Hot deformation and microstructural damage mechanisms in extra-low interstitial (ELI) grade Ti–6Al–4V

Abstract

The hot deformation behavior of extra-low interstitial (ELI) grade Ti–6Al–4V alloy with Widmanstatten preform microstructure over wide temperature (750–1100oC) and strain rate ranges (0.001–100 s−1) has been studied with the help of processing maps. In the lower temperature and strain rate regime (850–950°C and 0.001–0.1 s−1), globularization of the lamellar structure occurs while at higher temperatures (980–1100oC) the β phase exhibits large-grained superplasticity. The tensile ductility reaches peak values under conditions corresponding to these two processes. A dip in ductility occurs at the β transus and is attributed to a possible nucleation of voids within prior β grains. At lower temperatures and strain rates below about 0.1 s−1, cracking at the prior β grain boundaries occurs under mixed mode conditions. At strain rates higher than 1 s−1 and temperatures lower than about 950oC, the material exhibits a wide regime of flow instabilities. On the basis of these results, a temperature–strain rate window for hot working this material without microstructural defects is identified.