In the context of extending the operational lifetime of nuclear power plants (NPPs), currently operating in France, a materials ageing surveillance strategy is in place. It is essential for ensuring their mechanical properties. During the operation of the plant, materials are subjected to irradiation. Under this exposure, the internal structure of materials evolves, leading to the creation of numerous defects that degrade macroscopic properties and may result in a limitation of the long-time operation (LTO) of components. The proposed work is a fundamental study conducted on model materials, aiming to better understanding the behavior under irradiation of metallic alloys. It will contribute to the predictive modelling of materials, covering defects created at the nanoscale up to the level of nuclear components.
The irradiation of materials with high-energy particles such as neutrons, ions, or electrons generates a large number of defects called point defects (PD). These mobile PDs can migrate and aggregate to form 2D or 3D-objects like prismatic loops or cavities respectively. They can also be eliminated at PD sinks. The system is then submitted to PDs flows directed towards these sinks. These flows are then responsible for phenomena such as radiation-induced segregation (RIS) or precipitation (RIP) of solute atoms  . The presence of clusters of PDs and of PD flows alters the microstructure and can deteriorate the physical response of the irradiated materials. In particular, the formation of prismatic loops degrades the mechanical properties of materials as they can impede dislocations and induce embrittlement . In a previous study, we focused on vacancy defects in the form of cavities and investigated the facetting of defects formed in a weakly anisotropic metal, aluminum, using in-situ irradiations in a high-resolution transmission electron microscope (HRTEM).
The work aims to go further in the role of the elastic deformation field on the morphology of irradiation defects. More precisely, it aims to carry out a systematic study on different metals with different anisotropy coefficients. We have chosen reference metals with body-centered cubic (BCC) and face-centered cubic (FCC) structures with low or high anisotropy coefficients. The study will concern Cr and Fe with a BCC structure, and Al and Cu with a FCC structure and may be extrapolated to alloys of higher complexity such as high entropy alloys (HEA). The work will be mainly experimental but will also include a theoretical part. The effects of the crystal anisotropy on the morphology of prismatic loops will be carried out by phase field modelling . The spatial arrangement of the loops will be studied by Object Kinetic Monte-Carlo (OKMC) simulations , as recently done in aluminium.
The work will be mainly experimental. We will studied -oriented single crystals to avoid any surface effect on the shape of the objects formed. They will be irradiated with heavy ions at temperatures normalized with respect to their melting temperature either in-situ within the Jannus Orsay platform, or ex-situ within the Jannus Saclay platform . Loops will be imaged by conventional TEM or STEM with a FEI Tecnai and Jeol NeoARM type microscopes. The latter is equipped with a double spherical aberration corrector. The work will be carried out within the framework of the joint research laboratory (LRC) MAXIT.
The work will also include a modelling part. The effects of crystallographic anisotropy on the morphology of prismatic loops will be investigated using a phase-field code . The spatial arrangement of the loops will be studied using Object Kinetic Monte Carlo (OKMC) , as recently done in aluminum.
This work follows a 2-year postdoctoral fellowship scheduled to conclude in December 2023, during which deep learning (DL) approaches were developed to accelerate the automatic detection of defects created under irradiation . The utilization of these approaches will significantly enhance the statistical robustness and precision of the results.
Advantage for the student: The PhD is situated in a laboratory composed by 25 researchers and approximately 25 students (PhD, postdoctoral fellows), creating a simulating scientific environment. The activities involve both experimental and simulation sides, offering the opportunity to interact with experts from both sides.
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