First-principles study on the oxidation of MoAlB and Cr₂AlB₂

MAB phases exhibit excellent oxidation resistance, attributed to the formation of a protective Al₂O₃ scale on their surfaces. Therefore, it is crucial to study the oxidation behavior of MAB compounds computationally to gain a deeper understanding of their oxidation mechanisms and further optimize their performance in high-temperature applications. The initial stage of the oxidation process of the lowest-energy MoAlB and Cr₂AlB₂ surfaces was investigated at different temperatures and oxygen concentrations by utilizing first-principles

calculations. Surface energies were calculated for the low-index surfaces. We found that different surfaces exhibited distinct defect energetics, leading to different oxidation mechanisms and products. Diffusion barriers were determined for Al vacancies and O atoms, showing that Al vacancies exhibit lower diffusion compared to the O atoms and play an important role in the oxidation process in MoAlB. However, in Cr₂AlB₂, Al vacancy exhibits sluggish kinetics. We calculated the Gibbs free energy of various reactions to uncover possible oxidation reactions for MoAlB and Cr₂AlB₂. We conducted molecular dynamics simulations at various temperatures and O₂ concentrations to further shed light on the oxidation process. Our study provides valuable insight into the stability, structural properties, and oxidation behavior of MoAlB and Cr₂AlB₂ surfaces at the atomic level.