Thermodynamic models of low-temperature Mn-Ni-Si precipitation in reactor pressure vessel steels
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esearch Letters
Thermodynamic models of low-temperature Mn–Ni–Si precipitation in reactor pressure vessel steels Wei Xiong, Huibin Ke, and Ramanathan Krishnamurthy, Department of Materials Science and Engineering, University of Wisconsin, Madison, Wisconsin 53706 Peter Wells, Materials Department, University of California, Santa Barbara, California 93106 Leland Barnard, Materials Science Program, University of Wisconsin, Madison, Wisconsin 53706 G. Robert Odette, Materials Department, University of California, Santa Barbara, California 93106; Mechanical Engineering, University of California, Santa Barbara, California 93106 Dane Morgan, Department of Materials Science and Engineering, University of Wisconsin, Madison, Wisconsin 53706; Materials Science Program, University of Wisconsin, Madison, Wisconsin 53706 Address all correspondence to Dane Morgan at [email protected] (Received 30 May 2014; accepted 1 August 2014)
Abstract Large volume fractions of Mn–Ni–Si (MNS) precipitates formed in irradiated light water reactor pressure vessel (RPV) steels cause severe hardening and embrittlement at high neutron fluence. A new equilibrium thermodynamic model was developed based on the CALculation of PHAse Diagrams (CALPHAD) method using both commercial (TCAL2) and specially assembled databases to predict precipitation of these phases. Good agreement between the model predictions and experimental data suggest that equilibrium thermodynamic models provide a basis to predict terminal MNS precipitation over wider range of alloy compositions and temperatures, and can also serve as a foundation for kinetic modeling of precipitate evolution.
Irradiation enhanced precipitation hardening is the primary cause of in-service embrittlement of reactor pressure vessel (RPV) steels. Odette et al.[1–6] long ago predicted that at very high fluence Mn–Ni–Si (MNS), so-called late blooming phases, would form large mole fractions of nanoscale precipitates, even in low Cu RPV steels. These early predictions have since been qualitatively confirmed and refined.[2,3,7,8] Since MNS precipitates could result in severe and unanticipated embrittlement, and they are not treated in current regulatory models, these precipitates may limit currently planned plant life extension of up to 80 or more years. They are therefore arguably the most immediately important reactor materials aging-degradation challenge facing nuclear power’s continued carbon-free contribution to our electricity supply.[1,9] MNS precipitates do not develop under typical thermal aging conditions at temperatures relevant to light water reactor (LWR) operation (523–573 K) due to very slow thermal diffusion kinetics. However, they have been observed following irradiation and, most recently, in high Cu and Ni alloys after very long-time thermal aging at higher temperatures.[10] The challenge of producing these phases in the absence of irradiation has led to significant debate in the literature about the thermodynamic stability of MNS features and their potential for forming large precipitate mole
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