TiAl based intermetallic alloys

The emergence of TiAl-based intermetallic alloys has its origins both in the strong appeal of this new material with its unique properties, and in the economic and industrial context of cost reduction. A number of factors (weight reduction, lower fuel consumption, maintenance costs, environmental impact), linked to turbomachinery performance, justify aircraft manufacturers' search for lightweight materials capable of withstanding ever-higher operating temperatures for increased power. The selection criteria for these new materials are, on the one hand, changes in turbine inlet temperature, and on the other, changes in thrust/mass ratio.

Looking back over the 1980s, the most significant progress that could be hoped for in conventional titanium alloys lay in the incremental optimization of existing alloy transformation processes. The horizon was closed with these alloys, in particular because of the problems associated with oxidation above 600°C, which limits the temperature of use. At the same time, TiAl had interesting physical properties compared with titanium in terms of specific rigidity and fire resistance. In addition, its static and cyclic properties were potentially at least equivalent to those of nickel-based superalloys. The development of these new ordered intermetallics was considered highly promising, with an expected temperature capability of up to 850°C. Ti ₃ Al-based materials were the first titanium aluminides to be investigated in the 1980s, but they proved too limited in oxidation and creep resistance. Research into, and then development of, TiAl-based alloys began in the early 1990s and continued into 2020, in particular to achieve higher operating temperatures of around 850°C.

The aim of this article is to provide a basic understanding of TiAl-based intermetallic alloys, both for the curious reader wishing to know more about these materials, and for technicians and engineers seeking lightweight materials with reproducible properties for high-temperature application. There are many TiAl-based alloys with various additive elements, as evidenced by the hundreds of patents filed. Despite this wealth, this article hopes to demonstrate that chemical composition may not be the primary factor controlling the mechanical properties of TiAl. In fact, phase transformations are intrinsically correlated with the chemistry of these alloys, and it is these that govern the microstructure through elaboration processes and heat treatments. It is therefore necessary to understand all these factors in order to optimize the possible level of performance. Last but not least, we need to keep a critical eye on the technical and economic aspects when choosing the most suitable dies and the best alloys.

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