Flow behavior and microstructure in Ti–6Al–4V alloy with an ultrafine-grained α-single phase microstructure during low-temperature-high-strain-rate superplasticity

• Superplastic deformation mode in UFG Ti–6Al–4V alloy was examined.
• Grain boundary sliding mechanism dynamically changed with increasing strain.
• Flow-behavior with an unique curve was discussed in relation to several models.

Grain refinement of Ti–6Al–4V alloy (hereafter designated as Ti-64 alloy) is well recognized as a method for revealing the superplasticity at lower temperature or higher strain rates. This work examines the superplastic flow behavior of the ultrafine-grained (UFG) Ti-64 alloy (dα = 0.4 μm) consisting of single α phase in relation to microstructural evolution during deformation at 700 °C. Detailed microstructural evaluation reveals that the superplastic deformation mode of grain boundary sliding (tested at 700 °C-10−2 s−1) can be reasonably explained in relation to the Ball–Hutchison model at initial stage of deformation and the Gifkins Core–Mantle model at latter stage of deformation. During superplastic deformation, the β-precipitation occurs and contributes to accommodation mechanism of stress concentration at grain boundaries. This work also discusses the superplastic flow behavior in comparison with flow behaviors according to the Bird–Mukherjee–Dorn (BMD) generalized constitutive relation and the model which considers the effect of dynamic grain growth during deformation.