1. High pressure packs in atoms in high-entropy alloys

    High pressure could be the key to making advanced metal mixtures that are lighter, stronger and more heat-resistant than conventional alloys, suggests a new study by researchers at Stanford University.

    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 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 Cameron Tracy, a postdoctoral researcher at Stanford's School of Earth, Energy & Environmental Sciences and the Center for International Security and Cooperation (CISAC).

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

    "This suggests it's possible to tailor the material to give us exactly the mechanical properties that we want for a particular application."Cameron Tracy, Stanford University​

    In a paper in Nature Communications, Tracy and his colleagues report the successful creation of 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 within 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.

    High pressure 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. 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," Tracy said.

    Scientists have speculated that the reason high-entropy alloys don't undergo this shift naturally is because interacting magnetic forces between the metal atoms prevent it from happening. But high pressure seems to disrupt 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," Tracy said. "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."

    Interestingly, 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 co-author Wendy Mao, an associate professor of geological sciences at Stanford's School of Earth, Energy & Environmental Sciences.

    The team also discovered that by slowly cranking up the pressure, 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 said.

    For example, combustion engines and power plants run more efficiently at high temperatures but conventional alloys tend not to perform well under extreme conditions because their atoms start moving around and become more disordered. "High-entropy alloys, however, already possess a high degree of disorder due to their highly intermingled natures," Tracy said. "As a result, they have mechanical properties that are great at low temperatures and stay great at high temperatures."

    In the future, materials scientists 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.

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

    High pressure packs in atoms in high-entropy alloys
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