Processing map for the hot working of near-α titanium alloy 685

Abstract

Using a dynamic materials model, processing and instability maps have been developed for near-α titanium alloy 685 in the temperature range 775–1025°C and strain-rate range of 0.001–10 s⁻¹ to optimise its hot workability. The alloy's β-transus temperature lies at about 1020°C. The material undergoes superplasticity with a peak efficiency of 80% at 975°C and 0.001 s⁻¹, which are the optimum parameters for α-β working. The occurrence of superplasticity is attributed to two-phase microduplex structure, higher strain-rate sensitivity, low flow stress and sigmoidal variation between log flow stress and log strain rate. The material also exhibits flow localisation due to adiabatic shear-band formation up to its β-transus temperature with strain rates greater than 0.02 s⁻¹ and thus cracking along these regions.