Phase transformation kinetics of ω-phase in pure Ti formed by high-pressure torsion

High-pressure torsion (HPT) process is the only method which can obtain a 100 vol% of high-pressure omega-phase sample at ambient condition in pure Ti. In this paper, the mechanism of omega-phase stabilization by the HPT process is discussed on the basis of the reverse phase transformation kinetics of omega-phase in pure titanium formed by the HPT process and then measured using electrical resistivity and calorimetric experiments. The single omega-phase sample showed much higher electrical resistivity of 0.95 mu I (c) m at 350 K compared with that of the single alpha-phase sample (0.62 mu I (c) m). The omega-to-alpha reverse transformation behavior was clearly observed through both electrical resistivity and calorimetric measurements. The activation energy for omega-to-alpha reverse transformation, derived from the kinetics, showed a value close to that for the self-diffusion of Ti. The omega-phase obtained after the HPT process has an equiaxed submicron microstructure. The microstructure of reverse transformed alpha-phase showed no evidence of the occurrence of martensitic transformation. These results suggest that the mechanism governing omega-to-alpha phase transformation changed from diffusionless martensitic transformation to diffusion-controlled transformation after severe plastic deformation using the HPT process.