Post-irradiation annealing of dislocation microstructure and radiation-induced segregation in proton-irradiated stainless steels

Both microstructural effects (radiation hardening) and microcompositional effects (radiation-induced segregation (RIS)) have been identified as potential contributors to irradiation-assisted stress corrosion cracking (IASCC). However, the importance of each in IASCC is unclear. In an effort to isolate their roles, post-irradiation annealing of proton-irradiated 304L stainless steel samples has been studied. Model simulations of post-irradiation annealing at intermediate temperatures (400-650°C) indicate that microstructural features such as dislocation loops are removed faster than RIS. Simulations also predict that there exist time-temperature combinations that will significantly reduce the dislocation loop population while leaving the grain boundary segregation essentially unaffected. Ultra-high purity (UHP) 304L stainless steel samples have been irradiated with 3.2 MeV protons at 360°C to 1.0 dpa and then annealed in-situ as thinned TEM disks in the temperature range of 500°C-625°C for times between 20 and 60 minutes. RIS and dislocation loops were characterized before and after annealing in the same areas of each specimen. Post-irradiation anneals at 500°C for 45 minutes or 60 minutes resulted in no appreciable change in dislocation loop population or RIS. Annealing at 600°C for 20 minutes, or 40 minutes and at 625°C for 40 minutes, resulted in decreasing dislocation loop densities and increasing loop size with increasing annealing time or temperature, while the amount of RIS did not change significantly from the pre-annealed condition.