Researchers Get Closer to Producing High-Entropy Alloys with Better Properties

Researchers from the Stanford University have discovered that advanced metal mixtures that are stronger, lighter, and highly heat-resistant when compared to traditional alloys can be synthesized by means of high pressure.

For many millenniums now, metals have been blended together to produce alloys that have distinctive characteristics. However, conventional alloys commonly include one or two dominant metals and lesser amount of other metals or elements. A few classic examples include making bronze by adding tin to copper, and making steel by adding carbon to iron.

On the contrary, “high-entropy” alloys comprise of multiple metals blended in nearly equal amounts, resulting in lighter and stronger alloys. Such alloys are more resistant to radiation, corrosion, and heat, and may even have distinctive electrical, magnetic, or mechanical characteristics.

Although material scientists have given more focus toward high-entropy alloys, these alloys are still in the laboratory investigation phase and have not reached the market. A main cause of this is that researchers have not been successful in accurately regulating the arrangement (i.e. packing structure) of the constituent atoms. The arrangement of atoms in an alloy can have a remarkable impact on its characteristics, thus assisting in finding out whether the alloy is, for instance, strong or brittle, stiff or ductile.

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

A new structure

Until now, researchers have been successful in re-creating only two kinds of packing structures with most of the high-entropy alloys—namely, body-centered cubic and face-centered cubic—but they have not been able to re-create a third, common packing structure.

In this research, Tracy and his team report that they have successfully developed a high-entropy alloy formed of common and readily available metals. This alloy has a hexagonal close-packed (HCP) structure. The outcomes of the research have been published online in the Nature Communications journal.

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. What we managed to do is to make an HCP high-entropy alloy from common metals that are typically used in engineering applications.

Cameron Tracy, Postdoctoral Researcher, School of Earth, Energy & Environmental Sciences, Stanford University

It seems that the clue is to use high pressure. Tracy and his team used an instrument known as a diamond-anvil cell to apply a high pressure of around 55 GPa on small samples of a high-entropy alloy. This pressure is approximately equal to that inside 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,” stated Tracy.

Application of high pressure was seen to stimulate a transformation in the high-entropy alloy used by the researchers, which comprised of chromium, manganese, iron, cobalt, and nickel. “Imagine the atoms as a layer of ping pong balls on a table, and then adding more layers on top. That can form a face-centered cubic packing structure. But if you shift some of the layers slightly relative to the first one, you would get a hexagonal close-packed structure,” explained Tracy.

According to researchers, this transformation does not occur naturally in the high-entropy alloys because it is intercepted by the interacting magnetic forces among the metal atoms. However, high pressure was observed to disturb the magnetic interactions.

When you pressurize a material, you push all of the atoms closer together. Oftentimes, when you compress something, it becomes less magnetic. That’s what appears to be happening here: compressing the high-entropy alloy makes it non-magnetic or close to non-magnetic, and an HCP phase is suddenly possible.

Cameron Tracy, Postdoctoral Researcher, School of Earth, Energy & Environmental Sciences, Stanford University

Stable configuration

It was astonishing to note that the alloy retained the HCP structure even when the pressure was removed. “Most of the time, when you take the pressure away, the atoms snap back to their previous configuration. But that’s not happening here, and that’s really surprising,” explained Wendy Mao, who is an associate professor of geological sciences at Stanford’s School of Earth, Energy & Environmental Sciences as well as a coauthor of the study.

The researchers also found out that when the pressure was gradually increased, the amount of hexagonal close-pack structure in the alloy also increased. “This suggests it’s possible to tailor the material to give us exactly the mechanical properties that we want for a particular application,” stated Tracy.

For instance, in contrast to power plants and combustion engines that operate in a highly efficient manner at higher temperatures, traditional alloys do not perform well under drastic conditions as their atoms start to move and become highly disordered.

High-entropy alloys, however, already possess a high degree of disorder due to their highly intermingled natures. As a result, they have mechanical properties that are great at low temperatures and stay great at high temperatures.

Cameron Tracy, Postdoctoral Researcher, School of Earth, Energy & Environmental Sciences, Stanford University

Further research might enable materials scientists to enhance the characteristics of high-entropy alloys even more by blending together different elements and metals. “There’s a huge part of the periodic table and so many permutations to be explored,” stated Mao.

Other Stanford coauthors on the study include Rodney Ewing, senior fellow at the Stanford Freeman Spogli Institute for International Studies and a professor of geological and environmental sciences; and graduate students Sulgiye Park and Dylan Rittman; and colleagues at the University of Tennessee and Oak Ridge National Laboratory. The U.S. Department of Energy and the National Science Foundation funded the study.