Radiation induced solute segregation (RIS) of chromium, nickel, and minor elements (e.g., Si, P, S, etc.) in the vicinity of grain boundaries and its influences on intergranular stress corrosion cracking (IGSCC) of commercial purity AISI 304 and AISI 304L austenitic stainless steels were investigated in this study. High energy protons were used for producing irradiation environment.
Three millimeter TEM discs and miniature tensile specimens were irradiated at 550°C with 1 and 5 MeV H+ individually to doses of 0.01, 0.1, and 1 dpa respectively. Microstructural evolution and radiation-induced solute segregation were studied by using STEM/EDAX microscopy. Constant extension rate tensile (CERT) under various environments were performed using the irradiated miniature tensile specimens to study the relationship between solute segregation and SCC susceptibility of these materials.
Microstructural examination indicated that void swelling was observed in both 304 stainless steels irradiated specimens at the dose of 0.1 dpa and above. Precipitate analysis found that M23C6 carbides were the major precipitates in the as-received (AR) and sensitized (SAS) specimens. Irradiation enhanced precipitation(M23C6 carbides)along grain boundaries was also observed in irradiated AR specimens. It was found that in the irradiated specimens, both Cr and Fe were depleted and Ni and Si were enriched near the grain boundary areas. Proton irradiation induced the nucleation and growth of ferrite phase(bcc) was observed in AISI 304L stainless steels irradiated at 550°C. The size and density of ferrite increased with increasing fluence, and it is found that many M23C6 carbides formed along the interface of F-matrix and ferrite grains.
CERT test indicated that IGSCC accompanying with TG fracture existed in the irradiated region after 1 dpa irradiation and dimples in the unirradiated region. It is found proton irradiation induced the grain boundary etching and surface slip bands which may in turn affect the CERT results in the sense of initiating intergranular cracks.