Interface Energies for Carbide Precipitates in TiAl

Carbon has been extensively investigated as an additive to TiAl alloys to improve high-temperature mechanical properties. The ternary-carbide-precipitate phases that have received most attention are Ti₃AlC (cubic perovskite), and Ti₂AlC (hexagonal). The perovskite (P) precipitates in the form of needles along the 〈001〉 direction of γ-TiAl, and the hexagonal phase (H) primarily forms platelets perpendicular to 〈111〉 directions. Using the first-principles density functional theory code VASP, calculations of host-precipitate interface energies were performed for these two carbides. An atomic-scale formulation of the interface energy is adopted. Calculations for the coherent interfaces are performed to determine the preferred terminations and translation states. An approximate correction to the interface energy for the effect of misfit is applied to estimate the energy of incoherent interfaces. The relative interface properties of the P-type and H-type carbides are determined by, in addition to the misfit strains, (i) the stronger bonding of the P-type than the H-type precipitate with the host at the interface, and (ii) the more pronounced variations of the interatomic potentials as a function of parallel translation state across non-close-packed (100) P layers than those across close-packed (0001) H layers.