Microstructural changes induced by post-irradiation annealing of neutron-irradiated austenitic stainless steels

Irradiated EBR-II hexagonal ‘hex’ duct material fabricated from 304 stainless steel (SS) is characterized prior to and following in situ annealing in the transmission electron microscope (TEM) at temperatures between 400°C and 600°C for various lengths of time. The hex duct samples were irradiated at temperatures between 375°C and 389°C to doses up to 29 dpa over a range of dose rates. The pre-annealed microstructure of the irradiated hex ducts exhibited substantial radiation-induced dislocation development (networks and loops) and cavity formation (bubbles and voids). Following annealing at all temperatures, dislocation loop densities decrease significantly and the dislocation network density increases. Annealing at 500°C and 600°C led to the shrinking or disappearance of many larger faceted voids. In addition to shrinkage of larger voids, small spherical bubbles formed leading to an increase in the overall cavity density and a decrease in the average cavity size.