Solid-Solid Phase Transformations and Their Kinetics in Ti-Al-Nb Alloys

The application of light-weight intermetallic materials to address the growing interest and necessity for reduction of CO2 emissions and environmental concerns has led to intensive research into TiAl-based alloy systems. However, the knowledge about phase relations and transformations is still very incomplete. Therefore, the results presented here from systematic thermal analyses of phase transformations in 12 ternary Ti-Al-Nb alloys and one binary Ti-Al measured with 4-5 different heating rates (0.8 to 10 degrees C/min) give insights in the kinetics of the second-order type reaction of ordered (beta Ti)(o) to disordered (beta Ti) as well as the three first-order type transformations from Ti3Al to (alpha Ti), omega(o) (Ti4NbAl3) to (beta Ti)(o), and O (Ti2NbAl) to (beta Ti)(o). The sometimes-strong heating rate dependence of the transformation temperatures is found to vary systematically in dependence on the complexity of the transformations. The dependence on heating rate is nonlinear in all cases and can be well described by a model for solid-solid phase transformations reported in the literature, which allows the determination of the equilibrium transformation temperatures.