Preparation, mechanical, and thermal properties of CrNbO ₄: A novel dual-functional scale to protect RHEAs from oxidation and thermal attack

Cr-Nb containing refractory high-entropy alloys (RHEAs) excel in high strength beyond 1200°C but low density close to Ti-based alloys, which endow them promising for applications in aero engines. However, oxidation is the bottleneck that limits their practical applications. Recently, CrNbO₄has 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 herein for the first time the microstructure, mechanical, and thermal properties of CrNbO₄. Using atomic-resolution high-annular dark field and annular bright field techniques, we confirmed the rutile-type structure of CrNbO₄, identified the precipitation of Cr₂O₃ and observed the Cr segregation at interface boundary between CrNbO₄ and Cr₂O₃. The Young's modulus (<em>E</em>), shear modulus (<em>G</em>), and bulk modulus (<em>B</em>) of CrNbO₄ are 253, 100, and 180 GPa, respectively, while the Vickers hardness (<em>H</em>_V), flexural strength (σ_f), and fracture toughness (<em>K</em>_IC) of CrNbO₄ are 10.2±0.58 GPa, 205±8 MPa, and 1.54±0.12MPa·m^1/2, respectively. The measured melting point of CrNbO₄ is 2053±20 K. The anisotropic thermal expansion coefficients (TEC) are α_a=(5.38±0.09)×10^-⁶K^-¹,α_c=(7.44±0.14)×10^-⁶K^-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 declines to 0.45 W·m^-1·K^-1at 1473 K, being lower than most of the currently well-known thermal insulation materials. Consequently, CrNbO₄ can be regarded as a novel dual functional scale on top of RHEAs to protect them from oxidation and thermal attack.