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Atomic Trigger Shatters Mystery of How Glass Deforms

October 17, 2014

Joint Institute for Neutron Sciences Director Takeshi Egami, left, and Wigner Fellow Yue Fan, holding a metallic glass thumb drive, discovered the mechanism by which metallic glass deforms. Photo credit: Jason Richards, ORNL

Throw a rock through a window made of silica glass, and the brittle, insulating oxide pane shatters. But whack a golf ball with a club made of metallic glass—a resilient conductor that looks like metal—and the glass not only stays intact but also may drive the ball farther than conventional clubs. In light of this contrast, the nature of glass seems anything but clear.

A new study at the Department of Energy’s Oak Ridge National Laboratory, published Sept. 24 in Nature Communications, has cracked one mystery of glass to shed light on the mechanism that triggers its deformation before shattering. The study improves understanding of glassy deformation and may accelerate broader application of metallic glass, a moldable, wear-resistant, magnetically exploitable material that is thrice as strong as the mightiest steel and ten times as springy.

Whereas metals are usually crystalline, metallic glasses are amorphous in atomic structure. Amorphous metals, studied since the 1950s, have a tendency to crystallize when heated, which makes them extremely brittle. Metallic glass alloys that did not crystallize so easily were discovered at Tohoku University and Caltech in 1991 and introduced commercially in golf clubs in 2001.

Glass hangs in a metastable state in which the energy of the system is higher than the lowest-energy state the system could assume, a crystalline state. But its state is stable enough at room temperature to last a human lifetime.

“Exactly speaking, a metastable state cannot last; it evolves,” said project leader Takeshi Egami, a distinguished scientist/professor at ORNL and the University of Tennessee–Knoxville. “For instance, diamond is only metastable. Graphite is a stable state. Hollywood says ‘Diamonds Are Forever.’ Scientifically that is completely incorrect.”

Due to its relative stability, metallic glass can be melted and precision-cast into molds without going back to its most stable, crystalline state. The resulting parts do not need to be machined, which saves money. That said, metallic glass is pricey. (A club made of it can set a golfer back $800.) Regardless, it has been used in biocompatible bone implants, rust-resistant razors and scalpel blades, resilient coatings for refinery pipes, transformers with half the energy loss, Andre Agassi’s tennis racquet and the scratch-resistant logo of Apple’s iPhone 6.

It may gain wider application if dabbed on computer chips to reduce electromagnetic noise that produces heat. Wider deployment may drive down costs of metallic-glass watches, rings, skis and baseball bats. The material may be used in vehicle bodies and casings for smartphones and computers as well. But for metallic glass to achieve its promise, it must first overcome a long-standing problem.

“Metallic glasses are too brittle, meaning the materials easily break without significant ductile deformation,” said Yue Fan, a former Wigner Fellow at ORNL and the study’s lead author. “It is extremely important to understand the origin of deformation in metallic glasses to engineer solutions that would increase their usefulness.” Case in point: Initial prototypes of metallic-glass golf clubs shattered after as few as 40 hits. To improve ductility, which is a material’s ability to be deformed without fracture, manufacturers had to resort to a composite including crystalline material. While metallic glass golf clubs remain in demand today, manufacturers stopped making them 10 years ago because they are costly.