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 gamma -TiAl alloy (Ti-46. 5Al- 3Nb - 2Cr - 0.2W, in at,-%) were studied in two conditions: (a) after conventional forging in the alpha+gamma phase field and (b) after subsequen t isothermal forging in the alpha (2) + gamma phase field. Tensile tests me re conducted in the temperature range 800 - 1000 degreesC and strain rate r ange 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 gamma grains (g rain size of 3 to 20 mum) and 10 vol,-% of coarse grains or lamellar region s. The alloy in this condition showed a brittle to ductile transition at ab out 950 degreesC and extensive cavitation during deformation above the tran sition temperature. The microstructure in condition 2 was much more uniform and finer, and the transition temperature was decreased to 850 degreesC. T he alloy in condition 2 showed better deformability and cavitation resistan ce than that in condition 1 and superplastic behaviour at temperatures 900 - 1000 degreesC. (C) 2001 IoM Communications Ltd.