Enhanced Oxidation Resistance of Y-Doped High-Entropy Carbide Films via Oxidation Formation of Amorphous and...

Outstanding oxidation resistance is crucial for widely applying high-entropy carbides (HEC) under high-temperature conditions. However, due to the preferential oxidation, the traditional oxidation resistance strategies depending on generating a dense binary oxide layer present numerous limitations. In this work, we propose a rare earth Y-doping approach to enhance the oxidation resistance of (Ti, Cr, Nb, Al, Y) C HEC and obtain the optimal doping amount of 3.5 at % Y. Leveraging in situ high-temperature XRD, aberration-corrected scanning transmission electron microscopy, and first-principles calculations, we clarify the oxidation products at different oxidation stages and establish the oxidation resistance mechanism. In the initial oxidation stage (below 600 °C), the HEC maintains the original face-centered cubic (FCC) structure and presents no obvious oxidation due to its exceptional thermal stability. As the temperature rises to 600-700 °C of the interim oxidation stage, the FCC lattice collapses to form an amorphous oxide layer, which acts as a great barrier for oxygen diffusion to impede further oxidation. When it comes to the later stages (above 750 °C), as evidenced by the atomic image and the calculated Gibbs formation free energy, the metastable amorphous oxide transforms into a single-phase nanocrystalline high-entropy oxide (HEO) layer. Due to the single homogeneous structure and slow grain growth of the nanocrystalline, the HEO layer can maintain its compactness and provide robust oxidation resistance for the HEC.