SAN FRANCISCO, May 25 (Xinhua) -- A new study published online Thursday in the journal Nature Communications suggests that high pressure could be the key to making advanced metal mixtures that are lighter, stronger and more heat-resistant than conventional alloys.
Compared with traditional alloys, which typically consist of one or two dominant metals with a pinch of other metals or elements thrown in, such as adding tin to copper to make bronze, or carbon to iron to create steel, "high-entropy" alloys consist of multiple metals mixed in approximately equal amounts.
However, as they hold the promise to be stronger and lighter, more resistant to heat, corrosion and radiation, possibly with unique mechanical, magnetic or electrical properties, depending largely on how atoms in such an alloy are arranged, high-entropy alloys have yet to make the leap from the lab to actual products.
"Some of the most useful alloys are made up of metal atoms arranged in a combination of packing structures," study first author Cameron Tracy, a postdoctoral researcher at Stanford University's School of Earth, Energy & Environmental Sciences and the Center for International Security and Cooperation (CISAC), was quoted as saying in a news release.
Researchers have been able to recreate two types of packing structures with most high-entropy alloys, called body-centered cubic and face-centered cubic.
In the new study, Tracy and his colleagues report that they have 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," he 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.
The researchers 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, which appears to trigger a transformation in the high-entropy alloy the team used, which consisted of manganese, cobalt, iron, nickel and chromium.
"Imagine the atoms as a layer of ping pong balls on a table, and then adding more layers on top," Tracy said. "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."
The alloy retains an HCP structure even after the pressure is 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," said study coauthor Wendy Mao, an associate professor of geological sciences at the Stanford School of Earth, Energy & Environmental Sciences.
In addition, by slowly cranking up the pressure, the researchers found that they could increase the amount of hexagonal close-pack structure in their alloy.
"This suggests it's possible to tailor the material to give us exactly the mechanical properties that we want for a particular application," Tracy noted. For example, combustion engines and power plants run more efficiently at high temperatures but conventional alloys tend to not perform well in extreme conditions because their atoms start moving around and become more disordered.
Down the road, researchers may be able to fine-tune the properties of high-entropy alloys even further by mixing different metals and elements together. "There's a huge part of the periodic table and so many permutations to be explored," Mao said.
