High temperature, high strain deformation behavior of Ti–6.8Mo–4.5Fe–1.5Al

The elevated temperature deformation behavior of the metastable β titanium alloy Ti–6.8Mo–4.5Fe–1.5Al has been investigated within the temperature range 840–1290°C at constant strain rates between 2×10−4 and 1 s−1. The flow stress curves were characterized by discontinuous yielding, followed at the highest strain rates by slight strain hardening before flow softening and/or steady state flow. As the strain rate decreased, strain hardening was eliminated. Dynamic material modeling (DMM) was applied to establish flow stress–strain rate–temperature conditions corresponding to unstable and stable flow. Unstable flow was observed at all temperatures and strains at low strain rates (2×10−4–10−2 s−1) and was related to grain boundary cracking and cavitation. Stable flow was present at higher strain rates (10−2–1 s−1) and was associated with dynamic recovery below 940°C and dynamic recrystallization at higher temperatures. In this high temperature stable region, a bimodal grain size distribution was observed comprising large discontinuously recrystallized grains and small continuously recrystallized grains. The former were nucleated within the near grain boundary region at low strains, while the latter formed within the grain interiors.