The iconic 1967 movie, The Graduate, has an early scene in which “Benjamin Braddocks (Dustin Hoffman), 21, having just graduated from college, is attending his parents’ party. The guest list is classy encouraging elders and middle-class adults [read: their son will be counseled by their friends]. Ben is led outside to the pool where one of these sages tells Ben of a great career he could pursue in the future in “One word: plastics.”
high-entropy alloys change shape triggered by heat
could be used to make turbine blades more efficient
could be used to reduce exhaust nozzles size lower noise
Dr. Ibrahim Karaman, Chevron Professor I and head of the Texas A&M’s Department of Materials Science and Engineering, his co-authors are Demircan Canadinc, William Trehern, and Ji Ma of Texas A&M, and Fanping Sun and Zaffir Chaudhry, Technical Fellow of the United Technologies Research Center (UTRC) have published a study which may have a major impact on the future of aviation. Their advice for Benjamin is:
high-temperature shape memory alloys including high-entropy alloys
The researchers have discovered a set of “smart materials” (HTSMA) which hold “the potential to significantly improve the efficiency of fuel burn in jet engines, cutting the cost of flying …[and]… reduce airplane noise over residential areas. How?
The Texas A&M Shape Memory Alloy Research Team (SMART) are combining two relatively new areas of materials science involving metal alloys, or metals composed of two or more elements. The first area involves shape-memory alloys, “smart” materials that can switch from one shape to another with specific triggers, in this case temperature. Picture a straight metal rod that is bent into a corkscrew. By changing the temperature, the corkscrew turns back into a rod and vice versa.
The first application involves controlling the clearance, or space, between turbine blades and the turbine case in a jet engine. A jet engine is most fuel-efficient when the gap between the turbine blades and the case is minimized. However, this clearance has to have a fair margin to deal with peculiar operating conditions. HTSMAs incorporated into the turbine case could allow the maintenance of the minimum clearance across all flight regimes, thereby improving thrust specific fuel consumption.
“Another important potential application of HTSMAs is the reduction of noise from airplanes as they come in to an airport. Planes with larger exhaust nozzles are quieter, but less efficient in the air. HTSMAs could automatically change the size of the core exhaust nozzle depending on whether the plane is in flight or is landing. Such a change, triggered by the temperatures associated with these modes of operation, could allow both more efficient operation while in the air and quieter conditions at touchdown.”
“Karaman and his colleagues decided to try increasing the operating temperatures of HTSMAs by applying principles from another new class of materials, high-entropy alloys, which are composed of four or more elements mixed together in roughly equal amounts. The team created materials composed of four or more elements known to form shape-memory alloys (nickel, titanium, hafnium, zirconium and palladium), but purposefully omitted gold or platinum.
“When we mixed these elements in equal proportions we found that the resulting materials could work at temperatures well over 500 degrees C—one worked at 700 degrees C—without gold or platinum. That’s a discovery,” said Karaman. “It was also unexpected because the literature suggested otherwise.”
How do the new materials work? Karaman said they have ideas on how they operate at such high temperatures, but do not have solid theories yet. To that end, future work includes trying to understand what is happening at the atomic scale by conducting computer simulations. The researchers also aim to explore ways to improve the materials’ properties even further. Karaman notes, however, that many other questions remain.”
The A&M research is promising, but with all such advances there likely will be considerable time between the concept and its practical application. Another caveat to the Green Community: there should be no expectation of immediate noise reduction. There is a major time gap between theoretical metallurgical research and actual implementation of those possible solutions and actual real world noise reduction.
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