It is so rare that an entirely new class of materials like metallic glasses comes along, that just on a scientific level, it is very interesting to think about.
In the movie "Princess Bride," Inigo Montoya keeps telling Vizzini, "You keep using that word. I do not think it means what you think it means."
That's how it feels talking with Assistant Professor of Mechanical and Chemical Engineering Wendy Wright about "metallic glasses." It turns out many of us have probably misunderstood the word "glass" all our lives. To Wright, as to anyone who knows the proper definition, "glass" refers to a solid with no regular pattern to the arrangement of its atoms. Metallic glasses, then, are pieces of metal made up of irregularly arranged — or amorphous — metal atoms. This is not how metals are normally found in nature.
"All metals as found in nature are crystalline, which means that their atoms are arranged in regular repeating patterns," Wright says. In fact, metallic glasses were only created in the 1960s.
Metallic glasses are made by melting metals, and then cooling them so fast that the atoms can't get lined up in their usual crystalline form. They are alloys, a combination of two or more metals, and most metallic glasses contain three or more elements. "That is because putting in different elements makes the pattern that would be necessary to form a crystal very complicated," Wright says. "If we then cool it quickly enough from the liquid state, the atoms don't have enough time to arrange themselves into the crystal before the solid forms."
Metallic glasses look and feel like other metals, Wright says, but they are significantly stronger than most steels. Unfortunately, they are also quite brittle. A metallic glass will resist high stresses, and then break suddenly and without warning. Wright is working to understand why this is so. "We know very little about the deformation of amorphous metals compared to crystalline metals," she says.
Manufacturers have taken advantage of the strength of metallic glasses and of another trait, their elasticity, to make golf clubs and tennis rackets. "The high strength-to-weight ratio and elasticity of these materials make them ideal for sporting goods," Wright says. Due to their excellent formability, they also are used in cell phone and laptop casings. The hope is that, someday, metallic glasses will find widespread application as structural materials. But getting there will require a greater understanding of what makes them so brittle.
Such an understanding will eventually feed into efforts to alter metallic glasses for practical applications. But for now, Wright is excited just to be exploring unknown territory. "It is so rare," she says, "that an entirely new class of materials like this comes along, that just on a scientific level, it is very interesting to think about."
Posted Sept. 27, 2010
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