M. Vanderhasten, L. Rabet, Royal Military Academy, Brussels, Belgium; B. Verlinden, Katholieke Universiteit Leuven, Heverlee, Belgium

Summary:

A superplastic Ti-6Al-4V grade has been deformed at a strain rate of 5 10^-4 s^-1 and at various temperatures up to 1050°C. Structural mechanisms like grain boundary sliding, dynamic recrystallization, and dynamic grain growth, occurring during deformation, have been investigated and mechanical properties such as flow stress, strain hardening and strain at rupture have been determined. Dynamic recrystallization (DRX) brings on a decrease in the grain size. This could be of great interest because a smaller grain size allows a decrease in temperature for superplastic forming. For DRX, the driving force present in the deformed microstructure must be high enough. This means the temperature must be sufficiently low to ensure storing of enough dislocation energy but must also be high enough to provide the activation energy needed for DRX and to allow superplastic deformation. The best compromise for the temperature was found to be situated around 800°C; this is quite lower than the 925°C referenced in the literature as the optimum for the superplastic deformation. At this medium temperature the engineering strain that could be reached exceeds 400%, a value high enough to ensure the industrial production of complex piece by the way of the superplastic forming. Microstructural, EBSD and mechanical investigations were used to describe the observed mechanisms, some of which are concurrent.