Dynamic and static restoration processes in the α region of commercially pure titanium, and effects of oxygen and iron on these processes

Hot deformation behavior in the alpha region of commercially pure titanium (C.P. titanium) was investigated together with the effects of oxygen and iron on dynamic and static restoration processes. Hot compression tests were performed using a hot working simulator of THERMEC-MASTER-Z, and stress-strain (S-S) curves and dynamically restored microstructures were obtained under various hot deformation conditions covering the temperature from 873 to 1073K and strain rate from 1 X 10(-3) s(-1) to 1 s(-1). Static restoration behavior was investigated through variation of softening ratio with holding time obtained by intermittent two-steps hot deformation. Three types of S-S curves were obtained and classified by the deformation temperature and strain rate. Dynamic restoration process in the alpha region of C.P. titanium was confirmed to be dynamic recovery type from both of stress-strain curves and observation of dynamically restored microstructure. Both activation energy values in dynamic and static restoration processes were almost same as approximately 185 kJ/mol, which was very close to the activation energy for self-diffusion of titanium in alpha titanium. The effect of oxygen or iron with a respective content of 0.3% on flow stress and static restoration kinetics was relatively small, although the temperature-strain rate region for occurrence of dynamic recovery was slightly expanded by addition of these elements. These results appeared to be brought about by relatively high diffusivity of these elements in alpha titanium.