Effect of carbon addition on the microstructures and mechanical properties of gamma-TiAl alloys

The effect of carbon addition on the microstructures and mechanical properties of TiAl intermetallic alloys has been characterized. It is shown that carbon is not only an efficient solid solution strengthener in TiAl, it is also an efficient precipitation strengthener by fine dispersion of carbide. Transmission electron microscope observations have revealed that fine needle-like precipitates, which lie only in one direction parallel to the (001) axis, appear in carbon doped TiAl during aging at temperatures around 1073 K after quenching from 1523 K. Selected area electron diffraction analyses have shown that the needle-shaped precipitate is of a perovskite type, Ti3AlC (P-phase). The orientation relationship between the P-phase and the TiAl matrix is found to be (001)(P)//(001)TiAl and (010)(P)//(010)(TiAl). By aging at higher temperatures or for longer periods at 1073 K, plate-like precipitates of Ti2AlC (H-phase) with a hexagonal crystal structure are formed on the {111} planes of the TiAl matrix. The orientation relationship between the H-phase and the TiAl matrix is (0001)(H)//(111)(TiAl) and (<11(2)over bar 0>)(H)//(<(1)over bar 01>)(TiAl). TiAl doped with carbon hardens appreciably by the fine precipitation of carbide. The deviation from the stoichiometry has a significant effect on the aging behavior of TiAl. High temperature strength of TiAl increases appreciably by the precipitation of fine carbide. Dislocations bypass the carbide needles at further higher temperatures. The calculation of Orowan stress shows a good agreement with the experimentally measured result. (C) 1997 Elsevier Science Limited.