Hot deformation mechanisms in metastable beta titanium alloy Ti–10V–2Fe–3Al

Abstract

The mechanisms of hot deformation in the β titanium alloy Ti–10V–2Fe–3Al have been characterised in the temperature range 650–850°C and strain rate range 0·001–100 s^-1 using constant true strain rate isothermal compression tests. The β transus for this alloy is ∼790°C, below which the alloy has a fine grained duplex +β structure. At temperatures lower than the β transus and lower strain rates, the alloy exhibits steady state flow behaviour while at higher strain rates, either continuous flow softening or oscillations are observed at lower or higher temperatures, respectively. The processing maps reveal three different domains. First, in the temperature range 650–750°C and at strain rates lower than 0·01 s^-1, the material exhibits fine grained superplasticity marked by abnormal elongation, with a peak at ∼700°C. Under conditions within this domain, the stress–strain curves are of the steady state type. The apparent activation energy estimated in the domain of fine grained superplasticity is ∼225 kJ mol^-1, which suggests that dynamic recovery in the β phase is the mechanism by which the stress concentration at the triple junctions is accommodated. Second, at temperatures higher than 800°C and strain rates lower than ∼0.1 s^-1, the alloy exhibits large grained superplasticity, with the highest elongation occurring at 850°C and 0.001 s^-1; the value of this is about one-half of that recorded at 700°C. The microstructure of the specimen deformed under conditions in this domain shows stable subgrain structures within large β grains. Third, at strain rates higher than 10 s^-1 and temperatures lower than 700°C, cracking occurs in the regions of adiabatic shear bands. Also, at strain rates above 3 s^-1 and temperatures above 700°C, the material exhibits flow localisation.