The real time resistivity-temperature-time measurement system has been modified to operate under high vacuum system. Present study thus allowed to estimate the environmental effect, in particular, the oxygen effect on the α→$γ_L$ lamellar reaction in T-45.5at.%Al alloy. The results of the present study are in good agreement with previous results measured under Ar+He gas atmosphere in Ti-45.5at.%Al alloy except the data at relatively slow cooling rates, i.e., at cooling rates less than about $20℃s^{-1}$. The present data showed significantly lower α→γL lamellar start temperatures at these slow cooling rates. Apart from this difference, two results are very close each other. Namely the lamellar start temperatures are similar each other at relatively fast cooling rates. Similar results were also obtained with respect to the transition cooling rate$(70℃s^{-1})$ for the $α→α_2$ ordering reaction as well as to the ordering start temperature(1070℃).
The effect of B addition on the continuous cooling transformation behavior of Ti-45.5at.%Al alloy was also quantitatively measured. The addition of small amount of B(0.05at.%) raises the Tα transus temperature by 20℃ and enhances the lamellar formation kinetics. At the same time, the B addition significantly raises the lamellar stability by enlarging the lamellar formation regime toward much faster cooling rates. This is believed to be related to the effect of B to enhance nucleation and growth kinetics of lamellar formation in Ti-45.5at.%Al alloys. The addition of B tends to slightly lower the $α→α_2$ ordering start temperature.
The β→α transformation kinetics of CP-Ti during continuous cooling was also measured using the measurement apparatus. Unlikely from the pure Ti case, the massive transformation occurs at medium cooling rates, about 90℃/s to 600℃/s. Its start temperature is estimated to be about 890℃, which is close to the To 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 $T_o(β→α)$ vs. composition curve being parallel to the composition axis due to its retrograde solubility. The martensitic transformation starts to occur at cooling rate about 500℃/s, which is much slower compared with that (about 3000℃/s) reported in a pure Ti case. This retardation effect of martensitic transformation is also believed to arise from the presence of significant amount of Fe in CP-Ti, which is one of strong β-stabilizers.
