By ARL Public AffairsOctober 26, 2018
ABERDEEN PROVING GROUND, MD. -- Researchers from the U.S. Army Research Laboratory and Arizona State University have teamed up on a new material composition that super heroes, if real, would envy.
They've designed a super strong alloy of copper and tantalum that can withstand extreme impact and temperature. It's likely the closest material on earth to vibranium, a rare, fictitious metallic substance found in Marvel's Wakanda and used in Captain America's shield.
Its structure and deformation response make it a candidate for ballistic impact or protection applications for military vehicles or personal protection for Soldiers, said Dr. Kristopher Darling, a materials scientist with ARL's Lightweight and Specialty Metals Branch.
Darling said that even beyond the Army, "anywhere there's high strength and good electrical conductivity is required, these alloys can be thought of as a model system who's structure can be passed on to other alternative material systems. Materials based on iron or aluminum for instance could be used for protection and lethality applications."
The ARL team, which includes Darling, Drs. Cyril Williams and B. Chad Hornbuckle, joined with ASU's Professor Kiran Solanki, Professor Pedro Peralta and six doctoral students in materials science and mechanical engineering on the recently published paper on the alloy in Nature Communications: "Anomalous mechanical behavior of nanocrystalline binary alloys under extreme conditions."
Solanki, an associate professor in the Ira A. Fulton Schools of Engineering, is working on the copper and tantalum alloy, which has the potential to also be used on spacecraft for deep-space exploration. The same methodology can also be applied to other materials, such as nickel or iron, to develop more resilient transportation and sustainable infrastructures.
Most structural metals experience sudden deformation when subjected to extreme impact and temperature, such as the force from an automobile accident or the impact that occurs during a ballistic event. When a typical metal deforms at a fast rate, it loses its ability to deform in a ductile way and becomes brittle, absorbing relatively little energy prior to fracture or failure.
This instability has motivated the multidisciplinary research team to improve the toughness of coarse-grained metals and alloys to prevent metal deformation and failure. They created a nanocrystalline alloy of copper and tantalum with engineering-enhanced properties to make it maintain a relatively consistent level of mechanical strength and microstructure stability.
"The technical challenge was to make a material with an average grain size of about 50 nanometers (billionths of a meter) and remain stable when formed into usable parts or shapes," said Solanki, a co-author of the paper.
The unusual combination of properties in the copper-tantalum alloy results from a processing route that creates distinct nanoclusters of tantalum. As temperature increases, these nanoclusters don't significantly change in size or spacing, which leads to the material's notable stability and strength.
"Within these very small grains, we built in a microstructure that's even smaller than the grain size due to tantalum's nanoclusters," Darling said. "This doubles the material's strength and stability, making it immune to the deformation response."
The alloy can withstand high rates of impact and temperatures in excess of 80 percent of their melting point, which is higher than 1,073 kelvin or greater than 1,472 degrees Fahrenheit), with very little change in its microstructure.
ARL has had a robust program in designing thermally-stable bulk nanostructured metals for more than six years now, Darling said.
This research is based on a previous paper by co-authors Darling, Hornbuckle and Solanki, among others, published in Nature: "Extreme creep resistance in a microstructurally stable nanocrystalline alloy." The material system of the nanocrystalline copper-tantalum alloy is the same, but the properties and behavior are different.
Both papers cover an extreme type of deformation: a very slow strain rate where the material deforms gradually over long exposures to high temperatures versus a very fast strain rate where the material deforms rapidly. A slow strain rate can happen over the years such as when an infrastructure collapses due to its own weight. A fast strain rate can happen almost instantaneously, such as in a picosecond or nanosecond during ballistic events.
The copper-tantalum alloy was originally developed to replace copper-beryllium, a high-performance alloy known for its strength, conductivity, hardness and corrosion resistance. Copper-beryllium is critical for a range of applications, but the handling, manufacturing and machining of beryllium can cause a serious lung condition called chronic beryllium disease. Thus, the International Agency for Research on Cancer and the National Toxicology Program have designated the alloy as a carcinogen.
The team continues to work toward replacing copper-beryllium with an equally superior metal alloy with similarities in its mechanical properties of strength, conductivity, hardness and corrosion resistance. The copper-tantalum alloy is a step in that direction.
Through the teams' combined efforts, society may soon have access to materials with superhero strength without compromising the well-being and health of the people manufacturing them.
The work was supported by the U.S. Army Research Laboratory under contract 911NF-15-2-0038 and the National Science Foundation No. 1663287.
The U.S. Army Research Laboratory is part of the U.S. Army Research, Development and Engineering Command, which has the mission to ensure decisive overmatch for unified land operations to empower the Army, the joint warfighter and our nation. RDECOM is a major subordinate command of the U.S. Army Materiel Command.