Microstructural changes and solute segregation in austenitic Fe-Cr-Mn-Ni alloys have been studied after neutron irradiation to 25 dpa (maximum) in FFTF/MOTA. Voids were nucleated in all of the specimens in the temperature range of 420 to 650°C, and a swelling peak was observed to form at 550°C. The void suppression effect of nickel additions did not occur in these alloys. Precipitates were formed in the matrix and/or on grain boundaries and were mostly identified as M23C6. The composition in the grain boundary area changed, and the chromium (Cr) concentration near the precipitates became higher. On the other hand, manganese and chromium were depleted, and nickel was enriched in the grain boundary area, but without precipitation. Other phases such as ferrite and sigma could not be recognized. Thus, it was revealed that the addition of nickel to Fe-Cr-Mn alloy stabilizes the austenite even in the grain boundary area where segregation is marked.

Symposium held in Nashville, Tennessee, June 1990. Almost two-thirds of these 91 papers are authored by researchers outside of the US (including information on research in the former USSR, Japan, and Europe). Topics include: current commercial power reactor systems; microstructural characterization

The microstructural and microchemical evolution of Fe-15Cr-XNi and ISCr-85Ni austenitic alloys irradiated with fast neutrons at 510 and 538°C and heavy ions at 675°C was investigated to determine whether common mechanisms of nickel's influence on microstructural development existed under different irradiation conditions. A surprising commonality for these two types of irradiations was observed involving nickel-dependent void densities, dislocation and loop dominance of dislocation structures, and fluence dependence of swelling, void density, and solute segregation. Quantitative analyses of microstructure and microchemistry data snowed that the variation of dislocation sink strength with nickel addition may provide the major mechanisms by which swelling depends on nickel content. Other processes, such as nickel's influence on the mobility of vacancies and the different segregation behavior at sessile and mobile defects, also cooperate to produce the overall behavior.

The microstructure of Fe-Cu-Ni-P model pressure vessel steels after neutron irradiation and thermal aging has been characterized by atom probe field-ion microscopy and augmented by transmission electron microscopy. High densities of small, roughly spherical or disc shaped copper clusters/precipitates were observed in the neutron irradiated alloys that contained copper. Small spherical phosphorus clusters were observed in the irradiated copper-free alloys, and copper phosphides were observed in a high phosphorus Fe-Cu-Ni-P alloy. None of these clusters/precipitates were observed in the thermally aged materials. The increases in the tensile and yield strengths that were observed after neutron irradiation resulted from these clusters and other lattice defects.