Effect of alloying element modification on superplastic deformation properties of Ti–4Mo–4Al–2Sn–0·5Si (IMI550)

Abstract

The superplastic deformation properties of a commercial titanium alloy, IMI550 (Ti–4Mo–4Al–2Sn–0·5Si, wt-%) are compared with a specially melted ‘modified’ alloy in which 1 wt-%Mo (slow diffusing element) is replaced with 1 wt-%Fe (fast diffusing element) on the basis that the diffusional properties of the β phase, to which these alloying elements segregate, is a significant factor in determining the overall flow properties. It is shown that the superplastic deformation properties of both alloys in the temperature range 800–850°C develop with strain as the initial heterogeneous α + β structure develops into uniform duplex equiaxed microstructures. Thereafter, the modified alloy exhibits enhanced superplastic properties (reduced flow stresses in the strain rate range 5 × 10-5–5 × 10-3 s-1). The results are analysed quantitatively on the basis of the Ashby and Verrall model applied to the α and β phases combined according to the isostress model for two phase deformation which accounts for the interaction of the α and β phases.