Porosity of TiAl intermetallic compound with micro- and submicrocrystalline structure after superplastic deformation

Comparative investigations of porosity in micro- (d = 8 μm) and submicrocrystalline (d = 0.4 μm) TiAl after superplastic (SP) deformation have been performed. The experiments showed that the transition from microcrystalline (d ~ 1 μm) to submicrocrystalline (d < 1 μm) grain sizes in TiAl resulted in an abrupt reduction of porosity after SP deformation while retaining very similar values of SP parameters δ and m. To explain this effect, the probability of pore nucleation and pore growth rate were comparatively estimated in both states. These estimations were performed by taking into account both the changing microstructure and the change in grain boundary diffusion during the transition from “normal” SP deformation of the microcrystalline state to “low-temperature” SP deformation of a submicrocrystalline one. It can be concluded that the abrupt reduction of porosity in the course of SP deformation of submicrocrystalline TiAl is connected to the decrease of grain boundary stresses due to the grain and α2-Phase particle refinement. The considerable decrease in the temperature of SP deformation maybe also contribute to the suppression of porosity in the intermetallic compound since the growth of pores by diffusional mechanisms happens to be very difficult.