SEM/EBSD Analysis on Globularization Behavior of Lamellar Microstructure in Ti-6Al-4V During Hot Deformation and Annealing

Scanning electron microscope/electron back-scattering diffraction was used to investigate local misorientation development within an individual α plate of a Ti-6Al-4V alloy with an α lamellar microstructure during hot deformation at 1223 K (950 °C) and a strain rate of 0.1 s⁻¹. The correlation between the local misorientation development and the globularization behavior of α plates during subsequent annealing at 1223 K (950 °C) was discussed. The misorientation profile along an individual α plate showed that not only a continuous and smooth change in orientation but also a discontinuous change in orientation was developed by the hot deformation. We assume that the points where discontinuous change in orientation occurs, P _d, became α/α boundaries and resulted in splitting α plates the annealing. The mean length between adjacent discontinuous points, L _I, was determined and compared with the actual mean length of the α plates after hot deformation and subsequent annealing, L _a, as measured by optical microscopy. The two kinds of length parameters coincided at lower strains, but significant differences were observed at higher strains, i.e., L _I was larger than L _a. Further analysis showed that rotation axes (R.A.s) changed even within regions where orientation changes were continuous. By taking into account the points where the R.A.s changed, P _r and P _d, the mean length between adjacent points, L _II, appeared to coincide with L _a at higher strains. A higher lattice distortion is expected near points P _r at higher strains, which results in the formation of new α/α boundaries in subsequent annealing. Consequently, points P _d already developed by hot deformation were considered to become α/α boundaries and led to splitting α plates in annealing. New α/α boundaries formed at points P _r in subsequent annealing after a higher strain deformation, which led to a splitting of α plates as well.