Grain refinement in γ-TiAl-based alloys by solid state phase transformations

The colony size of a fully lamellar Ti–46Al–2Cr–2Mo–0.25Si–0.3B ingot was refined from 120 to 30–65 μm by well defined heat-treatments which exploit the suppression of the α → α + γ transformation or involve cyclic annealing around the α-transus temperature. In addition, an extremely fine lamellar spacing in the range of 30–40 nm was obtained. For coarse-grained fully lamellar Ti–46Al–9Nb a massive phase transformation was used for microstructural refinement. The thermal stability of the massively transformed material was tested by annealing treatments and characterized by hardness measurements and the variation of the c/a-ratio of the tetragonal γ-TiAl cell as obtained from X-ray diffraction. After annealing at 1200 °C α2-Ti3Al lamellae appear within the former massively transformed γ-TiAl grains parallel to all four (111)γ-planes causing an increase in hardness.