Ab Initio Simulations of Tritium Diffusion in Al-Rich Iron Aluminide Coating Phases

Abstract

Surface coatings of steels used in extreme conditions and corrosive environments generally aim to provide protection and increased durability. In the case of tritium-producing burnable absorber rods (TPBARs) used in nuclear reactors, a 316 stainless steel has been coated with Al. Scanning transmission electron microscopy (STEM) characterization of the coating found three Al-rich (>60 atom % Al) iron aluminide alloys identified as hexagonal FeNiAl5, monoclinic Fe4Al13, and orthorhombic Fe2Al5. Density functional theory simulations using nudged elastic band have been performed to investigate the diffusion of interstitial tritium in each Al-rich iron aluminide phase. While FeNiAl5 and Fe4Al13 can be viewed as the stacking of two layers, the structural peculiarity of Fe2Al5 is that channels of variable Al vacancy content are present along the c-axis. Therefore, three stoichiometries for Fe2Alx phase, namely, Fe2Al4, Fe2Al5, and Fe2Al6, have been considered to evaluate the impact of Al vacancy concentration on tritium diffusion behavior. Altogether, we found that at 600 K, tritium diffusion decreases from a faster rate in the channels of Fe2Alx phases (DT ≤ 10–11 m2·s–1) to Fe4Al13 (DT ≈ 10–12 m2·s–1), and finally in FeNiAl5 (DT ≈ 10–13 m2·s–1). We also find that interstitial tritium generally diffuses faster in Fe–Al coating phases than in the tritium breeding material γ-LiAlO2 (DT ≈ 10–14 m2·s–1) but slightly slower than in 316 stainless steel (DT ≈ 10–10 m2·s–1).