照射ステンレス鋼の粒界劣化機構に関する検討 －粒界破壊に及ぼす照射誘起偏析とキャビティ形成の影響－
A study on the mechanism of grain boundary degradation in irradiated stainless steels -Effects of radiation-induced segregation and cavity formation on grain boundary fracture –

三浦 照光（Terumitsu Miura） 山岡 祐樹（Yuki Yamaoka） 村上 健太（Kenta Murakami）藤井 克彦（Katsuhiko Fujii） 福谷 耕司（Koji Fukuya）

要約
照射誘起応力腐食割れの発生要因の一つと考えられる照射に伴う粒界強度の低下について，粒界での照射誘起偏析とキャビティ形成の影響を検討した．様々な条件でイオン照射したSUS316ステンレス鋼の粒界部の引張破壊挙動を室温の超微小引張試験で調べた結果，Cr欠乏やNi,Si 濃化が生じる照射誘起偏析と直径 1 nm 程度の微細なキャビティの形成が重畳した場合，キャビティ形成のみに比べて粒界破壊が起こり易いことが分かった．この傾向は，先行研究で調べられ
た中性子照射ステンレス鋼と He イオン照射材の粒界部の引張破壊挙動の傾向と類似しており，照射誘起偏析とキャビティの形成の重畳がステンレス鋼の粒界強度を低下させる要因であることが示唆された．

キーワード    ステンレス鋼，イオン照射，照射誘起偏析，キャビティ，超微小引張試験

Abstract
The effects of radiation-induced grain boundary segregation and cavity formation on the reduction of grain boundary cohesive strength associated with irradiation, which is considered one of the factors causing irradiation-assisted stress corrosion cracking, was investigated. The tensile fracture behavior of grain
boundary regions in SUS316 stainless steel subjected to ion irradiation under various conditions was examined by micro-tensile testing at room temperature. The results showed that when radiation-induced segregation, characterized by chromium depletion and nickel and silicon enrichment, and the formation of
fine cavities approximately 1 nm in diameter overlapped, grain boundary fracture occurred more easily compared to the case with cavity formation alone. This tendency was similar to the trends observed in previous studies on neutron-irradiated and helium ion-irradiated stainless steel specimens. These findings
suggest that the combination of radiation-induced segregation and cavity formation is a factor that reduces the grain boundary cohesive strength in stainless steels.

Keywords   stainless steel, ion irradiation, radiation-induced segregation, cavity, micro-tensile testing

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