Oscillatory Precipitation and Re-Dissolution of Mn-Ni-(Si)-Based Precipitates in Aged Reactor Pressure Vessel Model Steels

The irradiation-induced precipitation of Mn-Ni-rich precipitates (MNPs) or Mn-Ni-Si-rich precipitates (MNSPs) is the primary cause of embrittlement in reactor pressure vessel (RPV) steels. In this study, high-precision electrical resistivity (ER) measurements (10 nΩ·m accuracy) were employed to probe the thermal stability of aging-induced MNSPs in RPV model steels that were aged at 600 °C for 30 h. We report the discovery of oscillatory precipitation and re-dissolution of MNPs/MNSPs, evidenced by alternating ER peaks upon repeated thermal cycling to 950 °C. This oscillatory behavior is further confirmed by scanning transmission electron microscopy (STEM) and energy-dispersive X-ray spectroscopy (EDS) observations. Internal friction (IF) results indicate that the oscillatory precipitation and re-dissolution of MNPs/MNSPs should occur predominantly within the grain interiors rather than at grain boundaries (GBs).