High-resolution electron microscopy of γ-α₂ interfaces in titanium aluminide

Abstract

The structure of γ-α₂ interfaces in binary and Ta-containing Ti-Al alloys was analysed by high-resolution transmission electron microscopy and image simulations. Growth of α₂-Ti₃Al plates was found to be due to a ledge mechanism, consisting of Shockley partial dislocations on alternate (111)_γ 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)_γ planes, the c parameter (0·463 nm) of the α₂-phase. The islands were bounded by 90° (edge) and 30° (screw) Shockley partial dislocations. The 30° partial dislocation cores were localized, whereas the 90° 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° partials to 90° partials accomplishing the γ↔α₂ 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 α₂-phase. The image simulation studies show that the contrast of alternate (0002) planes in α₂-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 γ-α₂ interface is proposed, and this gives rise to γ-TiAl lamellae within the α₂-Ti₃Al structure.