Effect of hot working variables on microstructure and brittle to ductile transition in Ti-46.5Al-3Nb-2Cr-0.2W gamma titanium aluminide

The microstructure and mechanical properties of a γ-TiAl alloy (Ti-46.5Al-3Nb-2Cr-0.2W, in at.-%) were studied in two conditions: (a) after conventional forging in the +γ phase field and (b) after subsequent isothermal forging in the 2 +γ phase field. Tensile tests were conducted in the temperature range 800-1000°C and strain rate range of 10 -3 -10 -1 s -1 . The microstructure of the alloy in condition 1 was non-homogeneous consisting of about 90 vol.-% of small γ grains (grain size of 3 to 20 µm) and 10 vol.-% of coarse grains or lamellar regions. The alloy in this condition showed a brittle to ductile transition at about 950°C and extensive cavitation during deformation above the transition temperature. The microstructure in condition 2 was much more uniform and finer, and the transition temperature was decreased to 850°C. The alloy in condition 2 showed better deformability and cavitation resistance than that in condition 1 and superplastic behaviour at temperatures 900-1000°C.