Synthesis of atom probe experiments on irradiation-induced solute segregation in French ferritic pressure vessel steels

Microstructural changes due to neutron irradiation cause an evolution of th e mechanical properties of reactor pressure vessels (RPV) steels. This pape r aims at identifying and characterising the microstructural changes which have been found to be responsible in part for the observed embrittlement. T his intensive work relies principally on an atom probe (AP) study of a low Cu-level French RPV steel (Chooz A). This material has been irradiated in i n-service conditions for 0-16 years in the frame of the surveillance progra m. Under this aging condition, solute clustering occurs (Cu, Ni, Mn, Si, P, . . .). In order to identify the role of copper, experiments were also car ried out on Fe-Cu model alloys submitted to different types of irradiations (neutron, electron, ion). Cu-cluster nucleation appears to be directly rel ated to the presence of displacement cascades during neutron (ion) irradiat ion. The operating basic physical process is not clearly identified yet. A recovery of the mechanical properties of the irradiated material can be ach ieved by annealing treatments (20 h at 450 degrees C in the case of the RPV steel under study, following microhardness measurements). It has been show n that the corresponding microstructural evolution was a rapid dissolution of the high number density of irradiation-induced solute clusters and the p recipitation of a very low number density of Cu-rich particles. (C) 2000 El sevier Science B.V. All rights reserved.