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

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 (d_α=0.4μm) 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^-2s^-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.