Microchemistry and microstructure of proton-irradiated austenitic alloys: toward an understanding of irradiation effects in LWR core components

Abstract

Over 1200 measurements of grain boundary composition and microstructure have been made on 14 different austenitic Fe–Cr–Ni alloys following proton irradiation in the temperature range 200–600°C and in the dose range 0.1–3.0 dpa. From these data, a greater understanding of radiation induced segregation (RIS) and microstructure development has been gained. Grain boundary composition measurements revealed that Cr depletes at grain boundaries, Ni enriches and Fe can either enrich or deplete depending on alloy composition. Analysis of temperature and composition dependence of RIS revealed that the magnitude and direction of grain boundary segregation depends on alloy composition because the values of migration enthalpy of the alloy constituents are not the same, and diffusivities of the alloy constituents are composition-dependent. The dose dependence of segregation revealed ordering in Ni-base alloys and temperature dependence was used to show that RIS is consistent with a vacancy exchange mechanism. The dependence of segregation on composition is consistent with all known, relevant neutron data. RIS was found to be related to the development of the dislocation and void microstructures. Alloys in which the microstructure develops slower with dose also show slower changes in RIS. Similarly, it was shown that the dependence of swelling on composition is the same for neutron, ion and proton irradiation and all can be explained by the effect of RIS on defect diffusivity at the void nuclei. This paper illustrates the value of conducting carefully chosen irradiation experiments over several, well-controlled variables to elucidate the mechanisms underlying the microchemical and microstructural changes.