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 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.
KeywordsElectrical Resistivity Martensitic Transformation Select Area Diffraction Pattern Reverse Transformation Wire Electrical Discharge Machine
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).
- 4.Zilberstein VA, Nosova GI, Estrin EI (1973) Alpha–omega transformation in titanium and zirconium. Phys Met Metall 35:128–133Google Scholar
- 5.Zilberstein VA, Chistotina NP, Zharov AA, Grishina NS, Estrin EI (1975) Alpha–omega transformation in titanium and zirconium during shear deformation under pressure. Phys Met Metall 39:208–211Google Scholar
- 14.Kutsar AR (1975) T-p diagram of hafnium, and phase transitions in shock waves. Phys Met Metall 40:89–95Google Scholar
- 22.Todaka Y, Azuma H, Ohnishi Y, Suzuki H, Umemoto M (2010) Influence of strain amount on stabilization of ω-phase in pure Ti by severe plastic deformation under high-pressure torsion. J Phys 240:012113Google Scholar
- 30.Dyment F (1980) Self and solute diffusion in titanium and titanium alloys. In: Proceedings of the International Conference on Titanium, 1:519–528Google Scholar
- 37.Maki T, Tomota Y, Tamura I (1974) Effect of grain size on the transformation-induced plasticity in metastable austenitic Fe-Ni-C alloy. J Jpn Inst Met 38:871–876Google Scholar