Biaxial deformation of Ti-6Al-4V and Ti-6Al-4V/TiC composites by transformation-mismatch superplasticity

Gas-pressure bulge forming of unreinforced Ti-6Al-4V and TiC-reinforced Ti-6Al-4V was performed while cycling the temperature around the allotropic transformation range of the alloy (880-1020 degrees C). The resulting domes exhibited very large strains to fracture without cavitation, demonstrating for the first time the use of transformation-mismatch superplasticity under a biaxial state of stress for both an alloy and a composite. Furthermore, much faster deformation rates were observed upon thermal cycling than for control experiments performed under the same gas pressure at a constant temperature of 1000 degrees C, indicating that efficient superplastic forming of complex shapes can be achieved by transformation-mismatch superplasticity, especially for composites which are difficult to shape with other techniques. However, the deformation rate of the cycled composite was lower than for the alloy, most probably because the composite exhibits lower primary and secondary isothermal creep rates. For both cycled materials, the spatial distribution of principal strains is similar to that observed in domes deformed by isothermal microstructural superplasticity and the forming times can be predicted With existing models for materials with uniaxial strain rate sensitivity of unity. Thus, biaxial transformation-mismatch superplasticity can be modeled within the well-known frame of biaxial microstructural superplasticity, which allows accurate predictions of forming time and strain spatial distribution once the uniaxial constitutive equation of the material is known. (C) 1997 Elsevier Science S.A.