Flow behaviour and microstructure development of forged Ti-25Al-10Nb-3V-1Mo (super α2)

Abstract

Samples of super α 2(Ti-25Al-10Nb-3V-1Mo) have been hot upset forged after heating to either the ( α 2 + β)- or the β-phase field. Two different die temperatures and three strain rates, 1 s −1, 10 s −1 and 100 s −, were used. Detailed microstructural observations have been carried out on these forged samples. It was found that at low strain rate (<10 s −) flow softening of a super α 2 in the ( α 2 + β)-phase field is mainly associated with deformation heating, which causes some structural changes and dynamic recovery. At the higher strain rate (> 10 s −1) dynamic recrystallization is significant. The flow softening in the β-phase field consists of dynamic recovery and recrystallization coupled with a certain amount of grain boundary sliding or migration at both low and high strain rate. The average activation energies for deformation were calculated from the forging flow stress curves as 414 kJ mol −1 in the ( α 2 + β)-phase field and 235 kJ mol −1 in the β-phase field. These values confirmed that the controlling mechanism of deformation is dynamic recovery and recrystallization. Average strain rate sensitivity parameters of 0.29 and 0.36 were also obtained in the (α + β)- and β-phase field.