Phase diagram of the 2-phase; C15 Laves phase in RPV steel

松川 義孝*; 藤枝 秀斗*; 連川 貞弘*; 牟田 浩明*; 熊野 秀樹*; 笠田 竜太*; 吉田 健太*; 藪内 聖皓*; 山口 正剛   

Matsukawa, Yoshitaka*; Fujieda, Hideto*; Tsurekawa, Sadahiro*; Muta, Hiroaki*; Yuya, Hideki*; Kasada, Ryuta*; Yoshida, Kenta*; Yabuuchi, Kiyohiro*; Yamaguchi, Masatake

The 2-phase Ni3SiMn2 is one possible configuration of irradiation-induced Ni-Si-Mn precipitates (a.k.a. the late blooming phase) in the rector pressure vessel (RPV) steel of light water reactors. Unlike the other possible configurations such as the G-phase NiSiMn, this intermetallic compound is crystallographically characterized as the C15 Laves phase, which is of particular interest in fundamental irradiation damage study, specifically in relation to nucleation of interstitial-type dislocation loops. Although the dislocation loop is a planar, 2D cluster, recent simulation studies have suggested that its nucleus is not 2D but 3D, crystallographucally identical to the C15 Laves phase. Hence in the modeling of microstructure evolution in the RPV steel the relation between the 2-phase and the dislocation loop may be a key harnessing irradiation-induced precipitation and dislocation loop formation. We have recently succeeded in fabricating ingots of the 2-phase by means of arc melting, and examined the validity of existing Ni-Si-Mn phase diagrams. It appears that both experimental (Kuz'ma 1964) and simulation (Thermocalc TCNI12) diagrams are not valid.