That’s because the smaller the grain, generally the stronger, tougher and lightweight the metal can be, while less energy is consumed and emitted when it’s used in structural applications.
However, such tiny crystals require a lot of surface energy to stay small, so nanocrystals naturally combine and grow over time. But, when they grow, they lose their unique mechanical and multifunctional properties.
Researchers have tried to circumvent this problem by introducing a solute element that acts on the interfaces between nanocrystals, keeping this "binary alloy system" stable at high temperatures. But most structural materials have three or more elements, and efforts to stabilize these complex "multi-element or high-entropy alloy systems" degrade the metal.
Researchers at the University of Calgary’s Schulich School of Engineering recently published two papers aiming to tackle this issue, in the journals Materials Design and Small .
"This is a challenging area of research in materials science and metallurgy because of the complexities that come with having to deal with several elements in the design of stable grain boundary decorated alloys," says one of the lead researchers, PhD candidate Moses Adaan-Nyiak. "These elements interact with each other differently to form deleterious intermetallics, or second phases, that limit their widespread applications."
The researchers were the first to find different elements that make nanocrystalline "multi-element" high-entropy alloy materials more stable, particularly as they get hotter.
"The first paper shows how engineers can select constituent multi-component elements that will allow the segregation of one of these elements to the interface of the nanocrystals - grain boundary - without needing the traditional extra solute element," says Dr. Ahmed Tiamiyu , PhD, an assistant professor with the Department of Mechanical and Manufacturing Engineering.
"It’s more like a ’brick-and-mortar’ configuration." These elements not only stabilize the nanocrystals, he says; they improve corrosion resistance and prevent "grain boundary embrittlement."
The second paper outlines how the researchers developed a new framework for selecting the appropriate solute element to offset the "excess energy" that causes the nanograin instability. "Rather than simple binary alloys containing just two elements - solvent and minor solute - we showed our approach works for multi-element alloys; that is, alloys containing four or more elements," says Tiamiyu. "Until now, this has been very difficult to do."
Nanocrystalline or nanograin materials are in high demand in "multiple industries" because of their "many unique mechanical and multifunctional properties," says Tiamiyu. They’re stronger, they’re much more resistant to wear, fatigue and corrosion, and they have good magnetic properties. "These materials have a beneficial and unique combination of properties that are sometimes mutually exclusive."
The work is "very interesting," says Dr. Jian Luo, PhD, professor of materials science and engineering at University of California San Diego, who was not involved in this research. Not only does the work show how to select appropriate elements to stabilize nanocrystalline high-entropy alloys, Luo says, "it also supports a previously proposed theory that the formation of high-entropy grain boundaries can stabilize nanocrystalline alloys at high temperatures. This line of research opens up exciting opportunities to design and fabricate nanocrystalline high-entropy alloys for high-temperature applications."
These ultra-high-strength multifunctional stable nanograin materials can be used to develop lightweight and very strong components for high-temperature environments such as steam turbines and nuclear/chemical plants. They can also be used to develop lightweight surface metallic coatings for applications including pipelines, combustion liners, nuclear fuel dry-storage canisters, rotary blades for military helicopters and wind turbines, and other related energy systems operating in cold-climate conditions.
The researchers are working on consolidating these nanocrystalline powders into bulk components and metallic surface coatings. "We want to expedite their immediate use in various manufacturing applications," says Tiamiyu. "We’re seeking industrial partners to expedite this process."
Also involved with the research was Schulich alum Intekhab Alam, MSc’24, along with researchers from the Brookhaven National Laboratory (BNL) in New York.
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