Airbus aero engine
​The materials used in aero engines needs to cope with temperatures up to 1500 °C.​g
Image​ credit: Airbus

On the quest for high-entropy alloys that survive 1500 °C

​An aero-engine should operate at the highest possible temperature for the best output power and energy efficiency. But today’s metal alloys in the engines need cooling – otherwise they turn into powders. This causes alarming energy losses. Saad Sheikh is on the quest to design optimum alloys that survive ultra-high temperatures. 
High-entropy alloys (HEAs), or multi-principal-element alloys, is a new and growing field, and has gained enormous interest in recent years as potential ultra-high temperature materials. The materials and manufacture researcher Saad Sheikh focuses on developing HEAs with optimum tensile ductility and strength, superior than the current state-of-the-art nickel based superalloys. 

This work is driven by the need to improve the energy efficiency of aerospace and power-generation gas-turbine engines. For example, if cooling of aero-engines can be avoided, the aero-engine output power and energy efficiency would increase up to 50%. Other applications like solar power, fuel cells, materials processing and petro-chemistry can also benefit from the results. 

The aim is to be able to operate engines at higher temperatures than today. Today’s engines expose the nickel based superalloys inside to temperatures approaching 1200 °C, which is close to 90% of their melting points. In the hottest region of a turbine engine, temperatures are approaching 1500 °C. By using complex cooling systems and coatings the nickel based superalloys can exist in the hottest region but the efficiency gained from operating at higher temperatures is greatly reduced, as the cooling needs extra work.
Saad Sheikh
– The current situation of higher inefficiency losses is alarming, but also provides opportunity to look for new ground-breaking materials. It is a big but intriguing scientific challenge, says Saad Sheikh.

Saad Sheikh comes from a materials science background and did his Masters in Materials Processing at KTH in Stockholm. Before joining Chalmers University of Technology as a PhD student, he also worked on mechanical properties of cutting tools within the Swedish industry. He is very interested in alloy development and mechanical properties of new structural and high-temperature materials for sustainable energy systems. He explains the difference between HEAs and conventional alloys. 

– Conventional alloys are usually based on one or two principal elements. HEAs consist of at least four principal metallic elements with an atomic percentage of each element between 5 % and 35 %. These multi-component element alloys can enable formation of simple solid solution phases. 

In his research, Saad Sheikh has strived to improve HEAs in several ways. Firstly he has contributed with improved understanding of the solid solubility in HEAs. Secondly he has proposed a mechanism and route for increasing the ductility in refractory, or heat resistant, HEAs – so-called RHEAs.
True tensile stress-strain curve for Hf0.5Nb0.5Ta0.5Ti1.5Zr. The inset shows the microstructure at the fractured surface.

Thirdly, which has been the ultimate goal of his work, Saad Sheikh has addressed the balance of mechanical properties and oxidation resistance for RHEAs, aiming at high-temperature applications. 

– In studies I have found out that the insufficient oxidation resistance in existing ductile RHEAs is attributed to the failure in forming protective oxide scales accompanied by the accelerated internal oxidation leading to pesting corrosion. Aluminizing is a promising solution.

Image: True tensile stress-strain curve for the as-cast Hf0.5Nb0.5Ta0.5Ti1.5Zr. The inset shows the microstructure at the fractured surface.​

These studies provide important input to the further development of RHEAs as novel high-temperature materials and shed light on the design of refractory HEAs with optimal mechanical as well as heat and oxidation resistance properties.

FACTS

Saad Sheikh belongs to the division of Materials and Manufacture at the department of Industrial and Materials Science. He recently presented his doctoral thesis with the title: 

If you want to learn more about refractory high-entropy alloys, we recommend to read:

Saad Sheikh has been granted a postdoc fellowship by the Swedish Foundation for Strategic Research (SSF) and the Japan Society for the Promotion of Science (JSPS). He will be placed in Japan at the National Institute for Materials Science in Tsukuba, with focus on ultra-high temperature materials (alloy design and mechanical properties) for two years. 

Please contact Associate Professor Sheng Guo​, Saad Sheikh's supervisor for more information

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Text: Nina Silow
Images: Airbus, Nina Silow and Saad Sheikh

Published: Wed 27 Jun 2018. Modified: Mon 30 Jul 2018