Hopkins researcher draws on science and 'alchemy'
to make new alloys
Mention "glass," and a window pane comes to mind. But under certain conditions, a metal can also form as a glass, possessing properties that make it ideal for electric transformers, golf clubs and other products. Making useful metallic glasses is tricky, but a Johns Hopkins University researcher has taken on the challenge. Todd Hufnagel is trying to produce new metallic glasses in bulk form with superior strength, elasticity and magnetic properties.
In doing so, Hufnagel hopes to learn more about the microscopic events that occur when molten metal cools into a solid. This is the critical period when a metallic glass is born.
To scientists, a glass is any material that can be cooled from a liquid to a solid without crystallizing. Most metals do crystallize as they cool, arranging their atoms into a highly regular spatial pattern called a lattice. But if crystallization does not occur, and the atoms settle into a nearly random arrangement, the final form is a metallic glass.
Window glass possesses this same random atomic arrangement, although it is not metallic. Unlike window panes, metallic glasses are not transparent, yet their unusual atomic structure gives them distinctive mechanical and magnetic properties. Unlike window glass, metallic glass is not brittle. Many traditional metals are relatively easy to "deform," or bend permanently out of shape, because their crystal lattices are riddled with defects. A metallic glass, in contrast, will spring back to its original shape much more readily.
"If you rank materials for how springy they are, metallic glasses are off the chart," says Hufnagel, an assistant professor of materials science and engineering. "They're far and away better than anything else out there."
Hufnagel, whose studies are funded by the National Science Foundation and the U.S. Army Research Office, has set up a lab at Hopkins to test new alloys. He is trying to create a new metallic glass that will remain solid and not crystallize at higher temperatures, making it useful for engine parts. The new metallic glass may also have military applications as armor-piercing projectiles. Unlike most crystalline metal projectiles, which flatten into a mushroom shape upon impact, Hufnagel believes the sides of a metallic glass head will sheer away on impact, essentially sharpening the point and providing more effective penetration.
His work follows in the footsteps of scientists throughout history who have stirred together ingredients, trying to make valuable new materials. "Metallurgy has a long tradition of being a 'black art,'" says Hufnagel. "For a long time, people did things because they knew they worked, without understanding why. The real contribution of metallurgy is in starting to figure out why things work and how we can make them better.
"Part of what we're doing is still sort of alchemy, mixing up new combinations to see how well they form a glass. But the other part of this is science. We want to know how the crystallization works, what's going on there. If you can understand how the crystallization happens, then, presumably, you can design your alloy to avoid it. There's a lot of basic research to be done on this stuff."
Because they lack crystal defects, iron-based metallic glasses are very efficient magnetic materials. And like window glass and plastic, metallic glass softens as it is heated, making it easy to mold into a final shape.
In manufacturing, properties like these can have great appeal. But making a metallic glass in thick, bulk form is not easy because most metals rush to crystalize as they cool. To make a glass, the metal must harden before the crystal lattice has a chance to form. To create a glass from a pure metal, such as copper or nickel, one would have to cool it at about 1 trillion degrees Celsius per second, Hufnagel says. That's impractical.
In the 1950s, however, metallurgists learned how to slow the crystallization by mixing certain metals, such as nickel and zirconium. When thin layers of such alloys were cooled at 1 million degrees Celsius per second, they formed a metallic glass. Because of the rapid cooling requirement, this material could only be made as a thin ribbon, a wire or a powder.
More recently, however, scientists have created about a dozen metallic glasses in bulk form--bars, for example--by mixing four or five elements that possess atoms of varying sizes. That makes it tougher for the mixture to form crystal lattices. One of these new metallic glass alloys is being used commercially to make powerful golf club heads.
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