Description du sujet de thèse

A quantitative study of dislocation dynamics in refractory concentrated solid solutions by in-situ transmission electron microscopy

Contexte de travail

Since the dawn of humanity, people have known that combining different metals makes them more resistant. Thus adding a little tin to pure copper makes it harder. The alloy formed, a bronze, is a miscible mixture of the two chemical species, called solid solution, in which the atoms are distributed randomly on a specific crystallographic network. In metals and their alloys, the mechanical properties are largely determined by the multiplication and propagation at nanoscale of linear defects, called dislocations, and their interaction with the microstructure. Thus, solid solution hardening is attributed to the interaction between the minority atoms, the solutes, with the dislocations. Theorized in the 60s and 70s, however, hardening models still elude a satisfactory explanation, particularly in concentrated solid solutions, where the notion of solutes is more vague and ill-defined. Moreover, the presence of impurities atoms, such as oxygen, in interstitial sites greatly influence mechanical properties at high temperature where diffusion is significant.
This question of hardening has recently regained renewed interest with the discovery of complex multicomponent solid solutions, called high-entropy alloys, with promising mechanical properties, particularly at high temperatures. Within the ANR project DisMecHTRA (2025-2028), we aim at investigating the fundamental mechanisms that contribute to the hardening in such alloys containing refractory elements and that
form BCC concentrated solid solutions. Binary and ternary model alloys (Nb-Mo or Nb-Mo-Ti), with a controlled interstitial purity, will be considered in the project.
First, the alloys will be processed and optimized at the ICMPE in Thiais. Then, at CEMES, we will use the high spatial resolution of a Transmission Electron Microscope (TEM), to directly observe the motion of dislocations during deformation in the ultra thin parts of an alloy sample. This technique, called in-situ straining TEM, is a specificity of the lab. Due to the large variation in chemical landscape along the dislocation and on its trajectory, a jerky motion exhibiting anchoring points is expected. Finally, we will
use methods combining deep learning and computer vision, in order to statistically extract the main characteristics of the motion of the dislocations (distributions of the anchoring points, speed and local curvature of the dislocations, etc.). These observations should allow us to confront the predictions of the most recent atomistic models as well as local measurements by atom probe tomography for example.

Le poste se situe dans un secteur relevant de la protection du potentiel scientifique et technique (PPST), et nécessite donc, conformément à la réglementation, que votre arrivée soit autorisée par l'autorité compétente du MESR.