron probe microanalysis (EPMA) were used to obtain micrographs showing the microstructure and to collect compositional data for identifying intermediate phases and determining concentration profiles across the interdiffusion zone. Based on this characterization, the phases that developed in the diffusion zones were identified. In the Mo-Zr system, a large Zr solid solution layer developed in the couples annealed iv at and above 850C and a thin (~1-2 m) layer of Mo2Zr formed in all couples. Growth constants and concentration dependent interdiffusion coefficients were calculated for the Mo2Zr and Zr solid solution phases, respectively. In the Fe-Mo system, both the -Fe2Mo and -Fe7Mo6 phases were observed in couples annealed at 900C and below while -Fe7Mo6 and -Fe solid solution layers were observed in couples annealed above 900C. The relevant growth constants and activation energies for growth were calculated. In the Fe-Zr system, the couple annealed at 750C developed an FeZr2 and an FeZr3 layer while the couple annealed at 850C developed an Fe2Zr and Fe23Zr6 layer in the diffusion zone. The results of this analysis were then compared to available information from literature and the corresponding binary phase diagrams for each system. The results are discussed with respect to the effects of the allotropic transformations of Fe and Zr on the interdiffusion behavior in these systems.