Fatigue Crack Initiation in Proton-Irradiated Austenitic Stainless Steel

The effect of irradiation on slip band formation and growth and microcrack initiation behavior under low cycle fatigue in SUS316L austenitic stainless steel was investigated using accelerator-based proton irradiation and a low cycle fatigue test at room temperature in air. The mean space of the slip line in proton-irradiated specimens was 25-40% wider than that in unirradiated specimens under the same number of cycles, possibly due to localized deformation by proton irradiation. The microcrack initiation life of the proton-irradiated specimens was approximately 20% of that of the unirradiated specimens. While the microcrack initiation in the unirradiated specimens was observed at the grain boundary, twin boundary, slip band, and triple junction, that in the proton-irradiated specimens was observed only at the twin boundary and slip band, possibly due to irradiation hardening. The step-height of an extrusion near the microcrack was almost the same in the unirradiated and proton-irradiated specimens regardless of the initiation site (100-150 nm). Therefore, the microcrack initiation was considered to occur when the surface morphology change involving the extrusion exceeded the specific threshold value.