The Microstructural Characterization of NiSi-Rich Sub-precipitates Within Cementite in Isothermally Aged Reactor Pressur
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A reactor pressure vessel (RPV) plays a key role in reactor safety for serving as a pressure barrier for coolant water and a containment barrier for the radioactive fission products produced during a nuclear fission reaction.[1,2] Under in-serve reactor environments, RPV steels experience a wide range of microstructural and microchemical changes, which cause the irradiation embrittlement and hardening of the RPV steels.[3–5] The irradiation embrittlement is a
HAILONG LIU is with the School of Material Science and Engineering, Tsinghua University, Beijing 100084, P.R. China and also with the Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, P.R. China. XINGPING WU, RONG HU, and GANG SHA are with the School of Materials Science and Engineering, Nanjing University of Science and Technology, Jiangsu 210093, P.R. China. QIULIN LI is with the Graduate School at Shenzhen, Tsinghua University. Contact e-mail: [email protected] BEN XU and WEI LIU are with the School of Material Science and Engineering, Tsinghua University. GUOGANG SHU is with the State Key Laboratory of Nuclear Power Safety Monitoring Technology and Equipment, China Nuclear Power Engineering Co., Shenzhen 518172, P.R. China. Manuscript submitted January 26, 2019. Article published online July 9, 2019 3992—VOLUME 50A, SEPTEMBER 2019
very complex phenomenon. From a macroscopic point of view, this process usually manifests itself in an upward shift of the ductile-brittle transition temperature (DBTT) that marks the transition between the high toughness ductile (microvoid coalescence) and low toughness brittle (cleavage) fracture regimes. From a microscopic perspective, microstructural nanofeatures produced by neutron radiation are well known to contribute to irradiation embrittlement. These nanofeatures include the following: (1) nanoscale Copper-rich or manganese-nickel-rich precipitates (CRP/MNPs),[6–14] (2) stable matrix features (SMF),[15–18] and (3) unstable matrix defects (UMD) that are believed to be sub-nm point defect clusters.[3] Radiation-induced solute clusters or precipitations have been extensively reported in irradiated RPV steels. A high-volume fraction of nanoscale MNPs formed under neutron irradiation is responsible for irradiation hardening and embrittlement of low-copper RPV steels at high fluences.[19,20] Additionally, recent investigations reported that MNPs heterogeneously nucleated at the vicinity of defects, such as dislocations, grain boundaries, and small solute-defect complexes which were referred to as SMF.[13,21,22] These solute-defect complexes are thought to be precursors of the NMPs.[23] Generally, the MNPs are known to form only under irradiation due to irradiation-induced segregation, rather than under thermal aging. Recently, MNPs were found to form within the cementite of RPV model steel after 11700 hours of aging at 365 C.[24] However, these thermally formed MNPs have different compositions than irradiation-induced MNPs. Cleavage fracture will occur when the local stress exceeds the cleavage fracture
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