Hot deformation characteristics of Ti-6Al-4V

The flow behavior and hot deformation mechanisms of Ti-6Al-4V in both β and (α + β) phase regions have been studied by compression tests in the temperature range 750-1100°C and strain rate range 0.05-15 s^-1. The flow curves displayed discontinuous yielding at temperatures above 850°C and high strain rates, typically 5 and 15 s^-1. Flow oscillations were also observed under such conditions. The relative discontinuous flow stress drop increased with increasing temperature in the (α + β) phase region and reached a steady state in the β phase region. The discontinuous yielding is attributed to the sudden increase in the mobile dislocation density occurring due to the activation of β grain boundary sources during hot deformation. The apparent activation energy estimated using the standard kinetic analysis is about 246 kJ/mol in the β region and 621 kJ/mol in the (α + β) region while the strain rate sensitivity values are 0.34 and 0.18, respectively. The mechanism in the β region has been suggested to involve grain boundary sliding and dynamic recovery of the β phase while in the (α + β) region a transient mechanism, involving dynamic recovery followed by post deformation recrystallization, has been proposed.