Nanoscale tweaks help alloy withstand high-speed impacts

The research, published in Communications Materials, introduces nanometer-scale speed bumps that suppress a fundamental transition that controls how metallic materials deform.

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.

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.

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.

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.

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.

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.

“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?”

The research was supported by the National Science Foundation and the Army Research Office.