SURFACE STABILITES AND HELIUM TRAPPING OF NANO-SIZED OXIDE PHASES IN NANO-STRUCTURED FERRITIC ALLOYS: A FIRST PRINCIPLES STUDY

摘要

Nano-sized Y_2Ti_2O_7 and Y_2TiO_5 are primary precipitates in nano-structured ferritic alloys (NFAs). We report a first-principles study on the stabilities of polar Y_2Ti_2O_7 surfaces, and also the selective helium trapping in NFAs. Results show that (1) all variants of Y_2Ti_2O_7 (110) surfaces were found to be more stable than the (100) surfaces, and non-stoichiometric Y_2Ti_2O_7 (110) surfaces are always more stable than their stoichiometric counterparts. (2) The radiation induced helium prefers the octa-interstitial sites in Y_2Ti_2O_7 (or the open-channel along the b-axis of Y_2TiO_5) to vacancies in the iron matrix. (3) The He-He interaction in Y_2Ti_2O_7 is essentially repulsive. Helium dispersion would largely depend on the number and dispersion of these nano-oxides in NFAs. (4) Charge transfer occurs only between helium and neighboring oxygen anions, suggesting the generality of trapping helium in oxides. Helium trapping at the Fe(100) /Y_2Ti_2O_7(100) interface were also investigated.