ABERDEEN PROVING GROUND, Md. -- Imagine discovering a material in the future that is so resilient that it can withstand extreme pressures and temperatures that could lead to powerful new engines that resist wear and perform well in harsh environments, or armor only dreamed of in science fiction.
In a study recently published in Communication Materials - Nature, materials scientists from the U.S. Army Combat Capabilities Development Command’s Army Research Laboratory describe their development of a highly resilient nanocrystalline copper–tantalum alloy -- performing more than 300-percent better than expected.
“What we found was so profound that is has left many of the experts in the related fields of study to reconsider the fundamental basis for what is possible in designing materials for extreme applications,” said Dr. Kris Darling, a scientist in the lab’s Weapons and Materials Research Directorate.
Scientists make observations about the physical world. Things you can hold in your hand are called the macroscale. Going smaller, the microscale is what you can hold with a pair of tweezers. But the nanoscale is what you can see with the world’s most powerful microscopes and other advanced equipment. Nanoscale materials are only slightly larger than atoms and molecules. This is where Army materials scientists’ research is focused to yield these big discoveries.
The Army is interested in what this unique metal alloy can do because it exhibits exceptional resilience to a multitude of extreme stimulants, researchers said.
“Such a collection of properties, to my knowledge, has not been demonstrated before,” Darling said. “We see high thermo-mechanical strength, high resistance to deformation under constant, prolonged stress at high temperature, an exceptional ability to conduct heat and electrical current, and even resistance to nuclear radiation.”
The newest discovery is the alloy’s performance during shock loading, a process to measure its strength under extreme pressures at high deformation rate.
“Our experiments revealed that the stable microstructure is able to absorb high rate loading with minimal detrimental effects allowing the material to exhibit 300-percent higher spall strength than previously thought possible,” said article co-author Dr. Cyril Williams, also a researcher with the laboratory.
This research combined the expertise of the Shock Compression Science group and the Nano Metals group at the laboratory. The team, with Williams and Dr. Anit Giri as co-principal investigators, successfully performed shock experiments on a thermo-mechanically stabilized, bulk nanocrystalline metal to extreme pressures, 15 gigapascals. One gigapascal, or GPa, is equal to one billion pascals, which is a tremendous amount of pressure – 150,000 times the atmospheric pressure.
“Post mortem analysis revealed that the microstructure to be practically unchanged as compared to the un-shocked state as well as being devoid of any detrimental shock hardening affects,” said Dr. Chad Hornbuckle, the lead author of the paper.
What puzzled the researchers was the spall strength being approximately 300-percent greater than that reported for the same base metal with conventional micron-sized grains. Spallation is simply defined as rupture within a body due to tensile stresses in excess of the material’s tensile strength. Single crystals, because of their perfect atomic structure, approach the theoretical limit for what can be expected for a material’s spall strength.
“For a nanocrystalline metal, which has an atomic structure the polar opposite of a perfect single crystal displaying such high spall strength, this has experts asking a lot of questions,” Darling said.
Bulk stabilized nanocrystalline copper-tantalum alloys have been developed and produced only at the lab. All the previous experimental efforts on shock studies of nanocrystalline materials were limited to thin film samples or computer simulated microstructures, not bulk samples. The results reported by researchers at the laboratory on nanocrystalline CuTa are vastly different from those in the open literature.
The Army has had a robust program in designing thermally-stable, bulk nanostructured metals for more than eight years at the Rodman Materials Research Laboratory at APG. This research program yielded publications in Nature (2016), Nature Communications (2018), and Materials Today (2019).
“The ultimate spall resistant material does not currently exist,” Williams said. “This research could open doors to develop novel spall resistant, protection materials that would be of great benefit to future Soldiers and the U.S. Army.”
The copper-tantalum is a model alloy system, Hornbuckle said. The research will be extended to design other iron- and nickel-based materials, which may have wider application space in terms of temperature and strength.
What makes this research effort different is the Army has succeeded in scaling up the processing of nanocrystalline CuTa beyond the laboratory quantities as seen by the production of large billets of the material. These billets allow for a variety of applications specific testing to be performed, which has never happened before with stabilized nanocrystalline materials. Thus, establishing the material as a viable engineering material. The laboratory owns the rights to several existing patents and ongoing disclosures making it available for licensing.
Another publication showing similar results when experiencing pressure up to as high as 34 GPa generated via laser-driven flyer plate shock will be published in Applied Physics Letters and has been promoted as an editor’s pick publication.
CCDC Army Research Laboratory is an element of the U.S. Army Combat Capabilities Development Command. As the Army’s corporate research laboratory, ARL discovers, innovates and transitions science and technology to ensure dominant strategic land power. Through collaboration across the command’s core technical competencies, CCDC leads in the discovery, development and delivery of the technology-based capabilities required to make Soldiers more lethal to win the nation’s wars and come home safely. CCDC is a major subordinate command of the U.S. Army Futures Command.