| Mar 05, 2025 |
Researchers devised a new method for designing metals and alloys that can withstand extreme impacts, which could lead to the development of automobiles, aircraft and armor that can better endure high-speed impacts, extreme heat and stress.
(Nanowerk News) A Cornell University-led collaboration devised a new method for designing metals and alloys that can withstand extreme impacts, which could lead to the development of automobiles, aircraft and armor that can better endure high-speed impacts, extreme heat and stress.
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The research, published in Communications Materials (“Suppressed ballistic transport of dislocations at strain rates up to 109 s–1 in a stable nanocrystalline alloy”), introduces nanometer-scale speed bumps that suppress a fundamental transition that controls how metallic materials deform.
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The project was led by Mostafa Hassani, assistant professor of mechanical and aerospace engineering, in collaboration with researchers from the Army Research Laboratory (ARL). The paper’s co-lead authors were doctoral candidate Qi Tang and postdoctoral researcher Jianxiong Li.
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| This laser confocal microscopy reconstruction shows the impression of a spherical microprojectile impact. (Image: Cornell University)
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When a metallic material is struck at an extremely high speed – think highway collisions and ballistic impacts – the material immediately ruptures and fails. The reason for that failure is embrittlement – the material loses ductility (the ability to bend without breaking) when deformed rapidly. However, embrittlement is a fickle process: If you take the same material and bend it slowly, it will deform but not break right way.
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That malleable quality in metals is the result of tiny defects, or dislocations, that move through the crystalline grain until they encounter a barrier. During rapid, extreme strains, the dislocations accelerate – at speeds of kilometers per second – and begin interacting with lattice vibrations, or phonons, which create a substantial resistance. This is where a fundamental transition occurs – from a so-called thermally activated glide to a ballistic transport – leading to significant drag and, ultimately, embrittlement.
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Hassani’s team worked with the ARL researchers to create a nanocrystalline alloy, copper-tantalum (Cu-3Ta). Nanocrystalline copper’s grains are so small, the dislocations’ movement would be inherently limited, and that movement was further confined by the inclusion of nanometer clusters of tantalum inside the grains.
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To test the material, Hassani’s lab used a custom-built tabletop platform that launches, via laser pulse, spherical microprojectiles that are 10 microns in size and reach speeds of up to 1 kilometer per second – faster than an airplane. The microprojectiles strike a target material, and the impact is recorded by a high-speed camera. The researchers ran the experiment with pure copper, then with copper-tantalum. They also repeated the experiment at a slower rate with a spherical tip that was gradually pushed into the substrate, indenting it.
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In a conventional metal or alloy, dislocations can travel several dozen microns without any barriers. But in nanocrystalline copper-tantalum, the dislocations could barely move more than a few nanometers, which are 1,000 times smaller than a micron, before they were stopped in their tracks. Embrittlement was effectively suppressed.
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“This is the first time we see a behavior like this at such a high rate. And this is just one microstructure, one composition that we have studied,” Hassani said. “Can we tune the composition and microstructure to control dislocation-phonon drag? Can we predict the extent of dislocation-phonon interactions?”
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