Kinetics and oxide morphology of chromium–tantalate formation on a model alloy Cr-20Ta in low oxygen partial pressure

Abstract

This study investigates the oxidation behavior of a binary Cr-20 at.%Ta alloy in a nitrogen-free, reduced oxygen partial pressure environment at 1000 °C for 48 h, aiming to clarify the intrinsic formation and growth mechanisms of protective (Cr, Ta)O₂ oxides. Chromium outward diffusion primarily governs oxidation, leading to a duplex scale with an outer Cr₂O₃ layer and an inner (Cr,Ta)O₂ subscale. Two distinct (Cr, Ta)O₂ phases were identified: CrTaO₄ (rutile structure) in outer regions and CrTa₂O₆ (trirutile structure) closer to the substrate, with CrTa₂O₆ confirmed as thermodynamically more stable through post-oxidation heat treatment and calculations. Thermogravimetric analysis revealed the parabolic oxidation constant of Cr-20 at.%Ta for fine-grained samples was eight times lower than pure chromium, highlighting the beneficial effect of the (Cr, Ta)O₂ layer. The microstructure significantly influences the protectiveness: fine-grained alloys promoted a continuous (Cr, Ta)O₂ layer, leading to enhanced oxidation resistance, particularly after a transient period required for the protective subscale to establish. This research underscores the critical role of (Cr, Ta)O₂ and microstructure in developing advanced oxidation-resistant refractory alloys.