This depends on the alloy composition (Cu, P, Ni, etc.), fluence, and irradiation temperature. Copper forms very fine (1-3 nm) coherent precipitates; nickel and manganese amplify the hardening effect of copper. Phosphorus segregates to the grain boundaries (contributes no more than 10-20% of the total effect) and the rest to the particle-matrix interfaces (this contribution is very important). Figure 6.32a and b shows the effect of copper content on the change of DBTT and upper shelf energy, respectively.

Recent advances in atom probe technique have positively identified the presence of these Cu — and P-enriched clusters. An atom probe map of a neutron-irradiated KS-01 weld is shown in Figure 6.33.