Plasticity of Fe-Ni-Cr alloys under irradiation (PhD)

In nuclear plants structural reactor-core components undergo degradation of mechanical properties, due to the combined effect of neutron irradiation and contact with coolant (corrosion). This limits their operational lifetime and poses requirements for safe exploitation, calling for a quantification of the ageing of these structures. In support of the relevant safety evaluation, continuous progress in physical understanding of radiation damage phenomena and in computer science allows the development of multi-scale numerical tools to model the effects of irradiation on the mechanical properties of materials. The approach begins with the study of the production of atomic-scale defects in individual displacement cascades, initiated by neutrons, and ends with the application of plasticity and fracture mechanics, wherein the response of the material to changes in temperature and/or applied stress is of concern. Methods for modelling dislocations at the atomic and mesoscopic levels lie between these extremes. Mesoscopic models are known as dislocation dynamics (DD): they treat dislocations as strings governed by elasticity and local laws, thereby linking discrete defect behaviour to fracture mechanism models. Local laws (dislocation mobility and reactions with radiation defects) are to be provided by atomic-scale molecular dynamics (MD) simulations.

A first step towards the implementation of this approach has been reached within the FP6 PERFECT project, in which SCK·CEN has been one of the major partners. Relying on the existing PERFECT roadmap, the PERFORM60 Project has been launched in FP7, with the objective to improve existing multi-scale tools aimed at predicting the effects of irradiation on austenitic steels for internals, including corrosion effects, through the adoption of adequate model alloys as reference materials. SCK·CEN, through the SMA group of the NMS institute, is engaged to contribute to this project with a number of key tasks. One of them concerns the determination from atomic-level studies of dislocation mobility laws and an enriched description of the interaction between a dislocation and different types of radiation defects in austenitic alloys. Using this knowledge, DD simulations will be used by other partners in the project to evaluate the flow behaviour and the kinetics of strain localisation (clear bands formation) in individual grains. The present PhD thesis proposal is made in this framework.

The objective of the proposed PhD thesis work is to study by MD the mobility laws of dislocations and their interaction with radiation-induced defects in Fe-Ni-Cr alloys, relevant for austenitic steels serving as structural components in nuclear power plants.

The microstructure of irradiated austenitic steels is known from detailed experimental studies (e.g. those performed at SCK·CEN in the framework of the PhD thesis of X. Li). The defects formed under irradiation depending on the type of steel and irradiation conditions (dislocation loops, stacking fault tetrahedra, etc) are thus known. On the other hand, the tools and techniques to carry the study of dislocations and dislocation-defect interactions are currently available and well experimented within the SMA group. Thus, the proposed PhD thesis will focus on similar, extensive MD studies of dislocation-defect interaction, preceded by a study of the mobility laws, in austenitic alloys, instead of pure elements.

Crucial, for this purpose, is the availability of an adequate FeNiCr interatomic potential. An FeNi potential has been already produced within PERFECT at SCK·CEN, in the framework of the PhD thesis of G. Bonny and the production of an FeNiCr potential is foreseen for the beginning of the PERFORM60 project. In accordance with the planning of PERFORM60, the work would proceed progressively, addressing first Fe-Ni and Fe-Ni-Cr defect free matrices and then Fe-Ni-Cr containing radiation defects.

For this work the candidate will be able to count on the support of an established team in the SMA group, as well as on the collaboration with other partners within PERFORM60, in a highly international environment. The results of the work will be implemented in DD models, in collaboration with EDF (coordinator of the PERFORM60 project).