This schematic illustration of the new palladium-containing high-entropy alloy shows the presence of large palladium clusters (blue atoms). Image: Ting Zhu.
This schematic illustration of the new palladium-containing high-entropy alloy shows the presence of large palladium clusters (blue atoms). Image: Ting Zhu.

High-entropy alloys, which are made from nearly equal parts of several primary metals, could hold great potential for creating materials with superior mechanical properties. But with a practically unlimited number of possible combinations, one challenge for metallurgists is figuring out where to focus their research efforts in a vast, unexplored world of metallic mixtures.

A team of researchers from China and the US, including from the Georgia Institute of Technology, has developed a new process that could help guide such efforts. Their approach involves building an atomic-resolution chemical map to help gain new insights into individual high-entropy alloys and characterize their properties.

In a paper in Nature, the researchers report using energy-dispersive X-ray spectroscopy to create maps of individual metals in two high-entropy alloys. This spectroscopy technique, used in conjunction with transmission electron microscopy, detects X-rays emitted from a sample during bombardment by an electron beam, in order to characterize the elemental composition of the sample.

The maps show how individual atoms arrange themselves within the alloys, allowing researchers to look for patterns that could help them design new alloys with specific properties. For example, the maps could give researchers clues to understand why substituting one metal for another could make an alloy stronger or weaker, or why one metal outperforms others in extremely cold environments.

"Most alloys used in engineering applications have only one primary metal, such as iron in steel or nickel in nickel-based superalloys, with relatively small amounts of other metals," explained Ting Zhu, a professor in the George W. Woodruff School of Mechanical Engineering at Georgia Tech. "These new alloys that have relatively high concentrations of five or more metals open up the possibility of unconventional alloys that may have unprecedented properties. But this is a new compositional space that has not been explored, and we still have a very limited understanding of this class of materials."

The term ‘high entropy’ refers to the lack of uniformity in the mixture of metals, as well as to the many different and somewhat random ways the atoms from the metals can be arranged as they are combined. The new maps could help researchers determine whether there are any unconventional atomic structures such alloys can adopt that might be leveraged for engineering applications, and how much control researchers could have over the mixtures in order to ‘tune’ them for specific traits, Zhu said.

To test the new imaging approach, the research team compared two high-entropy alloys containing five metals. One alloy was a mixture of chromium, iron, cobalt, nickel and manganese, a combination commonly referred to as a ‘Cantor’ alloy. The other was similar but substituted palladium for the manganese, and that one substitution resulted in much different behavior in how the atoms arranged themselves in the mixture.

"In the Cantor alloy, the distribution of all five elements is consistently random," Zhu said. "But with the new alloy containing palladium, the elements show significant aggregations due to the much different atomic size of palladium atoms as well as their difference in electronegativity compared to the other elements."

In the new alloy with palladium, the mapping showed that the palladium tended to form large clusters, while the cobalt seemed to collect in places where the iron was in low concentrations. Those aggregations, with their sizes and spacings in the range of a few nanometers, provide strong deformation resistance and could explain the differences in mechanical properties from one high-entropy alloy to another. In straining tests, the alloy with palladium showed a higher yield strength, but similar strain hardening and tensile ductility as the Cantor alloy.

"The atomic scale modulation of element distribution produces the fluctuation of lattice resistance, which strongly tunes dislocation behaviors," said Qian Yu, a co-author of the paper and a professor at Zhejiang University in China. "Such modulation occurs at a scale that is finer than precipitation hardening and is larger than that of traditional solid solution strengthening. And it provides understanding for the intrinsic character of high-entropy alloys."

These findings could allow researchers to custom design alloys in the future, leveraging one property or another.

"We believe that this work is really important, as local chemical ordering in these extremely high profile, high-entropy alloys is critical to dictating their properties." said Robert Ritchie, another co-author and a professor at the University of California, Berkeley. "Indeed, this presents a way to tailor these materials to attain optimal properties by atomic design."

This story is adapted from material from the Georgia Institute of Technology, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.