Nature of defect clusters in neutron-irradiated iron-based alloys deduced from small-angle neutron scattering

The nature of the defect-solute clusters and/or atmospheres responsible for the irradiation-induced degradation of the mechanical properties of reactor pressure vessel steels is still subject of debate. It is therefore helpful to study model alloys, where the diversity of those features is constricted due to composition and where dominant effects can be identified or even isolated. The present investigation is focussed on Fe-based model alloys with intentionally varied Cu levels. The aim is to interpret deviations of the measured ratio of magnetic and nuclear scattering cross sections from the ratios calculated for pure vacancy clusters and pure Cu clusters. For the case of the low-Cu alloy the SANS results suggest the average scatterer to be an Fe-Cu-vacancy cluster of about 1 nm radius the composition of which is constricted according to given inequalities. For the case of the Cu-enriched alloy the SANS results are consistent with Cu-rich clusters of about 1.5 nm radius containing 15% vacancies per bcc lattice site.