Alloys based on the ferromagnetic τ-phase, which may be established in the Mn-Al system for near equiatomic composition [1], exhibit high magnetocrystalline anisotropy and coercivity and are of interest for advanced permanent magnet applications [2], Their technologically important properties depend strongly on the microstructure and defect structure development associated with the transformation from the disordered hexagonal (A3) ε-phase to the ordered tetragonal (L10) τ-phase. The structure, morphology and chemistry of the (ε/τ) interphase interfaces have been studied by methods of optical and electron microscopy (OM, SEM and TEM) in order to determine the nature of this technically important and scientifically interesting phase transformation. The optimization of processing-property relationships for this class of ferromagnetic materials requires a detailed understanding of the mechanisms facilitating the ordering process.

The τ-phase nucleates at prior ε-grain boundaries (ε-GB) and exhibits a serrated, faceted growth morphology (FIG. l).

It is the accepted wisdom that off-stoichiometric TiAl alloys with slightly Aluminum rich compositions, e.g. Ti-52 at.% Al and Ti-54 at.% Al, exist as single phase materials. However, previous studies have reported that Ti3Al5 ordering occurs in Al rich γ-TiAl of compositions 54-63 at.% Al [1,2]. The Ti3Al5 ordering involves the substitution of excess Al into selected Ti sites on the pure Ti plane (001) [1]. In the current work such Ti3Al5 ordering has been observed in TiAl alloys of compositions ranging from Ti-52 at.% Al to Ti-56 at.% Al which have been produced as described in earlier reports [e.g. 3].

Experimental and simulated selected area diffraction patterns (SAD) for the [111] zone axis of TiAl are shown in figure 1. A row of extra reflections which appears between the reflections corresponding to the Ll0 structure are clearly visible in the diffraction patterns obtained from the Ti-54 and 56 at.% Al alloys (Fig. 1b,1c).