Helium Interaction with Y 2 Ti 2 O 7 : A First Principles Study
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Helium Interaction with Y2Ti2O7: A First Principles Study Thomas Danielson1 and Celine Hin1,2 Virginia Polytechnic Institute and State University, Department of Materials Science and Engineering1, Department of Mechanical Engineering2, Blacksburg, VA 24060, U.S.A. ABSTRACT Helium embrittlement poses a great threat to materials used in both fusion and fission reactor systems due to (n,𝛼) transmutation reactions. Because of this, materials capable of moderating the helium content reaching grain boundaries and voids must be developed and improved to prevent catastrophic failure of reactor materials. Nanostructured ferritic alloys (NFAs) have shown great promise in preventing helium embrittlement due to the large number density of nanoscale precipitates acting as trapping sites for helium clusters and helium bubbles. In this study, we present density functional theory calculations on the interaction of helium with nanoscale precipitates found in NFAs as a preliminary study to furthering our understanding of the energetic mechanisms causing the precipitates to act as trapping sites for helium. INTRODUCTION NFAs are strong candidates for generation IV and future fusion reactor systems due to the desirable mechanical properties, which allow them to stand up to the high neutron flux, high temperatures and high pressures present in nuclear reactor systems [1]. However, helium, created by (n,𝛼) reactions, still remains a threat for NFAs due to its insolubility. Even very low concentrations of He are capable of causing embrittlement, void swelling and blistering posing catastrophic consequences if the nucleation and growth of He bubbles is not significantly moderated. Prior simulations have shown that He diffuses through the BCC Fe matrix with a very low migration barrier, reaching dislocation networks and further diffusing to grain boundaries, which are high-risk nucleation sites for He embrittlement [2,3]. NFAs have large number densities of complex oxide nanoclusters including, Y2Ti2O7, Y2O3, and Y2TiO5, which have the ability to trap He atoms at their surface, preventing He atoms from agglomerating at the grain boundaries. Because NFAs contain large number densities of nanoscale precipitates, the path of He diffusing through the matrix and along pinned dislocations intersects with the complex oxide precipitates, leaving He clusters and larger bubbles trapped at the precipitate surfaces. Helium implanter transmission electron microscopy studies have shown the surface of precipitates nearly completely covered by He bubbles, with the total percentage of bubbles reaching grain boundaries decreased by around 50% [4-6]. This is a strong indication that the moderation of He diffusing from the iron matrix to grain boundaries is significantly improved by the presence of the nanoscale precipitates. We investigate the stability of interstitial He in the Y2Ti2O7 lattice, and the resulting changes in the electronic structure using density functional theory (DFT) in order to gain insight on the interactio
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