Phase Equilibria of TiAl-Based Alloys Containing Ternary and Quaternary Additions of the Refractory Metals Nb, Mo and W

TiAl-based alloys have proven to be capable to replace Ni-based superalloys at temperatures up to about 800°C in applications such as, e.g., turbine blades in aero-engines. The effect of this 50% decrease in weight is a significant reduction in fuel consumption and with that a decrease in greenhouse gas emissions. However, to achieve the ambitious goals of climate-neutral aviation set by the ACARE council (Advisory Council for Aviation Research and Innovation in Europe) of the European Commission for the year 2050, the TiAl-based alloys currently in use and under development still need to be significantly improved, especially with regard to aspects such as their high-temperature formability and their phase and microstructural stability at application temperature above 800°C, i.e., in particular their creep resistance in the critical temperature region.
Nb is the most important alloying element for Ti-Al alloys. In addition to its beneficial effects on the high-temperature oxidation and creep resistance, it is also a β-phase stabilizer, i.e. it stabilizes the cubic (β-Ti) high-temperature phase and thus enables processing (e.g. hot forging of the blades) at very high temperatures. However, above a certain limiting alloying content, further Nb addition results in the formation of the ternary ω_o phase (which often but wrongly is described with the formula Ti₄Al₃Nb). This hexagonal phase has an embrittling effect and is therefore very detrimental for the mechanical properties and should be avoided. Alternative β-stabilizers are Mo and W both having the advantage that in the respective ternary systems no ω_o phase exists. Thus, it is also of particular importance to investigate and understand the combined effect of Nb and Mo or W on the stability and phase fraction of ω_o phase and on the phase relations, microstructure and properties of such TiAl-based alloys.
In this presentation, the results of our experimental phase diagram studies of the three ternary systems Ti-Al-Nb, Ti-Al-Mo, and Ti-Al-W in the temperature range between 700 and 1300°C will be summarized and compared. A focus then will also be on the effect of quaternary additions. Although it can be expected that the addition of Mo or W to Ti-Al-Nb alloys will reduce the amount and/or stability of the ω_o phase, there are no systematic studies on this issue. Therefore, some results obtained from two series of quaternary alloys containing 1-5 at.% Mo or W will be discussed.