MICROSTRUCTURAL EVOLUTION OF NEUTRON-IRRADIATED FE-CR-NI TERNARY ALLOYS AT HIGH HELIUM GENERATION RATES

A series of Fe-Cr-Ni ternary alloys were irradiated to 12-14 dpa in th e Oak Ridge Research Reactor (ORR). In this experiment, most of the he lium atoms were produced by the Ni-58(n, gamma)Ni-59(n, alpha)Fe-56 se quential reaction, and, therefore, the helium generation rate was almo st proportional to the nickel content, ranging from 27 to 58 appm He/d pa in 20 to 45 wt% Ni alloys. Irradiation temperatures were 330, 400, 500 and 600-degrees-C, and the specimen variables were the nickel leve l, the chromium level and cold-working. The high levels of helium, com bined with details of the temperature history, were found to have domi nated to microstructural development, sometimes overwhelming and even reversing the action of a given material variable. Microscopy examinat ion showed that there was an unprecedented refinement of cavity micros tructure at all temperatures, which accounts for the substantial alter ations in both swelling and mechanical properties observed in earlier studies.