Atomic-Scale Insights into Carbon Dissolution in α-, γ-, and θ-Al2O3: Phase-dependent Transport Dynamics from First-Principles Calculations

α-Al₂O₃ exhibits superior carburizing corrosion resistance compared to metastable γ-Al₂O₃ and θ-Al₂O₃ phases in high-temperature CO₂ environments, yet its atomic-scale origins remain unclear. Using first-principles density functional theory, we systematically investigate carbon dissolution and diffusion in α-Al₂O₃, γ-Al₂O₃, and θ-Al₂O₃, including the effects of oxygen (O) and aluminum (Al) vacancies. Our results show that α-Al₂O₃ consistently exhibits higher carbon solution enthalpies than γ-Al₂O₃ and θ-Al₂O₃ in both pristine and defective structures, indicating lower intrinsic carbon solubility in α-Al₂O₃. Vacancies significantly enhance carbon incorporation: O vacancies reduce solution enthalpy, while Al vacancies further amplify this effect, with a strong preference for carbon at Al vacancy sites. Carbon diffusion barriers are also highest in α-Al₂O₃, reflecting slower carbon mobility. Al vacancies increase diffusion barriers across all phases, while O vacancies raise barriers in α- and γ-Al₂O₃ but slightly lower them in θ-Al₂O₃. These results reveal a dual mechanism behind the carburizing resistance of α-Al₂O₃: reduced carbon solubility and elevated diffusion barriers. This work provides atomic-scale insights to guide the design of alumina-based materials with improved carburizing resistance through phase selection and defect engineering.