THE EFFECT OF ALLOY COMPOSITION ON RADIATION-INDUCED SEGREGATION IN FE-CR-NI ALLOYS

The effect of alloy composition on radiation-induced segregation (RIS) was investigated in austenitic iron-base and nickel-base alloys using proton irradiation. Specifically, RIS was studied by irradiation of N i-18Cr, Ni-18Cr-9Fe, and Fe-20Cr-9Ni over a dose range of 0 to 1.0 dpa and a temperature range of 200 to 500 degrees C. Grain boundary compo sition was measured using Auger electron spectroscopy and scanning tra nsmission electron microscopy with energy dispersive X-ray spectroscop y. Measurements from this study along with measurements from Fe-16Cr-2 4Ni, Fe-20Cr-24Ni, Fe-2rCr-24Ni, and Fe-24Cr-19Ni alloys irradiated wi th protons confirm that RIS is strongly dependent on the alloy composi tion. Trends in segregation behavior in Fe-base alloys are consistent with high temperature diffusion measurements, indicating that a vacanc y mechanism is the most likely primary driving force for RIS in austen itic steels. The migration energy for Cr is shown to be larger than th e migration energy of Fe, Segregation measurements in Ni-base alloys a re not consistent with high temperature diffusion measurements, indica ting that ordering forces may be significant in the segregation proces s, Comparison of model calculations to measured RIS data indicate that Fe, Cr, and Ni diffusivities axe composition dependent. This dependen ce on alloy composition limits the predictive ability of simple models because of the need for separate diffusion parameters for every alloy composition.