Thesis defense by Xingke Gao

Ferromagnetic shape memory alloys such as Ni-Mn-Ga exhibit strong coupling between temperature, magnetic field, and mechanical deformation, making them promising candidates for solid-state heating and cooling applications. This thesis investigates the kinetics and hysteresis of the martensitic phase transformation, focusing on the behavior of the austenite-martensite interface in single-crystal and polycrystalline alloys. Through calorimetric and magnetic measurements combined with in situ EBSD analysis, the transformation mechanisms are characterized. A custom setup applying a temperature gradient and magnetic field reveals distinct interface morphologies depending on the transformation direction. Results show that magnetic loading alters the interface structure and transformation temperatures, highlighting the interplay between thermal and magnetic energies. These findings provide new insights into the physical mechanisms of phase transformations and support the development of more accurate predictive models.

Composition du jury :

•    Tarak BEN ZINEB — Professeur des Universités — University of Lorraine — Rapporteur & Examinateur
•    Mohamed HABOUSSI — Professeur des Universités — Laboratoire des Sciences des Procédés et des Matériaux, CNRS — Rapporteur & Examinateur
•    Cristina BORMIO-NUNES — Associate Professor — University of São Paulo — Examinatrice
•    Habibou MAITOURNAM — Professeur des Universités — ENSTA-Paris — Examinateur
•    Olivier DE LA BARRIÈRE — Chargé de Recherche — SATIE, ENS Paris-Saclay, CNRS — Examinateur