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

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.

Phase Equilibria in the Ti-Rich Part of the Ti-Al-Nb System—Part I: Low-Temperature Phase Equilibria Between 700 and 900 °C

“…Additionally, in situ HEXRD measurements of alloy B11 prove that x o is present in the entire temperature range from 700 to 900 °C before it transforms to (bTi,Nb) o at a temperature above 900 °C. This was confirmed by DTA investigations, [8] which give equilibrium transformation onset temperatures of 908 and 906 °C for alloys B9 and B11 (corresponding to alloy A4 and A3 in Ref. [8]).…”

“…This was confirmed by DTA investigations, [8] which give equilibrium transformation onset temperatures of 908 and 906 °C for alloys B9 and B11 (corresponding to alloy A4 and A3 in Ref. [8]).…”

“…The measured overall compositions of the samples are indicated by black stars, the yellow lines mark the three-phase equilibria grey symbols and lines indicate the data from the diffusion couples, and blue symbols and lines mark the results from bulk alloys and the resulting phase boundaries of 908 and 906 °C, respectively (see the corresponding alloys A4 and A3 in Ref. [8]) The existence of x o in equilibrium with Ti 3 Al and TiAl at 900°C agrees with results reported by Xu et al, [22] who studied a diffusion multiple annealed at 900 °C, and Rackel et al, [53] who performed in situ HEXRD measurements of an alloy with the composition Ti-42Al-8.5Nb, which lies within our corresponding Ti 3 Al ? TiAl ?…”

“…They are thermodynamically unstable at high temperatures and decompose in solid state reactions above 900 °C. [8] In particular the O phase has attracted a lot of attention in recent years. Alloys based on this phase were found to have improved room-temperature ductility and fracture toughness compared to the above mentioned TiAl-based alloys.…”

“…Further information including heating and cooling cycles as well as evaluation of the measurement data can be found in Ref. [8].…”