HIGH-RESOLUTION ELECTRON-MICROSCOPY STUDY OF GAMMA-GAMMA-T TWIN INTERFACES IN THE LAMELLAR STRUCTURE OF TIAL-BASED ALLOYS

The so-called TiAl/Ti3Al lamellar structure has been studied in a two- phase TiAl-based alloy by using high-resolution transmission electron microscopy, in order to verify the existence of various mixed boundari es (MBs) in the gamma-phase of the lamellar structure. In this investi gation, particular attention was paid to the atomic structure of {111} twin boundaries (TBs) gamma-gamma(T). Although only a few cases allow ed us to examine this kind of interface, the existence of mixed twin b oundaries could be confirmed under particular conditions of specimen o rientation. This result strongly suggests that the other boundaries, n amely order-domain boundaries (ODBs) and pseudo-twin boundaries (PTBs) , can also exist in the mixed form. All these MBs can be formed throug h two distinct mechanisms: firstly the coupling of rather mobile antip hase boundaries with the other less mobile boundaries such as ODBs, TB s and PTBs which are primarily introduced in the lamellar structure an d secondly the meeting of two separately nucleated domains, leading to the direct formation of a MB; that is to say, it is during nucleation that an antiphase translation is introduced between the two atomic ar rangements, together with the other defect-inducing atomic stacking op erations (introduction of an order-domain relation due to the lattice tetragonality and/or introduction of twin relation due to opposite sta cking sequences ... ABCABC ... as against ... ACBACB ...).