Positron annihilation spectroscopy and small-angle neutron scattering characterization of the effect of Mn on the nanostructural features formed in irradiated Fe–Cu–Mn alloys

The size, number density, and composition of the nanometre-sized defects responsible for the hardening and embrittlement in irradiated Fe–0.9 wt% Cu and Fe–0.9 wt% Cu–1.0 wt% Mn model reactor pressure vessel alloys were measured using small-angle neutron scattering and positron annihilation spectroscopy. These alloys were irradiated at 290°C to relatively low neutron fluences (E > 1 MeV, 6.0 × 1020 to 4.0 × 1021 n m−2) in order to study the effect of manganese on the nucleation and growth of copper-rich precipitates and secondary defect features. Copper-rich precipitates were present in both alloys following irradiation. The effect of Mn was to reduce the size and increase the number density of precipitates in the Fe–Cu–Mn alloy relative to the Fe–Cu alloy. Vacancy clusters were observed in the Fe–Cu alloy, but not in the Fe–Cu–Mn alloy. These results suggest a strong effect of Mn on vacancy diffusion and clustering.