Analysis of high temperature flow stress of titanium alloys IMI 550 and Ti-10V-2Fe-3AI during isothermal forging

Abstract

High temperature flow stress data obtained from laboratory scale isothermal forging tests on two titanium alloys, IMI550 (Ti-4Al-4Mo-2Sn-0.5Si) and Ti-10V-2Fe-3Al, have been correlated using a generalised hot working equation. The (α + β) titanium alloy IMI 550 was studied at temperatures in the range 900–950°C, high in the (α + β) phase field, and over a range of strain rates from 4.2 × 10−4 S−1 to 4.2 × 10−2 S−l. The metastable β titanium alloy Ti-10V-2Fe-3Al was studied at temperatures in the range 740–780°C, high in the (α + β) phasefield, and also at 820–950°C in the β field, over the same range of strain rates. A good correlation between the experimental data and the hot working equation was observed for both alloys. Values of the activation energy for hot working have been considered, in conjunction with microstructural evidence, in relation to the dynamic restoration processes taking place. The activation energy for hot working of Ti-10V-2Fe-3Al in the β field has been found to be 185 kJ mol−1, which supports the view that dynamic recovery is the only restoration mechanism in the single phase field. In the(α + β) field, ‘effective’ activation energy values have been determined for both Ti-10V-2Fe-3Al and IMI 550, and these values were significantly higher than those for β forging, especially in the case of IMI 550. However, for both alloys, the dominant restoration mechanism at temperatures high in the (α + β) field is considered to be dynamic recovery of the β phase.