In-situ measurement of continuous cooling β → α transformation behavior of CP-Ti

Abstract

The ␤ → ␣ transformation kinetics of CP-Ti during continuous cooling was measured using a fully computer-controlled resistivity-temperature real-time measurement apparatus. Unlike the pure Ti case, the massive transformation occurs at medium cooling rates, about 90 ЊC/s to 600 ЊC/s. Its start temperature is estimated to be about 890 ЊC, which is close to the T 0 temperature. The reason for the appearance of massive transformation in CP-Ti is because CP-Ti contains a significant amount of Fe as an impurity, which leads to the T 0 (␤ → ␣ ) vs composition curve being parallel to the composition axis due to its retrograde solubility. The martensitic transformation starts to occur at a cooling rate of about 500 ЊC/s, which is much slower than that (about 3000 ЊC/s) reported in a pure Ti case. This retardation effect of martensitic transformation is also believed to arise from the presence of Fe in CP-Ti, which is a strong ␤ stabilizer.

Spontaneous Formation of Titanium Nitride on the Surface of a Ti Rod Induced by Electro-Discharge-Heat-Treatment in an N2 Atmosphere

“…On the other hand, electrical discharging with 3.0 kJ or greater significantly caused a β→α allotropic transformation, exhibiting a typical serrated morphology. The microstructure is very similar to that of the Ti which was annealed at 1000°C for 2 hours and air cooled [16]. This result implies that a temperature range extending over the β transus was generated by the electrical discharge at input energies of 3.0 kJ or higher.…”

Continuous cooling β-to-α transformation behaviors of extra-pure and commercially pure Ti

“…The start temperature of grade-4 CP-Ti initially decreases rapidly with cooling rate and exhibits a short range of high-temperature plateau before reaching a final low-temperature plateau, similarly to the case of grade-2 CP-Ti. [4] The three characteristic temperatures for the ␤-to-␣ transformation are summarized in a continuous cooling transformation (CCT) curve ( Figure 4). The CCT curve of EP-Ti is characterized by an almost constant starting temperature and a narrow transformation temperature range for the longrange diffusional transformations.…”

“…The grade-4 CP-Ti contains a significantly larger amount of Fe and O than the grade-2 CP-Ti, which was investigated in our previous b → a b → a study. [4] They were machined into sheet specimens of 4 ϫ 38 ϫ 0.4 mm using an electrodischarge machine.…”

“…[1][2][3][4] A careful analysis of data indicated that the detailed results are quite different depending on the investigation and that they are primarily due to the different impurity levels included in each investigation. [4] Furthermore, a recent in-situ measurement of grade-2 commercially pure (CP) Ti indicated that a massive reaction occurs at medium cooling rates at a temperature close to the transformation temperature, T 0 , [4] despite the fact that no massive reaction was reported to occur in a pure Ti.…”

“…[1][2][3][4] A careful analysis of data indicated that the detailed results are quite different depending on the investigation and that they are primarily due to the different impurity levels included in each investigation. [4] Furthermore, a recent in-situ measurement of grade-2 commercially pure (CP) Ti indicated that a massive reaction occurs at medium cooling rates at a temperature close to the transformation temperature, T 0 , [4] despite the fact that no massive reaction was reported to occur in a pure Ti. [1,2,3] The purpose of the present research was to quantitatively measure the continuous cooling transformation behavior of an extra-pure (EP) Ti using our fully computer-controlled resistivity-temperature real-time measurement apparatus in order to re-evaluate the kinetics of fast transformations, i.e., massive and martensitic transformations of an EP-Ti, and to estimate the effect of impurities such as Fe and O on these fast transformations.…”