Experimental study of irradiation creep in metals and alloys by coupling the MEMS technology to charged particles irradiation

Structural materials used in the PWR cores, such as austenitic stainless steels or zirconium alloys, are exposed to a significant neutron flux and, at the same time, a stress from various mechanical loadings. At the macroscopic scale, the mechanical behavior under irradiation is well characterized. However, at a microscopic scale, the deformation mechanisms under irradiation still remain unknown. Many irradiation creep mechanisms have been proposed from a theoretical point of view but the available experimental data have not, for now, permitted to identify the relevant mechanism leading to the deformation. The objective of this thesis is precisely to improve our understanding of the irradiation creep mechanisms of metals and alloys by the development of a novel experimental method. In this method, the irradiation is produced by the use of heavy ions. This kind of irradiation has the advantage of a fast damage rate without an activation of the material. However the irradiated area is confined in a few hundreds of nanometers. Such thickness requires a specific experimental device to apply a stress on the specimen. This device is based on the release of internal stress in a silicon nitride film to deform a metallic thin film. This method was designed and developed at the Universite catholique de Louvain in Belgium by the teams of Thomas Pardoen and Jean-Pierre Raskin. After proving the feasibility of the study and adapting the device to the irradiation environment, the method has been used with success to reproduce an irradiation creep experiment at room temperature on a model material: copper. A single creep power law with a stress exponent of 5 has been found under irradiation on 200 and 500 nm thick films. The SEM and TEM observations suggest that the deformation mechanism rely on the glide of dislocations assisted by climb. This law seems to be independent of the microstructure and the loading history. The dislocation climb, if it occurs, would not be controlled by diffusion process at long distance but by direct interaction between displacement cascades and dislocations. The mechanical behavior of unirradiated and irradiated copper films have also been assessed. The deformation mechanisms seem to be the same in both cases. At a moderate strain rate, the deformation is controlled by the intragranular glide of dislocations whereas at slow strain rate a change of mechanism takes place. The new mechanism still remains based on dislocations but a component of grain boundary sliding may appear. A post irradiation hardening has been observed on a 200 nm thick film due to the presence, in the irradiated samples, of a high density of SFT which act as obstacles against dislocation glide. (author)