High-temperature metals are essential for powering aircraft engines, gas turbines, X-ray systems, and other advanced technologies. Among the most heat-resistant are refractory metals like tungsten, molybdenum, and chromium, all of which have melting points around or above 2,000 degrees Celsius (~3600 degrees Fahrenheit). Despite their exceptional heat tolerance, these metals pose major challenges: they are brittle at normal temperatures and quickly oxidize when exposed to oxygen, leading to failure even at 600 to 700 degrees Celsius (~1100 to 1300 degrees Fahrenheit). Because of this, they can only be used in specialized vacuum environments, such as in X-ray rotating anodes.
To overcome these limitations, engineers have long relied on nickel-based superalloys for parts that must withstand hot air or combustion gases. These materials are standard in gas turbines and other high-temperature systems.
“The existing superalloys are made of many different metallic elements including rarely available ones so that they combine several properties. They are ductile at room temperature, stable at high temperatures, and resistant to oxidation,” explains Professor Martin Heilmaier from KIT’s Institute for Applied Materials — Materials Science and Engineering. “However — and there is the rub — the operating temperatures, i.e. the temperatures in which they can be used safely, are in the range up to 1,100 degrees Celsius maximum. This is too low to exploit the full potential for more efficiency in turbines or other high-temperature applications. The fact is that the efficiency in combustion processes increases with temperature.”
A Chance for a Technological Leap
Recognizing this performance limit, Heilmaier’s team set out to find a new solution. Within the German Research Foundation’s (DFG) “Materials Compounds from Composite Materials for Applications in Extreme Conditions” (MatCom-ComMat) research training group, the team developed a novel alloy combining chromium, molybdenum, and silicon. This refractory metal-based material, in whose discovery Dr. Alexander Kauffmann, now professor at the Ruhr University Bochum, played a major role, exhibits properties never seen before.
“It is ductile at room temperature, its melting point is as high as about 2,000 degrees Celsius, and — unlike refractory alloys known to date — it oxidizes only slowly, even in the critical temperature range. This nurtures the vision of being able to make components suitable for operating temperatures substantially higher than 1,100 degrees Celsius. Thus, the result of our research has the potential to enable a real technological leap,” says Kauffmann. This specifically remarkable as resistance to oxidation and ductility still cannot be predicted sufficiently to allow a targeted material design — despite the great progress that has been achieved in computer-assisted materials development.
More Efficiency, Less Consumption
“In a turbine, even a temperature increase of just 100 degrees Celsius can reduce fuel consumption by about five percent,” explains Heilmaier. “This is particularly relevant to aviation, as airplanes powered by electricity will hardly be suitable for long-haul flights in the next decades. Thus, a significant reduction of the fuel consumption will be a vital issue. Stationary gas turbines in power plants could also be operated with lower CO2 emissions thanks to more robust materials. In order to be able to use the alloy on an industrial level, many other development steps are necessary,” says Heilmaier. “However, with our discovery in fundamental research, we have reached an important milestone. Research groups all over the world can now build on this achievement.”
