Elucidating the Oxidation Mechanisms in Cr₂AlB₂ Using Density Functional Theory and Thermodynamic Modeling

We thoroughly investigated the oxidation behavior of Cr2AlB2 surfaces using a combination of density functional theory and thermodynamic modeling. Our analysis encompassed surface stability assessments, oxygen adsorption studies on various surfaces, thermodynamic insights into oxidation mechanisms, defect formation energetics as a function of distance to the surface layer and Al diffusion toward the surface layer. The (010) surface terminated with an Al layer exhibits the lowest surface energy among considered surfaces. O-2 is dissociatively adsorbed on all considered surfaces, with strong binding energies. Elevated temperatures reduce the favorability of oxidation, aligning with thermodynamics, and this trend holds consistently across different Cr2AlB2 surfaces. Low coverage surfaces favor oxidation and increasing pressure enhances oxidation, providing practical control strategies for O-2 partial pressure and temperature equilibrium. We also investigated vacancy formation energies and migration barriers, emphasizing the pivotal role of Al vacancy diffusion in initiating oxide formation. The kinetics of Al diffusion toward surface in Cr2AlB2 is found to be sluggish compared to MoAlB (1.45 versus 0.25 eV), where the Al diffusion is more facile due to the existence of two layers of Al between metal-B layers in the latter. This suggests that the formation of Al2O3 scale might necessitate higher temperatures for its occurrence, given that the oxide formation is not fueled by Al diffusion from the interior at lower temperatures. Analysis of vacancy formation energies unveiled surface-dependent variations due to distinct atomic arrangements, with defects forming more readily on the surfaces. These variations certainly influence the oxidation process and products formed. Our study provides valuable computational insights into corrosion processes in Cr2AlB2 and establishes a robust framework for understanding oxidation mechanisms in Al containing materials.