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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, 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 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 (E), shear modulus (G), and bulk modulus (B) of CrNbO4 are 253, 100, and 180 GPa, respectively, while the Vickers hardness (HV), flexural strength (σf), and fracture toughness (KIC) of CrNbO4 are 10.2±0.58 GPa, 205±8 MPa, and 1.54±0.12MPa·m1/2, respectively. The measured melting point of CrNbO4 is 2053±20 K. The anisotropic thermal expansion coefficients (TEC) are αa=(5.38±0.09)×10-6K-1,αc=(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.
Zhang et al. (Thu,) studied this question.