Contributions of Ni-content and irradiation temperature to the kinetic of solute cluster formation and consequences on the hardening of VVER materials

Abstract

The study of solute clusters induced by neutron irradiation is fundamental to the understanding of the embrittlement phenomena of Reactor Pressure Vessel (RPV) steels. In this paper, the influence of Ni-content and irradiation temperature on these cluster formation are investigated using atom probe tomography.VVER 1000-BM (base metal) and 1000-W (weld) steels were chosen for their high Ni content of 1.2 and 1.7 at%, respectively. They were irradiated up to doses of 0.28 dpa at two temperatures (265 and 300 °C). The irradiations were performed at SCK CEN in the BR2 reactor.After irradiation, Mn, Ni and Si rich clusters were observed. A careful study of their composition indicates that these clusters actually contain iron. Moreover, the evolution of Fe cluster concentration with the dose indicates a constant supply of solutes throughout the irradiation thanks to the flux coupling. With increasing dose, both the cluster size and number density increase.The effect of temperature has to be dissociated from that of Ni content. Both of them result in an increase of the cluster number density at a given dose. The irradiation temperature has a more pronounced effect than chemical composition (mainly Ni effect). This can be explained by the fact that more matrix damage is produced at lower irradiation temperature. The influence of Ni on cluster number density is significantly higher at the lower temperature. All these observations support the hypothesis of radiation-induced segregation, even if a thermodynamic contribution to solute clustering cannot be excluded, in particular in the higher Ni steel irradiated at low temperature.In all cases, the yield strength increases proportionally to the volume fraction of radiation-induced defects (√(Vf)).