Designing NbTiTa-Cr/Al refractory complex concentrated alloys with balanced strength-ductility-oxidation resistance properties: Insights into oxidation mechanisms

Limited tensile ductility, poor oxidation resistance, and the ductility-oxidation resistance trade-off are key bottlenecks besetting refractory complex concentrated alloys (RCCAs). Here, in this study, by alloying ductile NbTiTa RCCAs with low content of Cr or Al, we design two novel single-phase RCCAs Nb₄₀Ti₄₀Ta₁₀Cr₁₀ (Cr10) and Nb₄₅Ti₃₀Ta₁₅Al₁₀ (Al10). These two alloys demonstrate excellent tensile strength-ductility combination (∼800 MPa, >20%) and the best oxidation resistance at 800 °C (<7.2 mg/cm² after 20 h exposure) among all tensile-ductile RCCAs reported by far. Cr10 exhibits much slower oxidation kinetics compared with Al10, attributed to i) the dominant rutile-type Ti_1–2x(Nb,Ta)_xCr_xO₂ oxide that has lower volume mismatch with the alloy matrix compared with (Nb,Ta)₂O₅, as indicated by low Pilling-Bedworth ratio (PBR) values of 1.03–1.76; and ii) the alternative layer structure consisting of dense rutile and porous mixed oxides that is conducive to prohibiting stress cumulation and cracking. A new thermodynamic model connecting the Gibbs free energy of formation of a related oxide to the dynamically varied alloy compositions was proposed and used to unveil the dynamic formation mechanisms of oxides in Cr10 and Al10. The special alternative layer structure, mixed-oxides structure, voids, and/or cracks featured by oxide scales of Cr10 and/or Al10 are well-understood by combining detailed scanning transmission electron microscopy (STEM) studies, thermodynamic simulations, and stress analyses based on PBR evaluations.