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Ziraloy-4, a zirconium base alloy used extensively as cladding and core structural materials in water-cooled nuclear reactors, was examined by transmission electron microscopy (TEM) after quench-and-aging treatments, neutron irradiation, and postirradiation annealing. Phase instabilities found after quench-and-aging treatments include: (1) formation of the metastable precipitate Zr4(Fe,Cr) during quench from the β-phase field and (2) dissolution of metastable Zr4(Fe,Cr) and formation of the stable precipitate Zr(Fe,Cr)2 during aging in the α-phase field after quench. Rapid precipitation of Zr(Fe,Cr)2 occurs on aging at 750°C. The initial crystallographic structure of the precipitate is cubic, but the hexagonal structure predominates after aging. Microhardness reflects the microstructural changes that occur during annealing, but nodular corrosion resistance remains excellent throughout, except at the extremes of annealing temperature and time.
Phase instabilities detected by transmission electron microscopy after irradiation include disappearance of the metastable precipitate Zr4(Fe,Cr) in the β-quenched matrix. A singlephase solid solution is apparently achieved in the irradiated, β-quenched matrix and is maintained to high fluences without new visible precipitation and segregation. Iron in the matrix precipitates out at grain boundaries as the Zeta-phase during postirradiation annealing at 560°C. Both chromium and iron precipitate out as Zr(Fe,Cr)2 intermetallics on annealing at 750°C. In addition to the phase instabilities, irradiation also causes c dislocation development.
Zircaloy-4, beta-quenched Zircaloy, annealing, neutron irradiation, precipi-tation, aging, corrosion, hardness, electron microscopy
Principal engineer, GE Knolls Atomic Power Laboratory, Schenectady, NY
Manager, Fuel Materials Technology, GE Nuclear Energy, Vallecitos Nuclear Center, Pleasanton, CA