An effort is underway to explore the potential of Fe-Cr-Mn austenitic alloys as substitutes for Fe-Cr-Ni alloys in fusion reactors. This substitution is desired to achieve a reduced level of long-term radioactivity and thereby reduce the cost and safety hazards associated with disposal of reactor components at their end of life. Neutron-induced swelling data to 50 dpa are now available at 520°C for three reduced activation alloy series and for lower exposures at 420 and 600°C. These three series cover simple binary and ternary alloys, solute-modified ternaries, and a variety of commercially available Fe-Cr-Mn alloys. The swelling of these alloys is initially similar to that of Fe-Cr-Ni alloys but then changes in response to phase instabilities that develop in the Fe-Cr-Mn system.

Manganese-iron base and manganese-chromium-iron base austenitic alloys designed to have resistance to neutron irradiation induced swelling and low activation have the following compositions (in weight percent): 20 to 40 Mn; up to about 15 Cr; about 0.4 to about 3.0 Si; an austenite stabilizing element selected from C and N, alone or in combination with each other, and in an amount effective to substantially stabilize the austenite phase, but less than about 0.7 C, and less than about 0.3 N; up to about 2.5 V; up to about 0.1 P; up to about 0.01 B; up to about 3.0 Al; up to about 0.5 Ni; up to about 2.0 W; up to about 1.0 Ti; up to about 1.0 Ta; and with the remainder of the alloy being essentially iron.

It appears that the swelling of Fe-Cr-Mn alloys is remarkably insensitive to both irradiation temperature (420 to 600°C) and composition. The slight dependence of macroscopic swelling on manganese content is thought to be primarily the consequence of a composition-dependent densification, possibly associated with radiation-induced spinodal decomposition in the Fe-Cr-Mn Invar regime. The steady-state swelling rate of these alloys appears to be approx. 1%/dpa.

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

Whereas in earlier publications it appeared that a possible saturation of neutron-induced void swelling might be developing at temperatures of 400 and 427°C in annealed Fe-Cr-Ni model alloys in a manner that was dependent on composition, data obtained at higher neutron exposures in EBR-II have led to the conclusion that such an expectation was erroneous. At all temperatures in the range 400-650°C, the neutron induced swelling eventually reaches ~1%/dpa and continues to very high swelling levels thereafter without any hint of saturation.

The radiation-induced swelling of simple Fe-Cr-Ni austenitic alloys is known to be very sensitive to solute additions. It is shown in this paper that three of the most common solute elements (P,Si,Mo) exert a very complex and often non-monotonic influence on swelling with increasing solute level. The complexity of this influence and its dependence on other variables appears to be the result of a closely balanced competition between two or more roles played by each solute in its interaction with both vacancies and interstitials. This competition yields a variety of different swelling behaviors in response to changes in solute or solvent composition, displacement rate, and irradiation temperature.

The radiation-induced swelling of simple Fe-Cr-Ni austenitic alloys is sensitive to solute additions. It is shown in this paper that three of the most common solute elements (P, Si, Mo) exert a very complex and often non-monotonic influence on swelling with increasing solute level. The complexity of this influence and its dependence on other variables appears to be the result of a closely balanced competition between two or more roles played by each solute in its interaction with both vacancies and interstitials. This competition yields a variety of different swelling behaviors in response to changes in solute or solvent composition, displacement rate, and irradiation temperature.