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Iron-based precipitation-strengthened superalloys are being considered for use as structural materials in fast breeder reactors (FBR's). Appropriate application of these materials requires an understanding of the microstructural stability of precipitation-strengthening phases in the radiation damage environment. Recent theories modeling precipitate stability in-reactor balance two competing effects: (1) radiation-enhanced coarsening and (2) cascade-induced recoil or disordering dissolution. A direct consequence of these theories is the development of a stable precipitate particle size. Thus, it is predicted that microstructures developed at high neutron fluences will be insensitive to the microstructure prior to irradiation. Alloy properties such as swelling and creep should be unaffected by variations in the starting precipitate microstructure.
A test of these concepts is presented. Nimonic PE16, a γ' [Ni3(Al,Ti)] precipitate-strengthened alloy under consideration for fast reactor structural applications, has been reactor irradiated in three heat-treatment conditions: solution treated, aged, and overaged. After irradiation at 600°C to 5.4 × 1022 neutrons/cm2 (E > 0.1 MeV), or about 27 displacements per atom (dpa), specimens were characterized for ³' precipitate stability by transmission electron microscopy. The precipitate microstructures after irradiation were found to be very sensitive to heat treatment prior to irradiation. The precipitates present prior to irradiation remained stable. However, additional precipitation occurred in-reactor for each of the specimen conditions examined. The in-reactor ³ ' precipitation process decorated such microstructural features as voids, dislocations, and carbide precipitates. Therefore, it is apparent that a maximum stable precipitate particle size does not exist in this alloy under these irradiation conditions. It should be noted that 5.4 × 1022 n/cm2 represents only 25 percent of the targeted goal fluence for potential advanced FBR cladding and duct applications; however, these results can be considered to provide a realistic estimate of microstructural alterations to be expected at goal fluences.
It is apparent that the generally accepted models for precipitate stability during reactor irradiation do not successfully predict the experimental observations. Cascade-induced dissolution does not reduce the maximum stable precipitate particle size. Interpretations of early experimental results which indicated an invariant mean particle size after irradiation appear to be in error, because further precipitation in-reactor was not adequately considered. A more acceptable model for precipitate stability in-reactor must take cognizance of the fact that precipitate volume fractions change as a function of temperature, that solute segregation occurs at many microstructural features, probably in a dose-dependent manner, and that cascade dissolution of precipitates is not a dominant mechanism.
Nimonic PE16, gamma prime, precipitate stability, reactor irradiation, electron microscopy
Senior scientist, Hanford Engineering Development Laboratory, Westinghouse Hanford Company, Richland, Wash.