Phase diagram of Ni-Si-Mn precipitates in reactor pressure vessel steels

松川 義孝*; Terao, Masayuki*; 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*; 中森 文博*; et al.

no journal, , 

The primary cause of irradiation-induced embrittlement of the reactor pressure vessel (RPV) steel is precipitation of its alloying elements (Ni, Si and Mn) in the form of nano particles (2 nm). The RPV embrittlement practically limits the service life of the whole reactor; in other words, reactor's lifetime prediction is achieved by predicting the precipitation. Although precipitation under irradiation is a thermodynamically non-equilibrium reaction, recent studies have revealed that precipitation of those elements does occur even without irradiation. Hence the phase diagram of Ni-Si-Mn precipitates in steels has become a subject of interest in nuclear materials research. In this talk we demonstrate that the calculation phase diagram is still incomplete due to absence of experimental data about Ni-Si-Mn ternary compounds; there exist 10 in the phase diagram. We synthesized an ingot of the G-phase NiSiMn, which is one of the most frequently observed precipitates in the RPV steel, by arc-melting and examined the melting point and the composition range. The experimentally-determined melting point was inconsistent with simulation results, e.g., an old database (2016 version) resulted in an overestimation greater than 700 degree of Celsius. As for composition range, although off-stoichiometry is not considered in the calculation phase diagram, the G-phase certainly has it. The same thing happens to the 2-phase NiSiMn, which is another most frequently observed precipitates in the RPV steel.