HIGH-RESOLUTION ELECTRON-MICROSCOPY OF GAMMA-ALPHA-2 INTERFACES IN TITANIUM ALUMINIDE

The structure of gamma-alpha2 interfaces in binary and Ta-containing Ti-Al alloys was analysed by high-resolution transmission electron microscopy and image simulations. Growth of alpha2-Ti3Al plates was found to be due to a ledge mechanism, consisting of Shockley partial dislocations on alternate (111)gamma planes. The interface is atomically flat between the ledges and addition of 2 at.% Ta was found to transform arrays of growth ledges in the binary alloy into islands on the plate faces in the Ta-containing alloy for identical heat treatments. The height of the ledges and islands was always a multiple of two (111)gamma planes, the c parameter (0-463 nm) of the alpha2-phase. The islands were bounded by 90-degrees (edge) and 30-degrees (screw) Shockley partial dislocations. The 30-degrees partial dislocation cores were localized, whereas the 90-degrees partial dislocation cores appeared to be highly delocalized because of the presence of a high density of kinks, which in one case was found to be about 0.65 nm-1. The ratio of 30-degrees partials to 90-degrees partials accomplishing the gamma<-->alpha2 transformation was found to be 3 to 1, which differs from the ideal ratio of 2 to 1 found during f.c.c.<-->h.c.p. diffusional transformations. These results are interpreted in terms of the growth mechanisms and morphology of the alpha2-phase. The image simulation studies show that the contrast of alternate (0002) planes in alpha2-Ti3Al is due to beam tilt parallel to the interface plane rather than to actual ordering within the structure. A mechanism for creation of compositional faults at a pre-existing gamma-alpha2 interface is proposed, and this gives rise to gamma-TiAl lamellae within the alpha2-Ti3Al structure.