Researchers at Stanford University (Stanford, California) believe high pressure could enable the production of lighter, stronger alloys made from five or more mixed metals that are more resistant to heat, corrosion, and radiation than conventional alloys, according to results from a recent study.1
These new alloys, referred to as high-entropy alloys, are created by mixing multiple metals in approximately equal amounts and then exposing them to high pressure. This process contrasts conventional alloy production, which typically consists of one or two dominant metals with a pinch of other metals or elements thrown in. Classic examples of conventional alloys include the addition of tin to copper to make bronze, or carbon to iron to create steel.
“Right now, high-entropy alloys are mainly of interest for research,” says Cameron Tracy, a postdoctoral researcher at Stanford. “This is mainly because the field is still in its infancy. While this class of alloys was discovered a little over 10 years ago, it is only in the last few years that research interest has exploded, as some of the remarkable properties of these materials have been discovered.”
“So, while there are many prospects for the replacement of conventional alloys with high-entropy alloys possessing superior properties, the design, testing, and certification processes necessary for commercialization will take some time,” he adds.
Better Precision with Atomic Structure
In the initial years of testing these high-entropy alloys, the most common atomic structure found was called the face-centered cubic (FCC). This entails the arrangement of atoms in such a way that layers of atoms can easily slide past one another.
“This means that FCC materials are usually very ductile, because this sliding of atoms relative to one another allows the material itself to deform,” Tracy explains. “In contrast, atoms in a hexagonal close-packed [HCP] structure do not slide past one another so easily, meaning that HCP materials have low ductility and will usually not deform when pushed or pulled.”
“Imagine the atoms as a layer of ping pong balls on a table, and then adding more layers on top,” he adds. “That can form an FCC packing structure. But if you shift some of the layers slightly relative to the first one, you would get an HCP structure.”
The problem for researchers in the early testing was that they did not understand the link between creating an alloy with the FCC structure and creating one with the HCP structure.
“In industrial applications, ductility can be desirable or undesirable,” Tracy explains. “If you want to pound a metal into a thin sheet, you probably want high ductility. If a ship hits an iceberg, it is typically better for the ship’s hull to dent, due to its high ductility, then to crack, due to its low ductility.”
“However, if a component has to support a certain load without deforming too much—as in the case of a beam that holds a building together—high strength and low ductility are desirable,” he says. “By using pressure to control the ratios of the FCC and HCP structures in a high-entropy alloy, we can potentially tailor the ductility of the material to what is ideal for a specific application.”
Role of High Pressure
While some high-entropy alloys with the HCP structure were made in recent years, most contained exotic elements such as alkali metals and rare earth metals, Tracy says. Based on the rarity of these metals, they would likely be too expensive for commercial use.
But the team led by Tracy and Wendy Mao, an associate professor of geological sciences who co-authored the study, found a way to create HCP alloys from common metals that are typically used in engineering applications. In turn, based on doing this, they have a better understanding of what creates the FCC alloys as well.
The secret seems to be high pressure. Tracy and his colleagues used an instrument called a diamond-anvil cell to subject small samples of high-entropy alloys to pressures as high as 55 GPa. The team’s study included an alloy consisting of manganese, cobalt, iron, nickel, and chromium. The high pressure appeared to trigger a transformation in the alloy to an HCP structure, Tracy explains.
According to the researchers, scientists have speculated that the reason many high-entropy alloys don’t undergo this shift naturally is because interacting magnetic forces between the metal atoms prevent it from happening. However, high pressure seems to disrupt the magnetic interactions.
“When you pressurize a material, you push all of the atoms closer together,” Tracy explains. “Often times, 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.”
Even after the pressure was removed, the alloy in the study still retained its HCP structure.
“Most of the time, when you take the pressure away, the atoms snap back to their previous configuration,” Mao says. “But that’s not happening here, and that’s really surprising.”
Steps to Commercialization
The team believes full commercialization of these high-entropy alloys is still some time away, since the design, testing, and certification processes are still in the works. However, once those steps are complete, the researchers say they are very optimistic that these alloys will catch on quickly.
Specifically, Tracy says conventional alloys tend not to perform well in extreme temperatures because their atoms started moving around and become more disordered.
“High-entropy alloys, however, already possess a high degree of disorder due to their highly intermingled measures,” Tracy says. “As a result, they have mechanical properties that are great at low temperatures and stay great at high temperatures. One of the likely areas for the first commercial use of these materials is the aerospace industry, as there is a great need for strong metals that can withstand very high temperatures for use in gas turbine engines.”
The key to commercialization, of course, is replacing the rare earth elements used to create the initial high-entropy alloys with the HCP structure with cheaper and more abundant metals, such as the chromium, manganese, iron, cobalt, and nickel used in the Stanford study.
“High-entropy alloys that eventually find use in commercial applications will likely be made of elements like these and similarly cheap metals like aluminum or titanium,” Tracy says.
Going forward, materials scientists should be able to fine-tune the properties of high-entropy alloys even further by mixing different metals and elements together, according to the researchers.
“There’s a huge part of the periodic table and so many permutations to be explored,” Mao says.
Funding for the study was provided by the U.S. Department of Energy (Washington, DC) and the National Science Foundation (Arlington, Virginia).
Source: Stanford University, news.stanford.edu. Contact Cameron Tracy, Stanford University—E-mail: cltracy@stanford.edu.
Reference
1 C.L. Tracy, et al., “High pressure synthesis of a hexagonal close-packed phase of the high-entropy alloy CrMnFeCoNi,” Nature Communications 8, 15634 (2017).