Metal alloys used in nuclear applications are subjected to relatively low temperatures (below 450°C) for long periods of time (more than 10 years). At these temperatures, the kinetics of the diffusion-controlled microstructure transformations are slow. The appearance of certain undesirable phases, likely to embrittle the material, can occur after several years of service. Therefore, diffusion coefficients play a crucial role as input data for modeling the evolution of these microstructures using phenomenological models. However, experimental determination of diffusion coefficients at low temperatures (T < 600°C) is extremely tricky, especially because of the need to characterize nanometric diffusion lengths, a difficulty made all the more difficult in the presence of irradiation.
With the development of chemical analysis by transmission electron microscopy (TEM) and atom probe tomography (APT), it is now possible to experimentally access very small diffusion lengths and thus determine low-temperature diffusion coefficients using superlattices, which consist of stacking nanometric layers of different chemical compositions. We can even characterize the effect of irradiation on diffusion by performing ion irradiations, enabling us to simulate the changes caused by neutron irradiation without activating the materials. The aim of this thesis is to develop a methodology and characterize diffusion under and outside irradiation in a ternary system of interest (Ni-Cr-Fe), representative of the steels and high-entropy considered in the nuclear industry.
This thesis is an opportunity to work with cutting-edge experimental techniques, in close collaboration with a team of theoretician in the same department, as well as with teams specializing in the development of superlattices at UTBM in Belfort and CINAM in Marseille.