Positron annihilation spectroscopy and small angle neutron scattering characterization of nanostructural features in high-nickel model reactor pressure vessel steels

Irradiation embrittlement in nuclear reactor pressure vessel steels results from the hardening by a high number density of nanometer scale features. In steels with more than ≈0.10% Cu, the dominant features are often Cu-rich precipitates typically alloyed with Mn, Ni and Si. At low-Cu and low-to-intermediate Ni levels, so-called matrix hardening features are believed to be vacancy-solute cluster complexes, or their remnants. However, Mn–Ni–Si rich precipitates, with Mn plus Ni contents greater than Cu, can form at high alloy Ni contents and are promoted at irradiation temperatures lower than the nominal 290 °C. Even at very low-Cu levels, late blooming Mn–Ni–Si rich precipitates are a significant concern due to their potential to form large volume fractions of hardening features. Positron annihilation spectroscopy (PAS) and small angle neutron scattering neutron (SANS) measurements were used to characterize the fine-scale microstructure in split-melt A533B steels with varying Ni and Cu contents, irradiated at selected conditions from 270 to 310 °C between ≈0.04 and 1.6 × 1023 n m−2. The objective was to assess the character, composition and magnetic properties of Cu-rich precipitates, as well as to gain insight on the matrix features. The results suggest that the irradiated very low-Cu and intermediate Ni steel contains small vacancy-Mn–Ni–Si cluster complexes, but not large, well-formed and highly enriched Mn–Ni–Si phases. The hardening features in steels containing 0.2% and 0.4% Cu, and 0.8% and 1.6% Ni are consistent with well-formed, non-magnetic Cu–Ni–Mn precipitates. The precipitate number densities and volume fractions increase, while their sizes decrease, with increasing Ni and decreasing irradiation temperature. The precipitates evolve with fluence in stages of nucleation, growth and limited coarsening.