UNC-CH physicists find atoms of chilled metallic liquids chiefly move in lockstep

November 09, 1999

CHAPEL HILL - For the first time, atomic-scale measurements have revealed that atoms in a metallic liquid cooled significantly below the melting point - also known as a super-cooled liquid -- chiefly move together in clustered lockstep.

The effect is something like that of a phalanx of ancient Greek soldiers marching into battle or commuters moving on the platform of a crowded subway.

Such information is important in part because it helps reveal and explain the behavior of liquids having a strong tendency to form bulk metallic glasses, a new class of engineering materials with highly unusual properties, scientists say. First reported in 1993, the materials have thousands of potential uses in industry, the military, sports and other areas.

"What we have done is to shed some light on the mechanisms of atomic motion in the glass transition temperature region, around 650 degrees Fahrenheit in the materials we were working with," said Dr. Yue Wu, associate professor of physics and astronomy at the University of North Carolina at Chapel Hill. "This could be significant for understanding the nature of glass transition, which occurs when liquid cools rapidly and changes from a molten form to an amorphous solid without crystallization.

"The transition is a universal phenomenon that's been observed in almost all materials and plays a key role in many areas from food preservation to the forming of glass art objects."

A report on the research appears in the Nov. 11 issue of the journal Nature. Besides Wu, authors are Drs. X.-P. Tang, assistant research professor in physics at UNC-CH, Ulrich Geyer of Gottingen University in Germany, Ralf Busch of Oregon State University and William L. Johnson of the California Institute of Technology.

The study involved examining slow beryllium atomic motion on both microscopic scales using nuclear magnetic resonance (NMR) and macroscopic scales using diffusion measurements in zirconium-based bulk metallic glasses, alloys discovered by Johnson and a student at Cal Tech.

Tang and Wu employed a new NMR technique they developed to determine how local environments of beryllium atoms change when motion occurs and the rate of such changes.

"Atoms in a chilled metallic liquid manage to move more efficiently as a group than individual hopping, although the latter survives better at lower temperatures below the region of glass transition," Wu said.

"These glass-forming metals have great promise in a variety of technical applications ranging from springs and sports products to military applications," Johnson said.

"Together with earlier published work on this topic, the new paper illustrates that atomic diffusion in these liquids is a very complex phenomenon involving more than one type of mechanism," he said. "The NMR work illustrates that atomic motion of beryllium atoms occurs by at least two different mechanisms."

Experiments designed by Johnson have been carried out on liquid alloys aboard two space shuttle flights. He also has patented a five-metal alloy that is stronger than steel or titanium and written about the materials' armor-piercing potential.

His alloy has been used to make golf club heads, for example, that drive golf balls longer and straighter than older clubs. The new club heads allow that because almost 100 percent of the energy involved is transferred to the ball rather than a lower percentage due to more energy being absorbed by standard metals.

The U.S. Army Research Office, National Science Foundation and the U.S. Department of Energy supported the research.
Wu can be reached at 919-962-0307, yuewu@physics.unc.edu.
Contact: David Williamson, 919-962-8596.

University of North Carolina at Chapel Hill

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