Effect of prior thermal treatment on the microchemistry and crack propagation of proton-irradiated AISI 304 stainless steels

The effect of prior thermal treatment on crack growth was investigated on proton-irradiated Type 304 stainless steel (SS) of initially solution annealed (SA) and thermally sensitized (SEN) conditions. The Cr depletion profiles were measured by field emission gun transmission electron microscopy/energy dispersive spectroscopy (FEGTEM/EDS) in an attempt to correlate grain boundary chromium composition with the measured crack growth rate. The results showed that the crack growth of the 1-dpa-irradiated SEN 304SS is substantially higher than that of SA 304SS with the same irradiation dose. The unirradiated SEN material initially started with a shallow Cr depletion profile near grain boundary. After 1 dpa irradiation with proton, the Cr depletion profile becomes narrower and deeper. In contrast, the grain boundary Cr concentration in the SA specimen at the same irradiation dose was higher than that of the SEN specimen, mainly due to an initial Cr enriched condition. Consequently, the irradiated SEN specimen exhibited a higher degree of sensitization in electrochemical potentiokinetic reactivation test and faster crack growth rate in the stress corrosion crack test. The absence of irradiation enhanced crack growth in heavily thermal-sensitized 304SS is probably attributed to slower radiation-induced Cr depletion as a result of pre-existing thermally induced grain boundary Cr depletion. It is a clear indication that the inverse Kirkendall effect was hampered by the back diffusion of Cr due to initially depleted Cr concentration gradient near grain boundary.