On superplasticity of two phase α-titanium-intermetallic Ti-(Co,Ni)-A alloy

Both the Ti-12Co-5Al and Ti-6Co-6Ni-5Al alloys sheets with ultra fine α-Ti/Ti₂(Co,Ni) microstructure were obtained after a proper thermomechanical 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 elongations higher than 2000% were obtained within a wide strain rate range (< 3 × 10^-2s^-1) and at a relatively low temperature of 700°C. An obvious strain hardening during superplastic deformation was found for the two alloys. Superplastic deformation enhanced a structure coarsening of the alloys and the intercept lengths of the α-Ti grains and the intermetallic particles increased linearly with strain during deformation. The interphase boundary sliding between α-Ti and the intermetallic particles is considered to be the main process for the superplastic deformation of the investigated alloys. The microstructural changes during deformation resulted in the changes of m and y values with strain. The combination of strain rate hardening and strain hardening is the main reason that the alloys can be superplastic at relatively high strain rates and at relatively low temperatures.