Theory of interface properties for carbide precipitates in TiAl

Among the additives to TiAl alloys that have been investigated in recent years with the objective of improving high-temperature mechanical properties, particular attention has been given to carbon, which forms the carbide precipitates Ti₃AlC (cubic perovskite) and Ti₂AlC (hexagonal). Using the first-principles density-functional-theory code VASP, calculations of host-precipitate interface energies were performed for these two carbides. Calculations were first applied to coherent interfaces to determine the favored termination layers and parallel translation states. For the favored interface configurations, a correction is applied for the effect of misfit, to obtain an estimated interface energy for semicoherent interfaces. The correction is based on an approximate formulation recently presented by the authors. The perovskite is found to have a lower interface energy than the hexagonal phase, consistent with the experimental finding that the former nucleates homogeneously and the latter inhomogeneously.