Innovative metal alloy withstands extreme conditions

A new high-temperature material shows great potential – for example, in energy-efficient gas turbines.

Reducing the consumption of fossil fuels – a new material could help achieve this in aircraft engines and gas turbines in the future. A research team at the Karlsruhe Institute of Technology (KIT) has developed a refractory metal alloy with properties never before achieved. This novel combination of chromium, molybdenum and silicon is malleable at room temperature, remains stable even at extreme heat with a melting point of around 2,000 degrees Celsius, and is also resistant to oxidation.
High-temperature-resistant metallic materials are required in aircraft engines, gas turbines, X-ray machines and many other technical applications. The most resistant to high temperatures are refractory metals – that is, metals such as tungsten, molybdenum and chromium – which only melt at temperatures of around 2,000 degrees Celsius. However, their practical application is limited: they are brittle at room temperature and, upon contact with oxygen, begin to oxidise so severely as early as 600 to 700 degrees Celsius that they fail completely within a very short time. For this reason – for example, in the form of rotating X-ray anodes in medical technology – they can only be used under technically complex vacuum conditions.
Due to such challenges, nickel-based superalloys have been used for decades in components that come into contact with air or combustion gases at high temperatures. These are, for example, the standard material used in gas turbines. “Existing superalloys combine various metallic elements to bring together several properties. They are ductile at room temperature, yet strong and oxidation-resistant at high temperatures,” explains Professor Martin Heilmaier from the Institute of Applied Materials – Materials Science at KIT. “However – and here’s the catch – this is only the case at operating temperatures, i.e. within a range where they can be used safely, up to a maximum of 1,100 degrees Celsius. This is not high enough to exploit the full potential for greater efficiency in turbines or other high-temperature applications. This is because, in combustion processes, efficiency increases with temperature.”

Opportunity for a technological leap

Heilmaier’s research group set out to address this limitation of the materials currently available. As part of the Research Training Group “Materials Compounds from Composite Materials for Applications in Extreme Conditions” (MatCom-ComMat), funded by the German Research Foundation (DFG), the researchers succeeded in developing a new alloy consisting of chromium, molybdenum and silicon. This refractory metal alloy – the discovery of which at KIT also involved a significant contribution from Dr Alexander Kauffmann, now a professor at Ruhr University Bochum – offers previously unmatched properties. “It is malleable at room temperature, does not melt until around 2,000 degrees Celsius and – unlike previously known refractory alloys – oxidises only slowly, even in the critical temperature range. This makes it conceivable to use this alloy in components designed for operating temperatures significantly higher than 1,100 degrees Celsius. The research findings therefore have the potential to represent a genuine technological leap,” says Kauffmann – even though, despite major advances in the field of computer-aided materials development, it is not yet possible to predict oxidation resistance and formability with sufficient accuracy for the concrete design of new materials.

Greater efficiency, lower consumption

“In a turbine, even a temperature increase of 100 degrees Celsius can reduce fuel consumption by around five per cent,” explains Heilmaier. This is particularly relevant for the aviation sector, as electrically powered aircraft are unlikely to be suitable for long-haul flights even in the coming decades, and it will be crucial to significantly reduce fuel consumption. Stationary gas turbines in power stations could also be operated with lower CO₂ emissions thanks to more robust materials.
“Many more development steps are still needed before the alloy can be used in industry,” explains Heilmaier. “However, with our discovery in basic research, we have reached an important milestone. Research groups worldwide can now build on this.”
Source: KIT