Trapping helium in Y2Ti2O7 compared to in matrix iron: A first principles study

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.

Microstructure of HIPed and SPSed 9Cr-ODS steel and its effect on helium bubble formation

“…Once oxide-attached helium bubble grows larger than critical radius, the corresponding helium energy in bubble would be lower than in oxide [23]. Until then, helium atoms started to flow from internal of oxide to oxide-attached helium bubble and subsequently form to a larger bubble [23,36]. Thus, it could imagine that the final size of the bubble strongly depends on the size and number density of oxides.…”

“…It showed that helium atoms are more easily trapped at Y 2 Ti 2 O 7 oxide octahedral interstitial sites than FeeCr matrix sites due to lower corresponding energy (~0.94 eV/atom in Y 2 Ti 2 O 7 ,~2.34 eV/atom in matrix) [36]. At the beginning of helium implantation, initial helium could be deeply trapped inside the oxide.…”

“…A recent first-principles calculation implied that the size of helium bubbles in ODS steels can be managed by controlling the nano-oxides [36]. It showed that helium atoms are more easily trapped at Y 2 Ti 2 O 7 oxide octahedral interstitial sites than FeeCr matrix sites due to lower corresponding energy (~0.94 eV/atom in Y 2 Ti 2 O 7 ,~2.34 eV/atom in matrix) [36].…”

Effects of Cr and W additions on the stability and migration of He in bcc Fe: A first-principles study

“…Generally speaking, there exist plenty of vacancy defects in Fe host materials when subjected to irradiation, and He atoms tend to consume individual vacancy sites in Fe as much as possible [39]. In our previous work, we have already studied the vacancy effects on He behaviors including the vacancy-He interaction and vacancy trapping for multiple He atom in bcc Fe [12].…”

First principles assessment of helium trapping in Y2TiO5 in nano-featured ferritic alloys

“…25,17) and in iron 25,26 in Table II. We note that the most energetically favored site for helium in Y 2 TiO 5 is the open channel site (Type C), with E f ¼ 0.62 eV/atom.…”