When most people think of glass, the image of a window pane quickly comes to mind. But under certain conditions, a metal can also form as a glass, having qualities that make it ideal for electric transformers and golf clubs, for example. Todd Hufnagel, a researcher at Johns Hopkins University, Baltimore, is developing a way to produce metallic glasses with superior strength, elasticity, and magnetic qualities. Hufnagel hopes to learn more about the microscopic transformations that take place as molten metal cools into solids, when metallic glass forms.
To scientists, glass is any material that can be cooled from a liquid to solid without crystallizing. Most metals crystallize as they cool, arranging atoms into regular patterns called lattices. If crystallization doesn’t take place and the atoms remain random, a metallic glass forms.
Unlike window panes, metallic glasses aren’t transparent or brittle, yet their unusual atomic structure gives them distinctive mechanical and magnetic properties. Conventional metals are easy to deform or bend permanently out of shape because of the defects in their crystal lattices. Metallic glass, in contrast, more readily springs back to its original shape after deformed. The lack of crystal defects also makes iron-based metallic glasses efficient magnetic materials.
Under funding from the National Science Foundation and the U.S. Army Research Office, Hufnagel has set up a lab to test new alloys. He is trying to create a metallic-glass alloy that will remain solid and not crystallize at high temperatures, making it useful for engine parts. The material may also have military applications as armor-piercing projectiles. Unlike most crystalline-metal projectiles that flatten into mushroom shapes after impact, Hufnagel believes the sides of a metallic-glass head will sheer away and sharpen the projectile giving it better penetration.
Making a metallic glass in thick, bulk form is difficult because most metals rush to crystallize 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, it would have to cool at about 1 trillion degrees Celsius per second.
In the 1950’s, metallurgists learned how to slow crystallization by mixing certain metals such as nickel and zirconium. When thin layers of the alloys were cooled at 1 million degrees Celsius per second, they formed a metallic glass. But because of the rapid cooling requirement, they could only be made as thin ribbon, wire, or powder.
More recently, scientists have created a dozen metallic glasses in bulk form such as bars by mixing four or five elements with different atom sizes. Varying atom sizes makes it tougher for the mixture to form crystal lattices. One of these new alloys is being used commercially to make golf-club heads.