HIGH-RESOLUTION ELECTRON-MICROSCOPY OF TWIN INTERFACES IN MASSIVELY TRANSFORMED GAMMA-TIAL

Twin boundaries in the massively transformed gamma-TiAl phase, which i s formed in a Ti-48 at.% Al alloy quenched from the high-temperature a lpha-Ti phase field, have been investigated by high-resolution electro n microscopy. At the twin boundaries and ledges, a characteristic peri odic contrast with a spacing of about 0.7 nm is frequently observed, w hich corresponds the three-times periodicity of d{111}(gamma). This co ntrast is quite similar to those reported previously for severely defo rmed 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 simulati on, it is concluded that the contrast is caused by overlapping twin-re lated crystals along the incident beam direction and not by formation of the 9R structure. The configuration of overlapping twins is explain ed by complex ledge structures, which result from the formation of twi n-related crystals in the alpha --> gamma massive transformation durin g 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 t he alpha --> gamma massive transformation.