ON SUPERPLASTICITY OF 2 PHASE ALPHA-TITANIUM-INTERMETALLIC TI-(CO,NI)-AL ALLOY

Both the Ti-12Co-5Al and Ti-6Co-6Ni-5Al alloys sheets with ultra fine alpha-Ti/Ti-2(Co,Ni) microstructure were obtained after a proper therm omechanical treatment. The alloys under this state exhibit very large superplastic elongations at relatively low temperatures and relatively high strain rates. For the Ti-12Co-5Al alloy, the superplastic elonga tions higher than 2000% were obtained within a wide strain rate range (<3 x 10(-2) s(-1)) and at a relatively low temperature of 700 degrees C. An obvious strain hardening during superplastic deformation was fo und for the two alloys. Superplastic deformation enhanced a structure coarsening of the alloys and the intercept lengths of the alpha-Ti gra ins and the intermetallic particles increased linearly with strain dur ing deformation. The interphase boundary sliding between alpha-Ti and the intermetallic particles is considered to be the main process for t he superplastic deformation of the investigated alloys. The microstruc tural changes during deformation resulted in the changes of m and gamm a values with strain. The combination of strain rate hardening and str ain hardening is the main reason that the alloys can be superplastic a t relatively high strain rates and at relatively low temperatures.