Transmission electron microscopy study of the synergistic effects of Mn, Ni and Si on neutron irradiated RPV model steels

The microstructure of several reactor pressure vessel (RPV) model steels with varying contents of Ni, Mn and Si was examined using Transmission Electron Microscopy (TEM) before and after neutron irradiation conducted within the Lyra-10 program, up to 0.17 dpa at 286 °C. This study provides new insights into the synergistic effects of these alloying elements on irradiation-induced microstructural, particularly the density, size and distribution of dislocation loops. The results reveal a marked decrease in the density of visible dislocation loops with increasing Ni and Mn content, while the average loop size remains unaffected across all compositions. The effect of Mn seems to be more acute in the decrease of dislocation loop density and the trend is not influenced by variations in Si content. The observed reduction in dislocation loop density is hypothesized to be related to the formation of Mn-Ni-Si-rich solute clusters, which likely compete with loops for point defects or nucleate directly on them, hindering their growth. As a result, many dislocation loops fall below the resolution limit of TEM. These findings, supported by complementary techniques such as Atom Probe Tomography and Small-Angle Neutron Scattering, highlight the critical role of solute content in controlling irradiation-induced microstructures in the studied RPV steels.