Twin boundaries in massively transformed -TiAl, which formed in Ti-48at%Al alloy samples that had been quenched from the high-temperature -Ti phase field, were investigated by means of high-resolution electron microscopy. A characteristic periodic contrast with a spacing of about 0.7nm was frequently observed at twin boundaries and ledges. This corresponded to the 3-fold periodicity of d{111}. This contrast was similar to that reported for heavily deformed +2 2-phase Ti-Al alloys, and was attributed to the formation of a 9R structure with an ABC/BCA/CAB stacking sequence. With the aid of image simulation, it was shown that the contrast was caused by overlapping twin-related crystals along the incident beam direction rather than by the formation of the 9R structure. The configuration of overlapping twins was explained in terms of complex ledge structures which resulted from the formation of twin-related crystals in the    massive transformation during quenching. Extremely thin hexagonal close-packed plates, with a thickness of about 0.8 to 2.0nm, were found to exist in the massively transformed  phase.

E.Abe, S.Kajiwara, T.Kumagai, M.Nakamura: Philosophical Magazine A, 1997, 75[4], 975-91