Understanding Flow Behavior and Microstructure Evolution during Thermomechanical Processing of Mill-Annealed Ti-6Al-4V Titanium Alloy

Titanium alloy in the mill-annealed condition has a significant store of energy because it is not fully recrystallized and the grains possess a very high dislocation density. It is well established that the extent of softening mechanisms like dynamic recrystallization/recovery (DRX/DRV) are dependent on the stored energy during hot deformation. In this case, stored energy in the prior worked material may facilitate the softening. The hot deformation behavior of mill-annealed Ti-6Al-4V is studied by hot compression tests in the temperature range of 750 degrees C-950 degrees C and strain rate ((epsilon) over dot) range of 0.001-10 s(-1) for 60 % deformation. The true stress-strain curve of the compression tests exhibits the variation in flow stress (sigma) characteristics in a wide range of temperature and strain rate (T, (epsilon) over dot). The activation energy calculated from the phenomenological constitutive equation is significantly higher than the self-diffusion activation energy, which suggests DRX as the main softening process. However, alpha -> beta phase transformation below the beta-transition temperature during hot deformation substantiates the occurrence of a mechanism other than DRX. In the lower temperature regime T(< 850 degrees C), softening is caused by the DRX process at a lower deformation rate (epsilon) over dot (< 0.01s(-1)), whereas at a higher deformation rate, the grains are more deformed with very high grain average misorientation. In the high temperature regime, where alpha -> beta phase transformation is very pertinent, it subsequently forms the transformed microstructure. The processing map shows the highest efficiency in the lower (epsilon) over dot and temperature regime, whereas the least efficiency with instability is observed at high (epsilon) over dot. Microstructure analyses reveal that the DRX/DRV and phase transformation-assisted microstructure evolution in mill-annealed Ti-6Al-4V are the major deformation mechanisms.