Researchers from Stanford University and the University of Tennessee report that high pressure could be the key to making advanced high-entropy alloys (HEAs, near-equiatomic solid solutions of multi-principal-elements that are promising for high-temperature applications, earlier post) that are lighter, stronger and more heat-resistant than conventional alloys.
In an open access paper in Nature Communications, the team described the high-pressure synthesis of a hexagonal close-packed (hcp) phase of the prototypical high-entropy alloy CrMnFeCoNi. They found that the hcp phase was retained following decompression to ambient pressure, yielding metastable mixtures. This demonstrated a means of tuning the structures and properties of high-entropy alloys in a manner not achievable by conventional processing techniques.
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, HEAs 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 that might even possess unique mechanical, magnetic or electrical properties.
Combining simple structures with very high-chemical disorder, high-entropy alloy design represents a new strategy for the development of materials with combinations of properties that cannot be achieved in conventional alloys. These include simultaneous ductility and strength, with the latter sometimes exceeding those of bulk metallic glasses as well as high hardness, wear resistance, corrosion resistance and thermal stability. Complex magnetic behaviour and superconductivity have also been reported. In addition to their potential use as structural materials, these alloys have been investigated for diverse applications as thermoelectric, soft magnetic and radiation tolerant materials.
While high-entropy alloys encompass a wide range of compositions, their phase space remains constrained. The majority of these alloys reported to date adopt one of two simple structures: face-centered cubic (fcc) or body-centered cubic (bcc). Despite their prevalence among the transition metals, of which most high-entropy alloys are composed, hexagonal close-packed (hcp) phases are rare.—Tracy et al.
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.
To date, scientists have only been able to re-create two types of packing structures with most high-entropy alloys, called body-centered cubic and face-centered cubic. A third, common packing structure has largely eluded scientists’ efforts—until now.
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.—First author Cameron Tracy, Stanford postdoc
Tracy and his colleagues used 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.
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.—Cameron Tracy
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. 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
The alloy retains an hcp structure even after the pressure is removed. 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 deliver the mechanical properties desired for a particular application, Tracy said.
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.
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
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.
Funding was provided by the US Department of Energy and the National Science Foundation.
Cameron L. Tracy, Sulgiye Park, Dylan R. Rittman, Steven J. Zinkle, Hongbin Bei, Maik Lang, Rodney C. Ewing & Wendy L. Mao (2017) “High pressure synthesis of a hexagonal close-packed phase of the high-entropy alloy CrMnFeCoNi” Nature Communications 8, Article number: 15634 doi: 10.1038/ncomms15634