Adventures in alloy: Exploring the limits of advanced titanium alloy composites

TiAl specimen mounted in the MCL testing device during a temperature calibration experiment
TiAl specimen mounted in the MCL testing device during a temperature calibration experiment

Material performance improvements are a key enabler for making aircraft lighter, more efficient and therefore more environmentally friendly. In this Clean Sky project, Rolls- Royce plc explored the capabilities of Titanium Aluminide – which has high specific strength and modulus, plus good corrosion resistance.

In the TiAlCracks Clean Sky project, which received European Union funding and finished in October 2019, Rolls-Royce plc teamed up with the Materials Center Leoben Forschung GmbH (MCL) in Austria to carry out an evaluation of Titanium Aluminide (TiAl), a class of brittle intermetallic alloy, to gain a deeper understanding of its properties.

’Europe’s aviation industry is continually pushing to improve the fuel efficiency of gas turbine engines in order to reduce CO2 emissions and environmental impact,’ says Nigel Martin, materials specialist at Rolls-Royce in Derby, UK. ’Efficiency is determined by the design of turbomachinery, engine temperatures and the rotating mass, which drive the thermodynamics of the heat engine cycle.’

Titanium Aluminide has around half the density of nickel super alloys, making it an attractive candidate in aerospace due to its extremely good specific strength and modulus, where stiffness and light weight are required. TiAl alloys also offer good corrosion resistance. What this means for aircraft designers is the potential to produce components with a lower mass and higher performance which in turn increases engine efficiency and reduces emissions. However, before these materials can be deployed it is necessary to fully characterise their properties and behaviour to ensure high levels of product safety.

’With the introduction of a new material you have to understand it extremely well and show that it’s fit for purpose throughout the potential situations that could happen during the operation of a jet engine,’ says Martin. ’So you have to be very rigorous in testing and understand its performance before designing components and introducing them into jet engines.’

To undertake this testing the Materials Center Leoben set about probing the performance of TiAl at temperatures of up to 800°C under various loading conditions, verifying whether the material would be able to withstand the demands of an aircraft engine and examining how it performs in a variety of service conditions. ’There are only a few institutions in Europe that are able to perform the very specific type of tests we required at these high temperatures. For the Materials Center Leoben this was a great opportunity to start working with Rolls-Royce, and effectively develop that capability and testing facilities and get the experience of this technology,’ says Sven Eck, project manager at MCL.

Scanning electron microscopy (SEM) images of a TiAl specimen before and after a fatigue testing
Scanning electron microscopy (SEM) images of a TiAl specimen before and after a fatigue testing

’We’re all aware of the impact that environmental changes are having on our planet,’ says Martin. ’This project helps us understand what performance benefits can be gained through the application of new materials technology in order to minimise the impact of the aerospace industry. Clean Sky funding has supported this R&T project which has aimed at pushing fundamental materials understanding and therefore enabling more highly refined product to generate a competitive advantage for Europe.’

’The objectives of Clean Sky really mesh well with Rolls-Royce’s objectives for improving environmental performance, and improving engine efficiency, both because of the impact on the environment as well as airlines seeking to have more fuel-efficient aircraft, as fuel forms a significant percentage of the overall costs of running an airline. Projects like these fulfil the objectives of Clean Sky in terms of the environmental benefits but also the objectives of developing the capability, and the industry, within Europe, to maintain its competitiveness in a worldwide marketplace. That’s really the objective, and it’s facilitated by funding available for programmes where you can foresee that the technologies will meet those requirements,’ says Martin.

Engine designers now have a valuable learning resource regarding TiAl and its properties which they can then assess for applicability in future projects, as Martin explains: ’If the right opportunity comes around, we could use this material. The positives so far are that we’ve developed a fuller understanding of the capabilities of the material so that when we’re in a position where, if we do need to use it in the future, we have a very good basis on which to start. We’ve obviously had multiple benefits of developing a detailed understanding of the material and developing the capabilities and understanding within a European test facility, so these are the bigger picture benefits that it has as well, which align with the objectives of Horizon 2020 by bringing European companies together and developing their capabilities.’

Rosario Trillo Rivas, project officer at Clean Sky, adds that TiAl is much lighter in comparison to the other materials currently used, and with the understanding of its properties obtained from this project, the industry will be able to conduct further research on the subject. ’This will contribute to reducing fuel consumption and emissions with the application of such lighter and better performing materials. Rolls- Royce and MCL have collaborated very well in this project allowing them to improve their material knowledge. For Clean Sky and our partners, this is a step ahead!’

Testing innovative high-temperature materials for aircrafts engines MCL researchers work in the EU programme CleanSky together with Rolls-Royce plc
Testing innovative high-temperature materials for aircrafts engines MCL researchers work in the EU programme CleanSky together with Rolls-Royce plc

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