On the mechanism of radiation-induced segregation in austenitic Fe–Cr–Ni alloys

Abstract

The relative importance of the vacancy and interstitial contributions to radiation-induced segregation (RIS) in Fe–Cr–Ni alloys is studied to better understand the mechanisms causing changes in grain boundary composition and to improve the capability to predict RIS in austenitic Fe–Cr–Ni alloys. The primary driving mechanism for segregation in Fe–Cr–Ni alloys is shown to be the inverse Kirkendall (IK) mechanism, specifically the coupling between alloying elements and the vacancy flux. To study grain boundary segregation, seven alloys were irradiated with 3.2 MeV protons at temperatures from 200°C to 600°C and to doses from 0.1 to 3 dpa. Grain boundary compositions were measured using both Auger electron spectroscopy (AES) and scanning transmission electron microscopy with energy dispersive X-ray spectroscopy (STEM/EDS). Grain boundary compositions were compared to model predictions that assume segregation was driven either by preferential interaction of solute atoms with the vacancy flux alone or in combination with binding of undersized solutes to the interstitial flux. Calculations that assume the segregation is caused by preferential interaction of solute atoms with the vacancy flux generally followed the trends of the segregation measurements. However, the inclusion of interstitial binding to the IK model causes poor agreement between model predictions and segregation measurements, resulting in severe overprediction of Ni enrichment and Fe depletion. Comparisons of segregation models with RIS in alloys irradiated with neutrons also show that preferential interaction of solutes with the vacancy flux sufficiently describes segregation in Fe–Cr–Ni alloys.