Micro-mechanical investigations about grain boundary degradation factors in neutron-irradiated stainless steels

In this paper, we explain research activities to date that have examined the grain boundary (GB) degradation factors in neutron-irradiated stainless steels by using micro-tensile testing to elucidate the mechanism of irradiation assisted stress corrosion cracking (IASCC). First, we outline the micro-tensile testing method that we have developed, and then we introduce our findings on GB degradation in neutron-irradiated stainless steels obtained by applying the developed method. Finally, we show the mechanism of IASCC that is based on the presented information. Among the changes in microstructure and microchemistry of GBs that occur in neutron-irradiated stainless steels, the research activities revealed that the formation of GB cavities and the selective oxidation of GBs in high temperature water due to radiation-induced segregation degraded the binding force of GBs. For the IASCC mechanism, three steps were considered. In the first, oxidized GBs cracked by the action of local stress that increased due to localized deformation peculiar to irradiated materials on GBs whose binding force decreased due to the formation of cavities and oxidation. In the second, the micro-cracks on GBs grew by repeated oxidation and fracture of the GBs. In the final step, the grown cracks continued to progress on the GBs. (author)