Small-angle Al₂O₃ grain boundaries achieve ultralow oxygen diffusion

Al₂O₃ is extensively utilized as an oxidant barrier in aeroengines and gas turbines at elevated temperatures, and small-angle grain boundaries significantly reduce its oxygen permeation. However, experimentally observing and regulating these small-angle grain boundaries poses considerable challenges. In this study, we search for these small-angle Al₂O₃ grain boundaries via first-principles calculations. We identify two families of ultralow-oxygen-diffusion grain boundaries, namely, Gb(xxx)/(001) and Gb(xxx)/(110), whose diffusion rates are 2 to 4 orders of magnitude lower than those of conventional grain boundaries. On the basis of these findings, we summarize and analyze the influence mechanisms and relative importance of various factors affecting grain boundary permeation. Our results indicate that, in addition to the grain boundary angle, the average bond length and band gap play primary roles in impeding oxidant transport. Finally, the [0001] selective nucleation growth of alumina was achieved by stepwise sintering, and ultralow-oxygen-diffusion Gb(104)/(110) was successfully prepared, which was consistent with the simulation results. These findings provide valuable insights for the design of ultralow-oxygen-diffusion Al₂O₃ grain boundaries, thereby significantly enhancing the oxidation resistance of aluminum-containing superalloys.