Magnesium alloys are the lightweight structural materials with high strength to weigh ratio that permits their application in fuel economy sensitive automobile industries. Among the several flavors of of Mg-alloys, precipitation hardenable Mg-rare earth (RE) based alloys have shown good potential due to their favorable creep resistance within a wide window of operating temperatures ranging from 150°C to 300°C. A key aspect of Mg-RE alloys is the presence of precipitate phases that leads to strengthening of such alloys. Several notable works, in literature, have been done to examine the formation of such precipitate phases. However, there are very few studies that evaluated the effect stress induced deformation on the precipitation in Mg-RE alloys. Therefore, the objective of this work is to examine influence of deformation on the precipitation of Mg-Nd based alloys. To address this problem, precipitation in two Mg-Nd based alloys, subjected to two different deformation conditions, and was examined via transmission electron microscopy (TEM) and atom probe tomography (APT). In first deformation condition, Md-2.6wt%Nd alloy was subjected to creep deformation (90MPa / 177ºC) to failure. Effect of stress-induced deformation was examined by comparing and contrasting with precipitation in non-creep tested specimens subjected to isothermal annealing (at 177ºC). In second condition, Mg-4.0Y-3.0Nd-0.5Zr (wt %) or WE43 alloy (with comparable Nd content as model Mg-Nd system) was subjected to hot rolling deformation at a sub-solvus temperature.

Les raccords et connexions de caténaires ont un rôle important dans l’intégrité électrique et mécanique des caténaires. Ces pièces sont pour la plupart constituées d’un alliage de cuivre Cu-Ni-Si mis en forme par forgeage à chaud. Dans un contexte de fiabilisation du réseau ferroviaire, l’amélioration de leurs performances et de leurs procédés de fabrication est recherchée. Ces travaux se concentrent sur l’étape de mise en forme de ces pièces, et ont pour objectif de mieux comprendre les couplages matériaux-procédés des alliages Cu-Ni-Si.Une campagne expérimentale de compression est menée. Elle permet de développer un modèle rhéologique à base physique décrivant le comportement d’un alliage Cu-Ni-Si dans les gammes de température 800 - 950 °C et de vitesse de déformation 0.1 - 10 s−1. Il tient compte des phénomènes d’adoucissement par restauration et recristallisation dynamiques. Le modèle ainsi créé est ensuite implémenté dans un logiciel éléments finis.Un nouvel essai de formabilité est proposé. Il permet de reproduire les sollicitations mécaniques rencontrées lors d’un procédé de forgeage et vise à déterminer les conditions limites de formabilité d’un matériau. La mise en forme d’une pièce de référence est étudiée par le biais de cet essai. Sa caractérisation précise en matière de champ de déformations et de contraintes permet d’identifier les paramètres expérimentaux. Les résultats identifient une température limite de forgeage en-deçà de laquelle des fissures sont prédites dans la pièce. / Contact wire splices play a leading role in the electric and mechanical integrity of the catenary systems. Most of these parts are made of Cu-Ni-Si alloys and manufactured by a hot forging process. With the aim of increasing the reliability of the railway network, the improvement of their performances and their manufacturing processes is sought. This work is focused on the forming stage of these parts and aims to give a better understanding of the materials-processes couplings for the Cu-Ni-Si alloys.An experimental campaign including compression tests is conducted. It allows the development of a physically-based model to describe the rheological behavior of a Cu-Ni-Si alloy deformed in the temperature range between 800 °C and 950 °C, at strain rates in the range of 0.1 - 10 s−1. It takes into account the work-softening implied by dynamic recovery and dynamic recrystallization. The model thus created is then implemented in a finite element software.A new workability test is proposed. Its purpose is to reproduce the mechanical conditions encountered in a forming process and to determine the critical conditions of workability related to a material. The forging process of a reference part is studied by means of this test. An accurate characterization of the process in terms of strain magnitude and stress field allows the identification of the experimental parameters. The results lead to the identification of a limit temperature of workability below which fractures are predicted in the part.