Ferrite formation in neutron-irradiated type 304L stainless steel

Austenitic (γ) to ferrite (α) transformation was observed using transmission electron microscopy in type 304L stainless steel that had been irradiated at ~500°C to fast-neutron (E > 0.1 MeV) fluences greater than ~ 3 × 10²² n/cm². Previous studies on similar unirradiated stainless steels found no such transformation, indicating that the γ → αtransformation was irradiation-induced. The α phase appeared to nucleate on stacking faults, indicating that the presence of large Frank loops was the critical step in the transformation. After an entire grain of austenite had transformed, the only remaining γ phase existed as shells around voids. Coincidence of rapid swelling behavior with γ → α transformation indicated that the two were related, perhaps by reaction of both phenomena to the effects of irradiation and temperature on microchemical segregation. A volume expansion of about 2.5% was found to be associated with the transformation. Inferences are drawn relating this behavior in type 304L steel to the effects of radiation on other reactor structural materials such as type 316 stainless steel, which is also a metastable austenitic composition.