A new material might contribute to a reduction of the fossil fuels consumed by aircraft engines and gas turbines in the future. A research team from Karlsruhe Institute of Technology (KIT) has developed a refractory metal-based alloy with properties unparalleled to date. The novel combination of chromium, molybdenum, and silicon is ductile at ambient temperature. With its melting temperature of about 2,000 °C, it remains stable even at high temperatures and is at the same time oxidation resistant. The results are published in the journal Nature.
High-temperature-resistant metallic materials are required for aircraft engines, gas turbines, X-ray units, and many other technical applications. Refractory metals such as tungsten, molybdenum, and chromium, whose melting points are around or higher than 2,000 °C, can be most resistant to high temperatures. Their practical application, however, has limitations: They are brittle at room temperature and, in contact with oxygen, they start to oxidize causing failure within a short time at temperatures of 600 to 700 °C. Therefore, they can only be used under technically complex vacuum conditions — for example as X-ray rotating anodes.
Due to these challenges, superalloys based on nickel have been used for decades in components that are exposed to air or combustion gases at high temperatures. They are used, for example, as standard materials for gas turbines. “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,” explained Professor Martin Heilmaier, KIT Institute for Applied Materials — Materials Science and Engineering. “However — and there is the rub — the operating temperatures, i.e. the temperatures at which they can be used safely, are in the range of up to 1,100 °C 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.”
Here is an exclusive Tech Briefs interview, edited for length and clarity, with Heilmaier.
Tech Briefs: What was the biggest technical challenge you faced while developing this new material?
Heilmaier: Our motivation was to develop a refractory element-based structural material that could for the first time combine decent oxidation/corrosion resistance with some room temperature deformability (ductility). All commercially available Mo-, W-, Nb- or Ta-based alloys suffer from rapid oxidation at elevated temperatures, either forming volatile oxides (Mo, W) or bulky non-protective ones (Nb, Ta). However, the delicate combination of Cr (forming protective Chromia) and Mo plus some Si (but not too much, so as to avoid the formation of brittle Cr-silicides) surprisingly showed the desired property combination. This was simply not expected by us, nor could it have been predicted by first principles.
Tech Briefs: Do you have any set plans for further research/work/etc.? If not, what are your next steps?
Heilmaier: One of the greatest challenges will now be to check whether this alloy also possesses some ductility in tensile tests (we tested it under compression). There might be several impediments that hinder any application if there is not adequate tensile ductility. If so, we expect that we will need a grain size range that initiates both twin formation as well as conventional dislocation movement. So, to find the right grain size range is the challenge which we will tackle next.
The second challenge is to enhance the oxidation resistance even beyond the current limit, which we see at 1100 °C. We have some ideas on that, but do not want to reveal it presently.
The third challenge is to increase the high-temperature creep resistance without compromising the oxidation resistance. This can be done in essence by introducing a second strengthening phase such as precipitates or dispersoids. This is a task which the whole materials science community may work on.
All in all, these issues require modifications of the initial alloy composition, but this is a wide field or playground now!
Tech Briefs: Is there anything else you’d like to add that I didn’t touch upon?
Heilmaier: Of course, after all the fundamental investigations, at some point in time we need to do the next step in commercialization, which means to find a company which is willing to produce larger quantities of the alloy system. But this step could only be made after the above challenges have been addressed.
