Projet de thèse en 2MGE : Matériaux, Mécanique, Génie civil, Electrochimie
Sous la direction de Alexis Deschamps et de Hugo Van landeghem.
Thèses en préparation à Grenoble Alpes , dans le cadre de I-MEP2 - Ingénierie - Matériaux, Mécanique, Environnement, Energétique, Procédés, Production , en partenariat avec Science et Ingénierie des Matériaux et Procédés (laboratoire) depuis le 01-10-2017 .
L'objectif du travail de thèse proposé est de mettre en œuvre la méthodologie de fabrication de matériaux à gradient et de caractérisation des microstructures à gradient obtenues dans des alliages base fer (aciers). L'enjeu scientifique sera d'une part l'interaction entre éléments d'alliage et l'interface de transformation allotropique austénite / ferrite, celle-ci étant la brique essentielle de contrôle de la microstructure des aciers multi-phasés en plein développement.
Development of compositional-gradient metallic alloys for combinatorial investigation of microstructures
Diffusional growth of ferrite from austenite in steels has been largely investigated during the last decades. This transformation is of considerable interest in controlling the final properties of steels, such as Dual Phase and TRansformation Induced Plasticity where the final mechanical properties are linked to the presence of the two phases. Despite tremendous efforts in understanding the mechanisms controlling ferrite formation, the role of substitutional elements during ferrite growth and their interaction with the migrating α/γ interface remain unclear. In ternary systems Fe-C-X, and due to the large difference between the diffusivity of interstitial C and substitutional elements, ferrite growth can occur with or without partitioning of the alloying element X. Several models have been developed to describe growth kinetics in ternary systems, under partitioning (local equilibrium, LE) and non-partitioning (LENP and PE) modes. Recent experimental observations showed that a transition in interface conditions (from PE to LENP) may occur during ferrite growth. A new approach based on solute interaction with the migrating interface was developed to explain the observed transition. Solute drag based models assume that a part of the available driving force is consumed by the segregation of atoms at the migrating interface. The effect of multiple substitutional solutes on this complex behavior combining a thermo-kinetic growth and solute drag effect are still unknown. The developed models require an experimental validation. While studying the effect of temperature is relatively simple, evaluating the effect of composition is time-consuming and costly since it requires multiple castings with different compositions. A high- throughput alternative is to fabricate samples that contain macroscopic gradients of composition, and to perform time- and space- resolved high energy X-ray diffraction experiments to gather phase transformation kinetics in many points of the compositional space. The objective of the present study is to develop a combinatorial methodology to fabricate compositionally-graded materials. Samples with different compositions have been joined together using hot compression and treated at high temperature to generate gradients ofcomposition. The obtained gradients have been extended using cold-rolling, and the microstructure after different annealing treatments has been characterized in the compositional space by classical methods (optical/electron microscopy). In a second stage, high energy X-ray diffraction will be used to study ferrite growth in the compositionally-graded samples. Atom probe tomography and Transmission Electron Microscopy techniques will be used on the treated samples to investigate the segregation at the migrating interface. The obtained data will be used to validate and optimize models describing ferrite growth kinetics.