Kinetics of biaxial dome formation by transformation superplasticity of titanium alloys and composites

By thermally cycling through their transformation temperature range, coarse-graned polymorphic materials can be deformed superplastically, owing to the emergence of transformation mismatch plasticity (or transformation superplasticity) as a deformation mechanism. This mechanism is presently investigated under biaxial stress conditions during thermal cycling of unalloyed titanium, Ti-6Al-4V, and their composites (Ti/10 vol. pct TiC_p, Ti-6Al-4V/10 vol. pct TiC_p, and Ti-6Al-4V/5 vol. pct TiB_w). During gas-pressure dome bulging experiments, the dome height was measured as a function of forming time. Adapting existing models of biaxial doming to the case of transformation superplasticity where the strain-rate sensitivity is unity, we verify the operation of this deformation mechanism in all experimental materials and compare the biaxial results directly to new uniaxial thermal cycling results on the same materials. Finally, existing thickness distribution models are compared with experimentally measured profiles.