Design of an Alumina Forming Coating for Nb-Base Refractory Alloys

Refractory multi-principal element alloys (RMPEAs) promise to significantly enhance operating temperatures in advanced propulsion systems, but their poor oxidation performance inhibits their implementation. Alumina-forming bond coat alloys can provide oxidation protection, but discovering suitable chemistries remains a challenge. We employed a design methodology that screens for alumina-formation capability using Al activity and phase constitution predictions from CalPhaD (Thermo-Calc). Alloy down-selection from approximately 7,800 alloys in the Nb-Si-Ti-Al-Hf system was conducted by visualizing calculated thermodynamic properties through number-density topology style maps. We validate our approach by creating and testing the composition Nb₁₂Si₂₃Ti₂₄Al₃₆Hf₅, which forms protective alumina scales up to 1400°C and resists pesting at 800°C. Further, the alloy has an average coefficient of thermal expansion of ~10.1 ppm/K, making it well matched to refractory alloys.