Ta site occupation in O-Ti₂AlNb phase: A synergistic study combining first-principles calculations and transmission electron microscopy

The site occupancy behavior of Ta atoms in O-Ti₂AlNb phase was investigated through integrated first-principles calculations and experimental validation. A distinct hierarchical site occupancy preference of Ta was revealed: Ta preferentially occupies Nb sites, secondarily substitutes Ti sites, while exhibiting complete absence from Al sites. Such a phenomenon can be attributed to the atomic size compatibility, electronegativity similarity, and equivalent valence electron configuration between Ta and Nb atoms, consistent with the Hume-Rothery solid solution criteria, which stabilize the structure of Ti₂Al(Nb,Ta). Thermodynamically, the Ta-Nb substitution configuration exhibits enhanced stability over (Ti,Ta)₂AlNb and Ti₂(Al,Ta)Nb systems. This study provides critical theoretical guidance for designing advanced Ti₂AlNb-Ta alloys with optimized microstructural stability and properties.