Origin of enhancement of oxidation resistance of near β-Ti(Al) with addition of Ta: a combinatorial experimental and DFT analysis

Abstract

The study examines the effect of alloying tantalum (Ta) with the β-Ti(Al) solid solution on oxidation resistance at the temperatures of 1000 – 1200°C in air. Addition of 7 at.% Ta to β-Ti(Al) enhanced oxidation resistance by 97%. The oxide growth followed a linear growth mechanism in the initial stage, transitioning to a parabolic growth mode as time progressed. Transition from linear to parabolic growth occurred earlier as Ta content increased in the β-Ti(Al) phase. Both Ta-free and Ta-containing β-Ti(Al) formed an external oxide zone (EOZ) consisting of Ta-free rutile (TiO₂) and alumina (α-Al₂O₃). However, the Ta-containing β-Ti(Al) also developed an additional phase, β-Ta₂O₅, in the inward region of the EOZ along with the Ta-containing TiO₂. Presence of inert yttria markers at the oxide/air interface for the Ta-free β-Ti(Al) indicated that oxygen was the primary diffusing component contributing to the oxide growth. In contrast, markers inside the EOZ in the case of Ta-containing β-Ti(Al) suggested that both oxygen and metallic ions were involved in the oxide formation. Microstructural analysis by EBSD revealed columnar and equiaxed grains of TiO₂ growing on either side of the β-Ta₂O₅ grains in EOZ, further supporting the involvement of both oxygen and metallic ions in oxide growth. The enhanced oxidation resistance of Ta-containing β-Ti(Al) was attributed to a change in the defect concentration of TiO₂ as analysed via density functional theory (DFT). A Ti-Al-Ta-O phase diagram at 1200°C was established, and a comprehensive oxidation mechanism was proposed, integrating both experimental and computational insights.