First-principles study of oxygen diffusion in Ti/Zr/Ta-alloyed Hf6C5 carbides

The present work applied the first-principles calculations to investigate the diffusion behavior of oxygen atoms in Ti/Zr/Ta-alloyed Hf6C5 carbides. Two series of structural units, including Hf-Ti-based carbides (Hf4Ti2C5, Hf4Zr1Ti1C5, Hf3Zr2Ti1C5) and Hf-Ta-based carbides (Hf4Ta2C5, Hf4Zr1Ta1C5, and Hf3Zr2Ta1C5), were constructed to study the oxygen diffusion between two neighbored vacancies in C side. The substitution of Ti for the base Hf can decrease the diffusion activation energy of O from E-a = 6.889 eV in Hf6C5 to E-a = 6.828 eV in Hf4Ti2C5, resulting in poor oxidation resistance due to the decline in the charge accumulation between Hf and C or O. While the substitution of Zr-1 for the Ti can enhance the charge transfer among elements and then increase the activation energy of O diffusion (E-a = 7.110 eV) in Hf4Zr1Ti1C5, which means that the oxidation resistance is effectively improved. By contrast, the further addition of Zr in Hf3Zr2Ti1C5 reduces the activation energy of O diffusion, which is ascribed to the weak bonding between Zr/Hf and C/O. For Hf-Ta-based carbides, the variation of activation energies of O diffusion is similar to that in Hf-Ti-based carbides, in which the lower E-a values (6.231 similar to 7.087 eV) in Hf-Ta-based series indicates the relatively-weaker oxidation resistance than the Hf-Ti series.