Contribution of dislocation loop to radiation-hardening of RPV steels studied by STEM/EDS with surveillance test pieces

Microstructure and radiation-hardening of surveillance test pieces of reactor pressure vessel steels have been studied by STEM/EDS and Vickers micro-hardness measurements. In a low-Cu steel (0.06 wt%), the microstructures irradiated with fluence of 5.62 × 10²³ n/m² were found to include nano-size Ni–Si-rich solute clusters (SCs) and dislocation loops (DLs). Most of the DLs were sustained after post-irradiation annealing at 500 °C for 0.5 h, while the number density of SCs decreased. The stability of the DLs is suggestive of entanglement with solute atoms. On the other hand, Cu-rich SCs were observed in high-Cu steels (0.17 wt%) irradiated with fluence of 1.01 × 10²³ n/m² and 6.11 × 10²³ n/m², however, no DLs observed. The Cu content in the matrix seems to be crucial for the nucleation of the DLs, which suggests that the mobility of the vacancy coupled with Cu atom dominates the nucleation/annihilation of self-interstitial atom. The contribution of DLs and SCs are discussed on the basis of the Russell-Brown and Orowan models, respectively.