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The results of a theoretical and experimental study of the effect of ion irradiation on phase stability in austenitic Ni-C and Fe-Ni-C alloys are presented. Calculations modeling the precipitation of graphite or cementite from Ni-C or Fe-Ni-C alloys indicate that irradiation may induce precipitation of either phase in alloys that are thermally undersaturated in carbon. Graphite precipitation occurred readily in supersaturated thermally aged specimens. Although the theoretical calculations predict that rapid graphite nucleation may occur in a range of undersaturated and supersaturated alloys as a result of 4-MeV Ni2+ ion irradiation, transmission electron microscopy (TEM) studies reveal no indication of such precipitation. The irradiated alloys remained single phase even for one time-temperature cycle, which produced the beginning of graphite precipitation under purely thermal aging. We conclude that a trapping mechanism involving the redistribution of carbon from solid solution to radiation-induced dislocation loops, voids, and other traps, perhaps via a solute drag effect, operates in such a manner as to suppress the precipitation promoted by irradiation induced vacancies. The results of this study have important implications on the radiation stability of austenitic steels that lack strong carbide formers.
radiation effects, phase stability, precipitation (metals), nucleation (metals), Fe-Ni-C alloys, Ni-C alloys
Formerly, research assistant, Massachusetts Institute of Technology, Cambridge, MA
Professor, Massachusetts Institute of Technology, Cambridge, MA