Abstract: It is well-known that the formation of Mn-Ni-Si precipitates in reactor pressure vessel (RPV) steels during extended service in nuclear power plants results in hardening and embrittlement. However, the migration and local aggregation behavior of solute atoms in the early service period remains elusive due to limitations in characterization techniques. In this work, the migration and aggregation behavior of solute atoms, along with the thermodynamic characteristics of C-bearing Mn-Si-rich phases (C-MSPs) precipitated along grain boundaries (GBs) during the early aging period of RPV steels were systematically investigated using internal friction (IF), transmission electron microscopy (TEM), and small-angle scattering (SAS) techniques. The IF results reveal three primary peaks, designated as P₁, P₂, and P₃, where the P₁ and P₂ correspond to C-Snoek peaks influenced by Mn, Ni, and Si atoms, while the P₃ represents GB relaxation modulated by the content of C-MSPs precipitated at GBs. During short-term aging (≤ 5 h), the height of the P₁ decreases consistently, whereas the P₂ initially increases and then decreases, indicating that C atoms initially interact with substituent atoms (Me=Mn, Ni, and Si) to form C-Me pairs and subsequently co-precipitate as C-MSPs at GBs, with solute atoms aggregation and precipitation occurring concurrently over an extended period (≤ 100 h). The height of the P₃ decreases with increasing aging time and temperature, reflecting a gradual increase in C-MSPs content at GBs, consistent with TEM and SAS observations. Activation energies of 0.87 and 1.30 eV for solute aggregation and precipitation of C-MSPs, respectively, suggest that the growth of C-MSPs is predominantly governed by GB diffusion. Furthermore, a schematic diagram illustrating solute atom migration, C-MSPs nucleation, and growth, along with a time-temperature-transformation (TTT) diagram for C-MSPs evolution is provided. This work portrays the landscape map of the microstructural evolution of RPV steels during the early service period, offering valuable insights for guiding the compositional design of RPV steels.

Key words: Internal friction, RPV steel, C-bearing Mn-Si-rich precipitates, GB relaxation, Snoek relaxation