Metallic Glass: Material Of The Future?

Mention "glass," and a window pane comes to mind. But undercertain conditions, a metal can also form as a glass, possessingproperties that make it ideal for electric transformers, golfclubs and other products. Making useful metallic glasses istricky, but a Johns Hopkins University researcher has taken onthe challenge. Todd Hufnagel is trying to produce new metallicglasses in bulk form with superior strength, elasticity andmagnetic properties.

In doing so, Hufnagel hopes to learn more about the microscopicevents that occur when molten metal cools into a solid. This isthe critical period when a metallic glass is born.

To scientists, a glass is any material that can be cooled from aliquid to a solid without crystallizing. Most metals docrystallize as they cool, arranging their atoms into a highlyregular spatial pattern called a lattice. But if crystallizationdoes not occur, and the atoms settle into a nearly randomarrangement, the final form is a metallic glass.

Window glass possesses this same random atomic arrangement,although it is not metallic. Unlike window panes, metallicglasses are not transparent, yet their unusual atomic structuregives them distinctive mechanical and magnetic properties. Unlikewindow glass, metallic glass is not brittle. Many traditionalmetals are relatively easy to "deform," or bend permanently outof shape, because their crystal lattices are riddled withdefects. A metallic glass, in contrast, will spring back to itsoriginal shape much more readily.

"If you rank materials for how springy they are, metallic glassesare off the chart," says Hufnagel, an assistant professor ofmaterials science and engineering. "They're far and away betterthan anything else out there."

Hufnagel, whose studies are funded by the National ScienceFoundation and the U.S. Army Research Office, has set up a lab atHopkins to test new alloys. He is trying to create a new metallicglass that will remain solid and not crystallize at highertemperatures, making it useful for engine parts. The new metallicglass may also have military applications as armor-piercingprojectiles. Unlike most crystalline metal projectiles, whichflatten into a mushroom shape upon impact, Hufnagel believes thesides of a metallic glass head will sheer away on impact,essentially sharpening the point and providing more effectivepenetration.

His work follows in the footsteps of scientists throughouthistory who have stirred together ingredients, trying to makevaluable new materials. "Metallurgy has a long tradition of beinga 'black art,'" says Hufnagel. "For a long time, people didthings because they knew they worked, without understanding why.The real contribution of metallurgy is in starting to figure outwhy things work and how we can make them better.

"Part of what we're doing is still sort of alchemy, mixing up newcombinations to see how well they form a glass. But the otherpart of this is science. We want to know how the crystallizationworks, what's going on there. If you can understand how thecrystallization happens, then, presumably, you can design youralloy to avoid it. There's a lot of basic research to be done onthis stuff."

Because they lack crystal defects, iron-based metallic glassesare very efficient magnetic materials. And like window glass andplastic, metallic glass softens as it is heated, making it easyto 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 easybecause most metals rush to crystalize as they cool. To make aglass, the metal must harden before the crystal lattice has achance to form. To create a glass from a pure metal, such ascopper or nickel, one would have to cool it at about 1 trilliondegrees Celsius per second, Hufnagel says. That's impractical.

In the 1950s, however, metallurgists learned how to slow thecrystallization by mixing certain metals, such as nickel andzirconium. When thin layers of such alloys were cooled at 1million degrees Celsius per second, they formed a metallic glass.Because of the rapid cooling requirement, this material couldonly be made as a thin ribbon, a wire or a powder.

More recently, however, scientists have created about a dozenmetallic glasses in bulk form--bars, for example--by mixing fouror five elements that possess atoms of varying sizes. That makesit tougher for the mixture to form crystal lattices. One of thesenew metallic glass alloys is being used commercially to makepowerful golf club heads.

Related Web site -- Hufnagel's Home Page:http://www.jhu.edu/~matsci/people/faculty/hufnagel/hufnagel.html

Editor's note: Color slide available; contact Phil Sneiderman