As anyone who lives too close to a baseball field knows, glass can be frustratingly fragile.
But a new type of glass, made from opaque titanium and zirconium instead of transparent silicon, is harder and tougher — and weighs less — than stainless steel. The California scientists who developed and tested the opaque glass hope it could one day replace steel and aluminum in a wide variety of products, from golf clubs to airplanes.
"The problem with most [types of] glass is that they have very bad fatigue resistance," meaning they break easily, said Maximilien Launey, a scientist at the Lawrence Berkeley National Laboratory, who, along with Douglas Hofmann, William Johnson, and Robert Ritchie, detail their new material in this week's issue of the Proceedings of the National Academy of Sciences.
"What we've done is introduce a second phase that is able to improve the fatigue resistance by a factor of 10. Now it's as good as other high-strength alloys like steel," he said.
All glass, including window glass and opaque metallic glass, lacks a uniform crystalline structure. Instead, the atoms are randomly grouped together, with lots of atoms in one area but relatively few in another. Those groups of atoms dovetail to create a strong, hard surface, but a vulnerable one -- even the tiniest cracks can grow enough to destroy the entire structure.
By contract, crystalline structures are uniform, allowing groups of atoms to slide past one another with relative ease, which stops cracks from forming and spreading.
The difference between crystalline and non-crystalline materials is like the difference between a stack of playing cards and a stack of rugs, said William Nix, a professor of material science at Stanford University who was not involved in the new research.
Playing cards slide easily across one another, but the rough texture of the rugs grab a hold of each other, making it more difficult to slide them apart.
To create the tough, metallic glass, the scientists first heat an alloy of half titanium and half zirconium to 800 degrees Celsius (1,472 degrees Fahrenheit) until it melts. The liquid metal is allowed to cool slowly for a minute or two, and then cooled very rapidly after that.
By cooling slowly and then quickly, the scientists allowed tiny Christmas tree-like crystalline structures to grow through the metal. Rapidly cooling the rest of the metal maintains the glassy atomic structure.
The crystalline structures stop any cracks that from moving more than two to five microns in length. The glass gives the material its strength, while the Christmas tree-structures give it toughness.
In subsequent stress tests the 1 to 10-centimeter pieces of glass were dramatically tougher than any previous metallic glass, a full order of magnitude more resistant to breaking and stronger than steel.
"This is a major accomplishment," said William Nix of Stanford University. "One big advantage [of the new glass] is that it can be cast right to shape. Just pour it into a mold, let it freeze, and you have properties that are comparable to high-strength steel right from the get-go."
Since the alloy is titanium and zirconium instead of steel, it weighs significantly less than steel. It also melts at much lower temperatures than steel, making it easier and faster to produce.
"We could make an airplane out of metallic glass," said Launey. "But the problem is always the same. Processing these materials is kind of expensive."
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