Dynamic globularization kinetics during hot working of Ti-17 alloy with initial lamellar microstructure

The dynamic globularization kinetics of Ti-17 (Ti-5Al-2Sn-2Zr-4Mo-4Cr) alloy with initial lamellar microstructure over wide temperature (780-860 °C) and strain rate ranges (0.001-10 s^-1) had been quantitatively characterized and investigated using isothermal hot compression tests. The experimental results showed that the dynamic globularization kinetics and the kinetics rate were sensitive to deformation conditions. The fraction of dynamic globularization approximately increased with strain in an Avrami type sigmoid way, and also increased with increasing temperature and decreasing strain rate. The strains for initiation (the critical strain) and completion of dynamic globularization were predicted to be about 0.04-0.47 and 1.34-5.36, respectively, and they both increased with decreasing nominal test temperature and with increasing strain rate. The critical strain of dynamic globularization was higher than the peak strain on the flow curves mainly due to the kink, rotation of lamellae and the forming of intraphase boundaries before dynamic globularization initiation. The kinetics rate of dynamic globularization increased up to a peak value at a strain corresponding to about 40% globularization, and then decreased with the increasing strain. The peak value of the kinetics rate was higher at the conditions of higher temperature and lower strain rate, and it suggested that the rate of migration of intraphase boundary markedly restricted the kinetics rate of dynamic globularization.