The transformation between the γ and α phases in binary and ternary γ-based titanium aluminides

As-cast TiAl binary alloys with compositions between 49 and 53 at.% Al and ternary TiAlX alloys (X = silicon, boron) were homogenised at 1200°C and subsequently annealed for various times at 1350° or 1420°C, after which they were either water quenched or furnace cooled. The resultant structures were characterised using optical microscopy, EPMA, TEM and STEM. In water quenched binary and silicon-containing samples it was found that the transformation from the face centred tetragonal (γ) to the h.c.p. (α) phase resulted in the diffusional formation of laths of α. This transformation obeyed the usual crystallographic relationship of {111}_f.c.c. {norm of matrix}(0001)_h.c.p., 〈110〉_f.c.c. {norm of matrix} 〈1120〉_h.c.p.. The kinetics of transformation of the binary alloys were found to be qualitatively similar to the analogous transformation in AlAg with rapid α plate nucleation and α plate lengthening kinetics and slow thickening kinetics. A reverse transformation from α to γ, which does not involve long range diffusion was also observed, which results in the formation of plates of γ within the prior α lath. The addition of silicon was found to result in the formation of silicon-rich α laths with a greater lath density but msaller length, thickness and volume fraction than those in the binary alloys. The α laths either reverted back to γ or ordered to α₂ during water quenching. The boron containing samples showed no lath formation and the lamellar structure is stable. Reaction between the TiB₂ particles and the matrix changed the boron and titanium concentration in the matrix and thus stabilised the initial microstructure.