The effects of high fluence fast neutron irradiation on the fatigue behavior, microstructure, swelling, and microhardness were evaluated for annealed Type 304 stainless steel which had attained fluences near 1.6 × 1023 neutrons (n)/cm2 (> 0.1 MeV) at irradiation temperatures from 370 to 470°C (698 to 878°F). Unnotched fatigue specimens remotely prepared from the irradiated material were cycled at resonance in reverse bending in air at constant amplitude at 427 and 593°C (800 and 1100°F). At 427°C, the fatigue life was found to be strongly dependent on irradiation temperature. A decrease in fatigue life from 2 × 105 to 1.1 × 104 cycles was seen with increasing irradiation temperature within the range investigated. For irradiation temperatures below approximately 427°C, the fatigue life was found to decrease with increasing fluence in the range from 1.3 × 1022 to 1.5 × 1023 n/cm2 (> 0.1 MeV). The fatigue behavior of specimens tested at 593°C was characterized by a similar irradiation temperature dependency. At 593°C, however, little effect of fluence was observed in the irradiation temperature range below 427°C. At both test temperatures the nuclear exposure produced an increase in fatigue life at irradiation temperatures below 450°C.
Transmission electron microscopy was employed to characterize the radiation-induced void structure in the material subsequent to the fatigue tests. A maximum swelling of 10.4 percent was determined by immersion density measurements and independently confirmed from the microscopy data. Ambient temperature microhardness measurements showed that the hardness increment, based on unirradiated thimble material, was in general agreement with calculated microhardness values based on the microscopy data. The calculations indicate that the radiation-induced voids were the primary microstructural component responsible for the measured microhardness. Comparison of the fatigue data with the void, swelling, and microhardness results suggests that the fatigue lives of the specimens at both test temperatures directly reflect the microstructural effects measured by the microhardness as influenced by the void parameters.