Grain boundary impurity segregation and neutron irradiation effects in ferritic alloys

Segregation of alloying and impurity elements to grain boundaries in ferritic steels and alloys is known to modify the mechanical properties. This paper considers segregation of such elements, in particular phosphorus and carbon, that occur in ferritic nuclear pressure vessel steels subject to neutron irradiation and temperature typical of that encountered in service. Models are presented that allow the prediction of equilibrium and non-equilibrium segregation of phosphorus to grain boundaries and also take into account synergistic interaction with carbon under various combinations of neutron-irradiation temperature. These are related to a wide range of experimental observations compiled from data in the literature for mainly phosphorus and carbon measured at grain boundaries in neutron-irradiated ferritic vessel steels and alloys. The predictions from the segregation models are compared with these experimental data. The discussion provides a rationalization for the apparent variability in the measured grain boundary phosphorus compositions and thereby fracture susceptibility for various nuclear pressure vessel ferritic steels.