Six austenitic stainless steel heats (three heats each of 304SS and 316SS) neutron-irradiated at 275C from 0.6 to 13.3dpa were carefully characterized by transmission electron microscopy and their hardness measured as a function of dose. The characterization revealed that the microstructure was dominated by a very high density of small Frank loops present in sizes as small as 1nm and perhaps lower, which could be of both vacancy and interstitial-type. Frank loop density saturated at the lowest doses characterized, whereas the Frank loop size distributions changed with increasing dose from an initially narrow, symmetrical shape to a broader asymmetrical shape. Although substantial hardening was caused by the small defects, a simple correlation between hardness changes and density and size of defects did not exist. These results indicated that radiation-induced segregation to the Frank loops could play a role in both defect evolution and hardening response.

Evolution of Fine-Scale Defects in Stainless Steels Neutron-Irradiated at 275C. D.J.Edwards, E.P.Simonen, S.M.Bruemmer: Journal of Nuclear Materials, 2003, 317[1], 13-31