Plastic Flow and Microstructure Evolution during Low-Temperature Superplasticity of Ultrafine Ti-6Al-4V Sheet Material

Abstract

The low-temperature superplastic (SP) flow behavior of two lots of Ti-6Al-4V sheet, each with an ultrafine microstructure, was established by performing tension tests at temperatures of 775 °C and 815 °C and true strain rates of 10−4 and 10−3 s−1. The as-received microstructures of the two materials comprised either equiaxed or slightly elongated alpha particles in a beta matrix. The material with equiaxed alpha particles exhibited flow hardening, which was correlated with concurrent (dynamic) coarsening. The rate of dynamic coarsening was rationalized in terms of static coarsening measurements and the enhancement of kinetics due to pipe diffusion. By contrast, the material with initially elongated alpha particles exhibited comparable flow hardening at the lower strain rate but a complex, near-steady-state behavior at the higher strain rate. These latter observations were explained on the basis of the evolution of the alpha particle shape and size during straining; dynamic coarsening or dynamic spheroidization was concluded to be most important at the lower and higher strain rates, respectively. The plastic flow behavior was interpreted in the context of a long-wavelength flow localization analysis.