Abstract
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 °C/min) give insights in the kinetics of the second-order type reaction of ordered (βTi)o to disordered (βTi) as well as the three first-order type transformations from Ti3Al to (αTi), ωo (Ti4NbAl3) to (βTi)o, and O (Ti2NbAl) to (βTi)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.
Highlights
For future high-temperature structural applications, light-weight intermetallic materials based on TiAl alloy systems play an increasingly important role
On the left-hand side (Figure 1a), the microstructure of alloy Ti-36.4Al-9.7Nb (A4) heattreated at 900 ◦C for 650 h is shown with the ωo phase as bright matrix phase and fine precipitates of Ti3Al and TiAl
The measured transformation temperatures were found to be independent of the heating rate, while in the other cases is confirmed by the present results
Summary
For future high-temperature structural applications, light-weight intermetallic materials based on TiAl alloy systems play an increasingly important role. These alloy systems have been investigated extensively in recent decades for their energy-saving and lightweight construction potential [1,2]. Parts manufactured out of TiAl-based alloys have been introduced in aviation and car industries over the years [2,3,4]. To further improve their properties, it is essential to create a well-defined and stable microstructure at application temperature. Since the mechanical properties are controlled by the phases and microstructure present at application temperature, a lot of experimental and theoretical studies have been performed to determine the phases, phase transformations, resulting microstructure morphologies, and the influence of certain alloying elements on mechanical properties [4,5,6,7,8,9,10,11,12,13,14]
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