Change in Mechanical Properties by High-Cycle Loading Up to Gigacycle for 316L Stainless Steel

At the J-PARC pulsed spallation neutron source, a target vessel made of 316L stainless steel suffers radiation damage under high proton and neutron radiation environment. In addition to the radiation damage, the target vessel suffers cyclic impact stress caused by the proton beam-induced pressure waves. We have observed cyclic hardening/softening phenomena during investigating gigacycle (109 cycles) fatigue strengths of solution annealed and cold-worked 316L by an ultrasonic fatigue test. In this study, to elucidate the cyclic hardening/softening phenomena up to the gigacycle loading, dislocation densities of the cyclic-loaded specimens by the ultrasonic fatigue method were measured using neutron diffraction method. The results showed that the dislocation of the cold-worked 316L was rearrangement and re-accumulate by cyclic loading whereas the dislocation densities of the solution annealed 316L was increased with increasing in the number of loading cycles. For both cases, change in the dislocation densities by cyclic loading were well correlated with the change of mechanical properties; hardness, residual strength.