When it comes to designing new civilian and military engines that will meet all the future dreams and demands of the aerospace industry and those who regulate it, there is a general consensus among experts that it will take vast advances and enhancements in materials technology to meet future efficiency and emissions requirements; requirements that can truly only be speculated about today. And in preparation of such challenges, the U.S. Army Research Laboratory (ARL) is starting small. Very small.
ARL scientists are focused on developing new bulk-structured nanomaterials that are stabilized to high temperatures, a particularly important characteristic considering that temperatures can rise to more than 2500°F (4532ºC) inside turbine engines.
"What we're seeing is a revolutionary property arising in a class of materials that we never thought was possible," said Dr. Kristopher A. Darling, a materials scientist with the ARL's Lightweight and Specialty Metals Branch. "The properties that we're seeing are startling I would say. They have the potential to revolutionize a lot of potential applications."
According to Darling, he and his team stabilized a copper alloy microstructure and found it to be strong at very high temperatures. "Normally this wouldn't happen. We're trying to make microstructures survive at the high temperatures of consolidation," he said.
The DoD's continued dependence on gas turbine engines means that engine materials will increasingly require high-structural strength with high thermal stability. "This particular material is copper, which is not a type of material that you wouldn't necessarily use in an engine," said Darling. "But what it demonstrates is that these types of microstructures are capable of achieving properties that are extraordinarily high in comparison to what you would normally see in a conventional type of material."
Ultimately, the team hopes to be able to pass the properties on to other types of materials.
"If we could do this in other materials like nickel, cobalt, or tantalum, it would definitely have a chance of revolutionizing engine technology," he said. "What's unique about this finding is that it takes what is currently known about material systems and sort of flips it on its head and says that things that people thought were impossible are possible. Now we have high strength at high temperature."
Darling said it's all about creep response, or how materials deform under continuous stress at elevated temperatures.
"We're seeing orders of magnitude improvements in the creep response," he said. "There is a six to eight orders of magnitude increase in creep response relative to what conventional nanocrystal materials can do."
Apparently, even the scientists on the team were somewhat perplexed during preliminary tests. "A lot of times unique discoveries are made by accident or by sheer luck," said Darling, "and this was one of those cases where we started to probe the material and we didn't see the response that we were expecting."
Darling reached out to colleagues at the University of North Texas and Arizona State University to help make sure what they were observing was correct.
"Frankly when I talked with them, they didn't believe it either," he said. "We're seeing these properties and they said there's no way that this could be possible. They began to test the samples themselves and then they were amazed. From there, all the excitement grew and we started doing more thorough studies."
Darling has been studying this material for about five years. He started working at the laboratory after earning his doctoral degree from North Carolina State University in 2009.
"Initially it started off about the time I was a postdoc," he said. "There have been a number of people within the Army Research Lab who were heavily involved initially in the research projects and still are. Previously, published work on this alloy led to [me] being introduced into some of the university collaborators that we work with now, specifically Professor Kiran Solanki at Arizona State and Professor Rajiv Mishra at University of North Texas. They've been heavily involved in doing some of the mechanical testing and characterization. It's been a real group effort."
The team hopes to maximize the material properties, or engineer it to its finest extent.
"We could still see large improvements in understanding how to manipulate the microstructure to improve the properties even further," said Darling.
Nanotechnology research in metals is a relatively young field. Going back just a couple of decades, people were just learning how to synthesize these materials in bulk form and trying to understand the basic fundamental properties of these materials.
"What we're showing is that these materials show properties reminiscent of the properties reported with nickel-based single crystals. The results are shocking in the sense that the behavior is so much better than what you would expect from a material like this," Darling said. "Materials like this could possibly open a door to revolutionizing engine technology, which would be of benefit to the Army as well as civilian applications."