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

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 Fe₂Al_x 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 Fe₂Al_x phases (D_T ≤ 10^–11 m²·s^–1) to Fe₄Al₁₃ (D_T ≈ 10–12 m2·s–1), and finally in FeNiAl5 (D_T ≈ 10^–13 m²·s^–1). Here, we also find that interstitial tritium generally diffuses faster in Fe–Al coating phases than in the tritium breeding material γ-LiAlO₂ (D_T ≈ 10^–14 m²·s^–1) but slightly slower than in 316 stainless steel (D_T ≈ 10^–10 m²·s^–1).