Grain boundary segregation in solution-treated nimonic PE16 during neutron irradiation

Solute segregation at grain boundaries has been examined in solution-treated Nimonic PE16 after irradiation in EBR-II to fluences of 6 and 10 x 1022 n/cm2 (E > 0.1 MeV) in the temperature range 425-6500C. Energy-dispersive X-ray spectrometry shows grain boundary enrichment of Ni, Ti, Al and Si and depletion of Fe and Cr in samples irradiated at temperatures above 5100C. Electron microscope examinations further show that a continuous 30 nm thick layer of γ' forms at grain boundaries and at incoherent twin boundaries in samples irradiated at temperatures at and above 5100C and that antiphase boundaries are clearly revealed in superlattice dark-field images of the γ'. The thickness of the radiation-induced γ' layer at boundaries do not appear to be sensitive to irradiation temperature and neutron fluence. In addition to γ', M23C6 particles also precipitate at grain and at incoherent twin boundaries and the precipitate size depends on the irradiation temperature and the neutron fluence. The post-irradiation ductility degradation of this alloy at elevated temperatures is attributed to the brittle cleavage failure of the continuous γ' layer present at the grain boundaries. (orig.)