Characterization of hot deformation and microstructure evolution of a new metastable β titanium alloy

The hot deformation characteristics of as-forged Ti-3.5Al-5Mo-6V-3Cr-2Sn-0.5Fe-0.1B-0.1C alloy within a temperature range from 750 to 910 degrees C and a strain rate range from 0.001 to 1 s(-1) were investigated by hot compression tests. The stress-strain curves show that the flow stress decreases with the increase of temperature and the decrease of strain rate. The microstructure is sensitive to deformation parameters. The dynamic recrystallization (DRX) grains appear while the temperature reaches 790 degrees C at a constant strain rate of 0.001 s(-1) and strain rate is not higher than 0.1 s(-1) at a constant temperature of 910 degrees C. The work-hardening rate theta is calculated and it is found that DRX prefers to happen at high temperature and low strain rate. The constitutive equation and processing map were obtained. The average activation energy of the alloy is 242.78 kJ/mol and there are few unstable regions on the processing map, which indicates excellent hot workability. At the strain rate of 0.1 s(-1), the stress-strain curves show an abnormal shape where there are two stress peaks simultaneously. This can be attributed to the alternation of hardening effect, which results from the continuous dynamic recrystallization (CDRX) and the rotation of DRX grains, and dynamic softening mechanism.