Preparation, mechanical, and thermal properties of CrNbO₄: A novel dual-functional scale to protect RHEAs from oxidation and thermal attack

Cr–Nb-containing refractory high-entropy alloys (RHEAs) have high strength above 1200 °C but low density close to that of Ti-based alloys, which makes them promising for application in aero engines. However, oxidation is the bottleneck that limits their practical application. Recently, CrNbO₄ has been found to effectively protect them from oxidation. Nevertheless, little is known about this oxide. To elucidate the protection mechanism of CrNbO₄ and explore its properties, we report for the first time the microstructure, mechanical, and thermal properties of CrNbO₄. Using atomic-resolution high-annular dark field (HAADF) and annular bright field (ABF) techniques, we confirmed the rutile-type structure of CrNbO₄, identified the precipitation of Cr₂O₃, and observed Cr segregation at the interface boundary between CrNbO₄ and Cr₂O₃. The Young’s modulus (E), shear modulus (G), and bulk modulus (B) of CrNbO₄ are 253, 100, and 180 GPa, respectively, whereas the Vickers hardness (H_V), flexural strength (σ_f), and fracture toughness (K_IC) of CrNbO₄ are 10.2±0.58 GPa, 205±8 MPa, and 1.54±0.12 MPa·m^1/2, respectively. The measured melting point of CrNbO₄ is 2053±20 K. The anisotropic thermal expansion coefficient (TEC) is α_a = (5.38±0.09)×10⁻⁶ K⁻¹, α_c = (7.44±0.14)×10⁻⁶ K⁻¹, and the average TEC is (6.07±0.12)×10⁻⁶ K⁻¹, which is close to that of refractory metals and RHEAs. Interestingly, the room temperature thermal conductivity of CrNbO₄ is 1.09 W·m⁻¹·K⁻¹ and decreases to 0.45 W·m⁻¹·K⁻¹ at 1473 K, which is lower than that of most of the currently well-known thermal insulation materials. Consequently, CrNbO₄ can be considered a novel dual-functional scale on top of RHEAs to protect them from oxidation and thermal attack.