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

Abstract

Abstract 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°C–950°C and strain rate (ε˙) 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 (σ) characteristics in a wide range of temperature and strain rate (T,ε˙). 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, α → β phase transformation below the β-transition temperature during hot deformation substantiates the occurrence of a mechanism other than DRX. In the lower temperature regime T(< 850°C), softening is caused by the DRX process at a lower deformation rate ε˙ (< 0.01 s−1), whereas at a higher deformation rate, the grains are more deformed with very high grain average misorientation. In the high temperature regime, where α → β phase transformation is very pertinent, it subsequently forms the transformed microstructure. The processing map shows the highest efficiency in the lower ε˙ and temperature regime, whereas the least efficiency with instability is observed at high ε˙. Microstructure analyses reveal that the DRX/DRV and phase transformation–assisted microstructure evolution in mill-annealed Ti-6Al-4V are the major deformation mechanisms.