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 alpha plate of a Ti-6Al-4V alloy with an alpha lamellar microstructure during hot deformation at 1223 K (950 A degrees C) and a strain rate of 0.1 s(-1). The correlation between the local misorientation development and the globularization behavior of alpha plates during subsequent annealing at 1223 K (950 A degrees C) was discussed. The misorientation profile along an individual alpha 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 alpha/alpha boundaries and resulted in splitting alpha plates the annealing. The mean length between adjacent discontinuous points, L (I), was determined and compared with the actual mean length of the alpha 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 alpha/alpha boundaries in subsequent annealing. Consequently, points P (d) already developed by hot deformation were considered to become alpha/alpha boundaries and led to splitting alpha plates in annealing. New alpha/alpha boundaries formed at points P (r) in subsequent annealing after a higher strain deformation, which led to a splitting of alpha plates as well.