THE ORIGIN OF AND CONTRAST FROM ANTIPHASE DOMAIN BOUNDARIES IN TIAL-BASED ALLOYS

Recent observations have shown that defects which show the contrast ex pected from antiphase domain boundaries (APDBs) are commonly observed within the gamma-phase in quenched samples of alloys based on TiAl, wh en imaged in the transmission electron microscope. Detailed examinatio n has been carried out on several alloys and it has been found that th e contrast is complex. The complexity is associated firstly with dynam ical interactions between fundamental and superlattice reflections, wh ich lead to contrast where two-beam theory would predict none. Secondl y, 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. Final ly, and most significantly, it is shown that the regions which show AP DB contrast generate extra diffraction maxima which originate from a t hin region of a 90 degrees domain at the APDB. The observation of APDB s 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 d egree and sequence of ordering taking place during cooling. the observ ation of thin regions of 90 degrees domains at APDBs will be discussed in terms of the relative energies of APDBs and 90 degrees domains.