Microsegregation observed in Fe-35.5Ni-7.5Cr irradiated in EBR-II

In another recent study of the compositional dependence of swelling in annealed Fe-Ni-Cr alloys, it was found that some annealed alloys densified during irradiation. No precipitate phases were observed to form however. At 5930C one alloy, Fe-35.5Ni-7.5Cr, which was particularly resistant to swelling, increased in density .9% at 7.6 x 1022 n/cm2 (E > 0.1 MeV). Examination by energy dispersive x-ray analysis revealed that substantial oscillations occur in the nickel content of the alloy, varying from 25 to 53% about the nominal level of 35.5%. These oscillations exhibit a period of approx. 200 nm. Regions enriched in nickel are depleted in chromium and iron, and the reverse is true in regions of low nickel content. This spinodal-like process produces a net densification and also appears to eventually destroy the swelling resistance of the alloy. Once voids form in the nickel-poor chromium-rich regions, further segregation of nickel to void surfaces is expected to accelerate the loss of swelling resistance