CRYSTAL-STRUCTURES AND MORPHOLOGIES OF CARBIDE AND NITRIDE PRECIPITATES IN TIAL

Morphologies and crystal structures of precipitates in L1(0)-ordered TiAl containing carbon or nitrogen have been investigated in terms of transmission electron microscopy (TEM). During aging at temperatures around 1073 K after quenching from 1423 K, TiAl hardenes appreciably by the precipitation of carbide or nitride. The increment of yield strength due to the precipitation of carbide is larger than that obtained by the precipitation of nitride. TEM observations revealed that needle-like precipitates, which lie only in one direction parallel to the [001] axis of the TiAl matrix, appear in the matrix and preferentially at dislocations. Selected area electron diffraction analyses have shown that the needle-shaped precipitate is of perovskite type, Ti3AlC or Ti3AlN (P-phase), which has not been expected from the accepted equilibrium phase diagram. 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 below 1073 K, long range ordering of carbon-vacancy and nitrogen-vacancy pairs in the perovskite carbide and nitride is formed and two variants of ordered domains coexist in a single needle thereby decreasing the accumulation of the anisotropic strain field around needle. By aging at higher temperatures or for a longer period at 1073 K, plate-like precipitates of Ti2AlC or Ti2AlN (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). The stability of P-phase decreases with increasing aluminum content in TiAl.