Abstract
Pyrochlore Y2Ti2O7 is a primary precipitate phase in nano-structured ferritic alloys (NFAs) for fission and fusion energy applications. We report a theoretical study for assessing the relative stability of trapping helium in Y2Ti2O7 versus in matrix iron. Various defect structures and the associated energies are examined and compared. Results reveal that helium can be deeply trapped in Y2Ti2O7 and that the corresponding self-interaction is essentially repulsive. Transmutant helium in NFAs prefers to occupy individual octa-interstitial sites in Y2Ti2O7, before forming small clusters in Y2Ti2O7. Helium partitioning in NFAs depends on the number and dispersion of Y2Ti2O7; and thus initially, bubble formation and growth in iron matrix can be largely suppressed. Charge transfer occurs from helium to neighboring oxygen anions, but not to neighboring metal cations, suggesting a general effectiveness of trapping helium in oxides. Reasons for the ultimate fate of helium to form small nm-scale interface bubbles are also discussed.
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