Phase transformation kinetics of ω-phase in pure Ti formed by high-pressure torsion
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High-pressure torsion (HPT) process is the only method which can obtain a 100 vol% of high-pressure ω-phase sample at ambient condition in pure Ti. In this paper, the mechanism of ω-phase stabilization by the HPT process is discussed on the basis of the reverse phase transformation kinetics of ω-phase in pure titanium formed by the HPT process and then measured using electrical resistivity and calorimetric experiments. The single ω-phase sample showed much higher electrical resistivity of 0.95 μΩ m at 350 K compared with that of the single α-phase sample (0.62 μΩ m). The ω-to-α reverse transformation behavior was clearly observed through both electrical resistivity and calorimetric measurements. The activation energy for ω-to-α reverse transformation, derived from the kinetics, showed a value close to that for the self-diffusion of Ti. The ω-phase obtained after the HPT process has an equiaxed submicron microstructure. The microstructure of reverse transformed α-phase showed no evidence of the occurrence of martensitic transformation. These results suggest that the mechanism governing ω-to-α phase transformation changed from diffusionless martensitic transformation to diffusion-controlled transformation after severe plastic deformation using the HPT process.
This work was supported by a Grant-in-Aid for Scientific Research on Innovative Area, “Bulk Nanostructured Metals,” (Contract No. 22102002) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. The neutron diffraction experiments were performed using the RESA-1 diffractometer of JRR-3 with the approval of the Japan Atomic Energy Agency (Proposal No. 2008B-A08).
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