Development of advanced austenitic steels study of the influence of titanium and niobium on swelling by ion irradiation
Résumé
Due to their good corrosion resistance and excellent creep properties between 400C and 700C, austenitic steels have been chosen as reference material for the cladding materials for the French Sodium Fast Reactor (SFR) program. However, due to the swelling phenomenon under irradiation, their lifespan is limited and new advanced materials are developed to increase their radiation tolerance for GEN IV reactors.Currently in France, the most optimized steel is the AIM1, a 15Cr-15Ni stabilized with titanium. A generic study is conducted around this material to understand the swelling mechanisms and will benefit for the future AIM2 design, an improved variation of the AIM1. Swelling behaviour depends on various microstructural properties such as the dislocation density, the shape and the nature of precipitates and the elements in solid solution. Their role in the voids formation and growth are complex. The objective of this study is to appreciate the role of these metallurgical parameters on the voids formation mechanisms. Model steels with different content in Ti and Nb, compared to AIM1, have been produced. In order to study different mechanisms of swelling, various metallurgical states have been generated with different thermomechanical treatment. To simulate neutron irradiation, selected samples were irradiated at 600C up to 120dpa with 2MeV Fe2+ in the JANNuS facility in CEA Saclay (France). The resulting microstructures were analysed by Transmission Electron Microscopy and different effects on void swelling were highlighted. In particular, the content of elements in solid solution and the dislocation density are the main metallurgical parameters which influence the voids formation in these irradiation conditions. Replacing the titanium with niobium, the swelling behavior is significantly different. Niobium seems to be a different swelling inhibitor compared to titanium. The coupled effects of titanium and niobium are discussed.