Effects of P on formation and growth of Mn-Ni-Si clusters in low-Cu reactor pressure vessel steel analyzed by atom probe tomography

The effects of P on the formation and growth of Mn-Ni-Si clusters in surveillance test specimens of a low-Cu reactor pressure vessel steel were analyzed using atom probe tomography. What appeared to be P clusters on the atom map were found in regions with low dislocation line densities at the initial stage of neutron irradiation. In contrast, P atoms were segregated to the dislocation lines in areas having high dislocation line densities, and few clusters were observed. What appeared to be P clusters acted as heterogeneous nucleation sites for the formation of Mn-Ni-Si clusters. Assessments of the neutron fluence dependence established that Mn-Ni-Si clusters enriched in P atoms were initially formed, following which Mn-Ni-Si clusters without enrichment of P atoms appeared. P was also found to promote the growth of Mn-Ni-Si clusters. What appeared to be P clusters at the initial stage of neutron irradiation on the atom map were probably P-decorated interstitial-type dislocation loops. The experimental results can be reasonably explained based on the assumption that fast-diffusing interstitial and P atom pairs are absorbed by dislocation lines in regions with high dislocation line densities, while these pairs are coalesced to from the P-decorated interstitial-type dislocation loops in areas with low dislocation line densities. Consequently, the Mn-Ni-Si clusters having higher P atom concentrations are believed to have formed complexes involving dislocation loops.