Precipitation behavior of nitrides in L10-ordered TiAl

A transmission electron microscope investigation has been performed on the precipitation behavior of nitrides in L10-ordered γ-TiAl with different stoichiometry. By aging at temperatures around 1073 K after quenching from 1523 K, fine needle particles of Ti3AlN and platelet particles of Ti2AlN appear in the TiAl matrix. The hardness of TiAl increases appreciably due to the fine precipitation of nitrides and overage softening occurs after prolonged aging or aging at higher temperatures. The perfect lattice coherency is maintained at the interfaces between the nitride particles and the TiAl matrix. Selected area electron diffraction analyses have shown that the needle-shaped precipitate is of perovskite type, Ti3AlN (P phase). The orientation relationship between the P-phase and the TiAl matrix was found to be (001)P//(001)TiAl and [010]P//[010]TiAl, needle//[001]TiAl. By aging at higher temperatures or for longer periods of time at 1073 K, plate-like precipitates of Ti2AlN (H-phase) with a hexagonal crystal structure are formed on the {111} planes of TiAl. The orientation relationship between the H-phase and TiAl is (0001)H//(111)TiAl and [112¯0]H//[1¯01]TiAl, plate//{111}TiAl.