β→α+β isothermal phase transformation kinetics in Ti-B19 metastable titanium alloy

The isothermal phase transformation kinetics of β→α+β at the temperature range 400°C∼700°C in Ti-B19 metastable titanium alloy have been studied by in-suit electronical resistivity measurement and synchrocyclotron X-ray diffraction. The results show that the isothermal phase transformation of Ti-B19 alloy is the process of nucleation and growth, controlled by long range diffusion, and can be described by JMA equation. The highest transformation rate appeared on 500°C isothermal treatment condition. When the temperature was above 500°C, the transformation rate decreased with the temperature increase due to the insufficient transforming driving force. When the temperature was below 500°C, it took more time to finish the phase transforming due to the influence of solute diffusion rate. The highest transformation rate in each isothermal temperature appeared in the stable transforming stage, the second stage in the kinetic curve. The highest a phase amount with 57.7% precipitated at 500°C, then the amount decreased with the isothermal temperature increase. Based on the results of electronical resistivity measurement, the time-temperature-transformation diagram (TTT diagram) have been plotted for the studied alloy.