An in-situ continuous cooling β to α transformation kinetics of extra-pure (EP) Ti and of grade-4 commercially-pure (CP) Ti were investigated using fully computer controlled resistivity-temperature real-time measurement apparatus and transmission electron microscopy. Martensitic transformation from β to α‘ occurs under near pure shear condition in both of the extra-pure Ti and grade-4 CP-Ti. The habit plane of lath-type martensite was determined to be ($41\bar{5}0$)α’ parallel to ($43\bar{3}$)β , on which a maximum shear stress is operating with the normal stress being negligible. As the oxygen content is increased the $M_s$ temperature is raised by increasing the $T_o$ temperature of pure Ti, while the iron greatly reduces the critical cooling rate of the martensitic transformation. The massive reaction is, for the first time, observed to occur in an EP-Ti as well as in a grade-4 CP-Ti. The starting temperature of the massive transformation was lower than To by about 30℃ in an EP-Ti and increased in CP-Ti by more than the $T_o$ difference expected from the oxygen effect. The estimation showed that the thermal stress generated in the specimen during rapid cooling can raise the $M_s$ temperature of pure Ti by about 20℃ and further suggested a possibility that it can induce a massive transformation by significantly raising the $T_o$ temperature.
In addition to the phase transformation effect of the O and Fe, effect of oxygen and iron on the high temperature deformation behaviors of CP-Ti was studied at temperatures from 750℃ to 950℃ using compression test at various strain rates from $0.001s^{-1}$ to $10s^{-1}$. High temperature deformation efficiencies and flow instability conditions were, in particular, evaluated from their high temperature flow curves. Work hardening rates were always higher in the grade 1 rather than in the grade 4 CP-Ti. This was mainly attributed to the difference in the extent of α+β two-phase field arising from the different Fe content. The optimum working condition was estimated to be the strain rate $10s^{-1}$ at 850℃ and to be the strain rate $10s^{-1}$ at 900℃ for grade 1 and grade 4 CP-Ti, respectively. A dynamic recovery mechanism was believed to be mainly responsible for the high deformation efficiency for both grades. As the oxygen content is increased the condition for the high deformation efficiency is shifted toward a higher temperature or lower strain rate condition. It is in a good agreement with the theoretical expectation for the shift direction of the condition for dynamic recrystallization. At the intermediate strain rates and at about 900℃, power dissipation efficiencies were very low in both specimens. This was believed to be closely related to the occurrence of α→β transformation during deformation.
