Study of irradiation and nano-structuring effects in austenitic stainless steels

Internal structures of pressurized water reactors, made of 304 and 316 austenitic stainless steels, undergo a high neutron flux. A degradation of macroscopic properties (hardening, loss of corrosion resistance), caused by this irradiation, involves cracking of bolts of internal structures by irradiation assisted stress corrosion cracking. Modification of macroscopic properties is attributed to an evolution of microstructure under irradiation: formation of point defect clusters (Frank loops, cavities and/or gas bubbles), radiation induced segregation at grain boundaries. However, only few studies concern neutron irradiation effects on solute distribution into grains in these materials. So, the first aim of this work is to observe, at the atomic scale, solute distribution after neutron irradiation by tomographic atom probe. Ni-Si clusters are thus revealed. Then, thanks to model ion irradiations and with the help of transmission electron microscopy and cluster dynamics modelling, some information about mechanism of solute cluster formation has been provided. Heterogeneous radiation-induced precipitation seems to be the more plausible assumption. The second aim is to elaborate an ultrafine grained austenitic steel. The larger the grain boundary surface is, the larger the elimination of point defects, responsible of microstructure evolution under irradiation, is. Microstructure of this material has been characterized after thermal ageing and after irradiation. Results show a limitation of intra-granular damage. (author)