Role of Ni on Cu-rich cluster formation in neutron irradiated reactor pressure vessel steels

The role of Ni on formation of Cu-rich clusters (CRCs) in neutron irradiated reactor pressure vessel steels is studied. The Ni concentration dependence of the size, the number density, and the chemical composition of the CRCs, as well as the residual Cu concentrations in matrix, are estimated by using atom probe tomography. Very low Ni samples exhibit a much lower CRC number density than higher Ni samples, while containing a significantly larger number of Cu atoms in the CRCs, which decrease drastically with increasing Ni content. These results are interpreted in terms of the reduction of the critical nucleation energy of the CRCs in the presence of Ni. In addition, the presence of Ni causes the Cu cores in the CRCs to become more diffused by mixing other solute elements. It is also found that the residual Cu concentrations in the very low Ni samples decrease even at low irradiation fluence compared with those in the higher Ni samples, indicating that the kinetics of the CRC formation under neutron irradiation are much faster in the very low Ni samples than those in the higher Ni samples. This phenomenon is partly attributed to the differences in the behavior of the irradiation-induced interstitial atoms in addition to well-known vacancy trapping by solute; the presence of Ni suppresses the radiation-enhanced diffusion of Cu due to the suppression of the interstitial migration by forming interstitial-solutes complexes including Ni. (233 words)