The origin of and contrast from antiphase domain boundaries in TiAl-based alloys

Abstract Recent observations have shown that defects which show the contrast expected from antiphase domain boundaries (APDBs) are commonly observed within the γ-phase in quenched samples of alloys based on TiAl, when imaged in the transmission electron microscope. Detailed examination has been carried out on several alloys and it has been found that the contrast is complex. The complexity is associated firstly with dynamical interactions between fundamental and superlattice reflections, which lead to contrast where two-beam theory would predict none. Secondly, it is associated with a significant displacement across the APDBs, in addition to the expected 1 2 〈101〉 displacement, which gives rise to contrast where the simple hard sphere model would predict none. Finally, and most significantly, it is shown that the regions which show APDB contrast generate extra diffraction maxima which originate from a thin region of a 90° domain at the APDB. The observation of APDBs will be discussed in terms of the structure at the high temperatures from which the samples have been rapidly cooled and in terms of the degree and sequence of ordering taking place during cooling, the observation of thin regions of 90° domains at APDBs will be discussed in terms of the relative energies of APDBs and 90° domains.