The effect of volume fractions of α and β phases on the superplasticity was investigated for the five Ti--Al-V alloys, of which chemical compositions being on the tie line including the Ti-6Al-4V at 900°C. The testing temperature was 900°C and the initial strain rate range was from 6.67 × 10-5 to 2.50 × 10-2/s.
In spite of wide distribution of the volume fraction of the α phase in the alloys, all showed a good superplasticity under certain strain rate conditions. The superplastic elongation became the maximum at about equal volume fractions of α and phases, and decreased with deviation from that fraction. The initial average grain size of the alloys was dependent on the volume fraction ratio of the phases and found to become the minimum at equal volume fractions.
At a strain rate of 6.67 × 10-4/s, the average grain size of alloys after fracture was independent of the initial average grain size and the volume fraction ratio of the phases of the alloys. The average grain size after fracture was 8 μm. At the same strain rate, the strain induced grain growth rate of alloys increased with decreasing initial average grain size.
