High pressure key to lighter, stronger metal alloys, Stanford scientists suggest

Subjecting complex metal mixtures called high-entropy alloys to extremely high pressures could lead to finer control over the arrangement of their atoms, which in turn can result in more desirable properties.

Humans have been blending metals together to create alloys with unique properties for thousands of years. But traditional alloys typically consist of one or two dominant metals with a pinch of other metals or elements thrown in. Classic examples include adding tin to copper to make bronze, or carbon to iron to create steel.

In contrast, “high-entropy” alloys consist of multiple metals mixed in approximately equal amounts. The result is stronger and lighter alloys that are more resistant to heat, corrosion and radiation, and that might even possess unique mechanical, magnetic or electrical properties.

Despite significant interest from material scientists, high-entropy alloys have yet to make the leap from the lab to actual products. One major reason is that scientists haven’t yet figured out how to precisely control the arrangement, or packing structure, of the constituent atoms. How an alloy’s atoms are arranged can significantly influence its properties, helping determine, for example, whether it is stiff or ductile, strong or brittle.

“Some of the most useful alloys are made up of metal atoms arranged in a combination of packing structures,” said study first author Cameron Tracy, a postdoctoral researcher at Stanford’s School of Earth, Energy & Environmental Sciences and the Center for International Security and Cooperation (CISAC).

A new structure
To date, scientists have only been able to re-create two types of packing structures with most high-entropy alloys, called body-centered cubic and face-centered cubic. A third, common packing structure has largely eluded scientists’ efforts until now.

In the new study, published online in the journal Nature Communications, Tracy and his colleagues report that they have successfully created a high-entropy alloy, made of common and readily available metals, with a so-called hexagonal close-packed (HCP) structure.

“A small number of high-entropy alloys with the HCP structure have been made in the last few years, but they contain a lot of exotic elements such as alkali metals and rare earth metals,” Tracy said. “What we managed to do is to make an HCP high-entropy alloy from common metals that are typically used in engineering applications.”

The trick, it appears, is high pressure. Tracy and his colleagues used an instrument called a diamond-anvil cell to subject tiny samples of a high-entropy alloy to pressures as high as 55 gigapascals – roughly the pressure one would encounter in the Earth’s mantle.

“The only time you would ever naturally see that pressure on the Earth’s surface is during a really big meteorite impact,” Tracy said.