Synergistic effects of Mn, Ni, and Si on the formation of Mn-Ni-Si-rich clusters in reactor pressure vessel model steels irradiated at high neutron fluence

The bulk composition of reactor pressure vessel (RPV) steels influences the formation of embrittling Mn-Ni-Si (MNS)-rich clusters or precipitates under neutron irradiation. As part of the STRUMAT-LTO European project, this study examines the individual and synergistic effects of Mn, Ni, and Si concentrations on MNS-rich cluster formation at high neutron fluences using atom probe tomography. The results indicate that cluster number density increases with bulk Ni and Mn content, with the effect of each element being more pronounced in alloys containing higher amounts of the other -highlighting a Mn-Ni synergy. In contrast, Si has a negligible impact on cluster formation in the investigated composition ranges. Comparisons between experimental data and predictions from Thermo-Calc and the MILAGROS tool suggest that the formation of MNSrich clusters involves radiation-induced segregation via flux coupling effects. These findings provide new insights into the mechanisms governing solute clustering in irradiated RPV steels, with implications for long-term reactor operation and material performance.