The Effect of Molybdenum on the Grain Boundary Segregation of Chromium Exhibited by Austenitic Stainless Steels after Neutron Irradiation in a BWR

摘要

Analytical investigations employing field-emission gun scanning transmission electron microscopy (FEGSTEM) have indicated that the degree of chromium depletion at grain boundaries is less pronounced in AISI type-316 when compared to type-304 stainless steel after neutron irradiation in Boiling Water Reactor (BWR) coditions to a fluence of 10~26 n.m~(-2) (E>1MeV).An explanation of this observation is proposed, which is based on the non-equilibrium segregation (NES) theory of grain boundary segregation. NES model predictions for the radiation-induced segregation of chromium and molybdenum in an austenitic matrix are presented together for the first time. The predicted segregation profiles are compared with the FEGSTEM data, whilst the predicted trends as a function of neutron dose are discussed in terms of possible microstructural evolution in a BWR environment. In particular it will be demonstrated how, according to NES theory, the binding of molybdenum to self-interstitial atoms (SIAs) will take precedence over chromium. In turn, the subsequent migration of molybdenum-SIA "complexes" retard the migration of chromium away from the grain boundary, thus moderating the magnitude of radiation-induced chromium depletion.