Microstructure and phase ratio effects on the superplasticity of Ti-6Al-4V

A study of the effect of the alpha and beta phase ratios on the stress/strain rate properties of Ti-6Al-4V at 870 C was conducted. The full spectrum of alpha content from 0 to 100 pct volume fraction was developed for evaluation by hydrogenation of Ti-6Al-4V and by producing a Ti-6Al alloy. The experimentally determined flow stress vs strain-rate properties were compared with analytical predictions by utilizing the Ashby-Verrall model, coupled with a rule-of-mixtures assumption to accommodate the different superplastic properties in each of the phases. It was observed that good correlation was achieved if the assumption of iso-strain rate in the two phases is used and if the Ashby-Verrall model is adjusted by a factor of 3. It is concluded that the maximum superplasticity for a fixed grain size and temperature should be observed for 100 pct beta phase concentration. The normally observed maximum in superplasticity at about 50 pct alpha is probably due to the finer grain size and grain size stability normally developed with this phase ratio. 13 references