Unveiling the Oxidation Mechanisms of High-entropy Carbides Through Atomic-scale Dynamic Observation.

Understanding the behavior of high-entropy carbides (HECs) under oxygen-containing environments is of particular importance for their promising applications in structural components, catalysis, and energy-related fields. Herein, the structural evolution of (Ta, Ti, Cr, Nb)C (HEC-1) nanoparticles (NPs) is tracked in situ during the oxidation at the atomic scale by using an open-cell environmental aberration-corrected scanning transmission electron microscope. Three key stages are clearly discerned during the oxidation of HEC-1 NPs at the atomic level below 900 °C: i) increased amorphization of HEC-1 NPs from 300 to 500 °C due to the energetically favorable formation of carbon vacancies and substitution of carbon with oxygen atoms; ii) nucleation and subsequent growth of locally ordered nanocluster intermediates within the generated amorphous oxides from 500 to 800 °C; and iii) final one-step crystallization of non-equimolar MeO and MeO (Me = metallic elements, Ta, Ti, Cr, and Nb) high-entropy oxides above 800 °C, accompanied with the reduction in atomic defects. This result is further confirmed by theoretical calculations that these observed high-entropy oxide phases are thermodynamically preferable to generate above 830 °C. The study provides direct evidence of the ordered-disordered-ordered structural transition of HECs during oxidation.