Cluster dynamics modeling of Mn-Ni-Si precipitates coupled with radiation-induced segregation in low-Cu reactor pressure vessel steels

Abstract

Formation of precipitates enhanced or induced by irradiation causes hardening and embrittlement of nuclear structural materials. Post-irradiation microstructure characterization of reactor pressure vessel (RPV) steels has shown that precipitation can be strongly associated with radiation-induced segregation (RIS) of solutes on dislocations. However, the effect of RIS on the kinetics of Mn-Ni-Si precipitation has not been quantified and coupled in previous model predictions. In this study, a new hybrid and spatially-dependent precipitation model is developed that couples cluster dynamics with RIS with transport coefficients of diffusion mediated by vacancies and interstitials. The new approach provides a unique way to account for concurrent evolution of heterogeneous cluster densities as well as solute and point defect concentration profiles. The model is applied to study the segregation of Mn, Ni, and Si on dislocations and heterogeneous nucleation of Mn-Ni-Si rich precipitates (MNSPs) in a low-Cu RPV steel. The RIS result shows that the segregation tendency is dominated by the vacancy mechanism for Mn, Ni, and Si. The coupled cluster dynamics modeling result shows that the onset of MNSP nucleation on dislocations depends strongly on RIS and occurs at the fluence of 2×1023n..m−2. The number density and mean radius can reach ∼1024m−3 and 1−2 nm at high fluence, respectively. These observations suggest that the role of RIS and heterogeneous nucleation can be significant for RPV steels under high neutron fluence. Results of simulations adopting various dose rates and dislocation densities show an increased fraction of MNSPs on dislocations at the condition of low dose rate, high dislocation density, and high fluence.