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This work explores the effect of microstructure on the oxidation behavior of an equiatomic TaTiCr RCCA after 24 h of exposure at 800°C, 1000°C, 1200°C, and 1400°C. Two microstructural conditions, both containing a bcc matrix with C15 Laves precipitates, with one condition having coarser precipitates (∼10.9 µm diameter) and one condition having finer precipitates (∼2.9 µm diameter) were studied. At all oxidation temperatures except 800°C, the finer-scale (TaTiCr-F) condition experienced more rapid oxidation kinetics, higher mass gains, and thicker oxide scales and internal reaction zones. However, at 800°C, the microstructure containing coarser precipitates (TaTiCr-C) formed a thicker oxide scale. Continuous, protective Cr2O3 layers were only observed in the coarse precipitate condition and correlations between Cr2O3 layer thickness, subsequent formation of complex refractory oxides, and the initial Laves precipitate size are described. It is proposed that the formation of continuous Cr2O3 is highly dependent on the size and distribution of the Cr-rich Laves phase and only mildly dependent on phase fraction. Oxidation mechanisms for each condition are discussed relative to the initial microstructures, observed oxide species, and related alloy systems. 

This study explores the oxidation behavior of a TaTiCr refractory complex concentrated alloy (RCCA) in the temperature regime of 800–1400 °C. The oxidation kinetics were found to be proportional with temperature. However, a mechanistic shift from linear oxidation kinetics at 800 °C and 1000 °C to sub-parabolic kinetics at 1200 °C and 1400 °C was observed. This was attributed to the establishment of continuous external scales of TiO2 and Cr2O3, with fairly continuous underlying complex oxide layers at the higher temperatures. While there were morphological differences in the oxide scales depending on exposure temperature, the oxide species were all similar, primarily consisting of an outer layer of TiO2, an intermediate layer of Cr2O3, and an inner layer of rutile-structured (Cr,Ta,Ti)O2. Internal nitridation was observed in all cases, with a severe degree of internal reaction at 1400 °C, reaching a depth of 750 μm. Overall, the best performance was observed at 1200 °C, where a favorable balance of kinetics and thermodynamics promoted the most continuous oxide layers, thereby enhancing oxidation resistance.