FRACTURE BEHAVIOR OF OXIDIZED GRAIN BOUNDARY IN NEUTRON-IRRADIATED STAINLESS STEEL

摘要

Effects of oxidation on cohesive strength of a grain boundary (GB) was investigated by micro-tensile testing for stainless steel specimen neutron-irradiated to 73 dpa and then immersed in simulated PWR primary water at 320°C. The micro-specimens were fabricated with an oxidized GB and a non-oxidized GB (as-irradiated). The specimen with the oxidized GB failed at 1100 MPa, whereas the specimen with the non-oxidized GB failed at 2600 MPa. It was found that GB oxidation would decreased the cohesive strength to one third that of the as-irradiated GB. By considering the stress concentration on GBs due to deformation constraint in a polycrystalline material and dislocation channeling deformation, it was suggested that the oxidized GBs would crack when the tensile stress was applied on the neutron-irradiated stainless steel used in high temperature water at the same level as the initiation threshold stress of the irradiation-assisted stress corrosion cracking (IASCC). Intergranular cracking propagation by repetition of further GB oxidation and its fracture was thought to be one of the initiation mechanisms of IASCC.