Mechanism of continuous dynamic recrystallization of Ti-6Al-4V alloy during superplastic forming with sub-grain rotation

The continuous dynamic recrystallization (CDRX) mechanism of Ti-6Al-4V alloys during the superplastic forming (SPF) was investigated, aiming to clarify the dominant mechanism for various misorientation evolution processes. The tensile tests were designed at 890, 920, and 950 degrees C with 10-4-10-2 s-1, the electron back-scattered diffraction (EBSD) observations and constitutive model were applied to describing the microstructure evolution. The geometrically necessary dislocations (GNDs) are responsible for the low angle grain boundaries (LAGBs) evolution. Furthermore, the mechanism of the transformation from LAGBs to high angle grain boundaries (HAGBs) includes: (1) the migration from HAGBs to LAGBs; (2) the remaining dislocations after dynamic recovery will move to the LAGBs; (3) the sub-grain rotation also promotes the grain nucleation. Based on the physical constitutive model, the sub-grain rotation is verified just before the transformation from LAGBs to HAGBs. Particularly, the dislocation motion is dominant before the misorientation reaches the saturated value during the grain boundary misorientation evolution, otherwise, the sub-grain rotation will be activated.