High-resolution electron microscopy of twin interfaces in massively transformed gamma-TiAl

Twin boundaries in the massively transformed gamma-TiAl phase, which is formed in a Ti-48 at.% Al alloy quenched from the high-temperature alpha-Ti phase field, have been investigated by high-resolution electron microscopy. At the twin boundaries and ledges, a characteristic periodic contrast with a spacing of about 0.7 nm is frequently observed, which corresponds the three-times periodicity of d{111}(gamma). This contrast is quite similar to those reported previously for severely deformed gamma + alpha(2) two-phase Ti-Al alloys, which was attributed to the formation of the 9R structure with ABC/BCA/CAB stacking sequence. From close examination of this contrast with the aid of image simulation, it is concluded that the contrast is caused by overlapping twin-related crystals along the incident beam direction and not by formation of the 9R structure. The configuration of overlapping twins is explained by complex ledge structures, which result from the formation of twin-related crystals in the alpha --> gamma massive transformation during quenching. Extremely thin hcp plates with a thickness of about 0.8-2.0 nm are found to exist among the massively transformed gamma phase. These thin hcp plates are considered to be a retained alpha phase in the alpha --> gamma massive transformation.