Effects of proton irradiation on the microstructure and microchemistry of type 304L stainless steel

Abstract

A research program has been undertaken to determine the origins of irradiation-assisted stress corrosion cracking (IASCC) in austenitic alloys in light water reactors, and the effect of impurities on IASCC susceptibility. Controlled purity alloys of 304L stainless steel were irradiated with protons at 400[deg]C to a dose of 1 dpa and analyzed via Auger electron spectroscopy (AES) and scanning transmission electron microscopy (STEM). The alloys investigated were an ultra-high purity (UHP) alloy and UHP alloys containing phosphorus (UHP + P), sulfur (UHP + S), or silicon (UHP + Si). Microstructural and microchemical changes were quantified and compared with literature results for other irradiating species. Following irradiation, the alloys showed dislocation loop formation and growth, "black dot" loops, and a change in the nature of the dislocation network. AES and STEM microchemical analysis of the alloys revealed Cr depletion of up to 6 at% and Ni enrichment of up to 6.6 at% at the grain boundaries of the alloys, with more segregation observed in the alloys containing impurities than in the UHP alloy. Significant grain boundary enrichment of P and Si in the UHP + P and UHP + Si alloys, respectively, was also observed. The results of the analyses of proton-irradiated samples are shown to compare favorably with previous studies on samples irradiated with neutrons at or near LWR conditions.