Scientists used a diamond anvil cell like this one to squeeze tiny samples of metallic glass. Under very high pressure, the samples switched from their amorphous, glassy state to form a single crystal—the first time this behavior has been seen in a glass. Image: Brad Plummer/SLAC |
Glass, by definition, is
amorphous; its atoms lack order and are arranged every which way. But when
scientists squeezed tiny samples of a metallic glass under high pressure, they
got a surprise: The atoms lined up in a regular pattern to form a single
crystal.
It’s the first time
researchers have glimpsed this hidden property in a glass. The discovery,
reported in Science, offers a new window into the atomic structure and
behavior of metallic glasses, which have been used for decades in products such
as anti-theft tags and power transformers, but are still poorly understood. The
more scientists learn about the structure of these commercially important
materials, the more effectively they can design new metallic glasses and tinker
with old ones to improve their performance.
“Maybe a lot of glasses
have this underlying structure, but we just didn’t know how to look for it,”
said paper co-author Wendy Mao, a mineral physicist at the Department of
Energy’s (DOE) SLAC National Accelerator Laboratory and Stanford Univ.
Daniel Miracle, a
metallurgist at the Air Force Research Laboratory in Ohio who was not involved in the research,
called the discovery “a really, really neat, important finding.” Not only will
it help researchers design better metallic glasses, he said, but it may help
explain why these materials can be so tough: If each piece of glass is a single
crystal at heart, it doesn’t have any of the weak spots at the boundaries
between crystals where fractures and corrosion tend to start.
Unlike familiar window
glass, metallic glasses are alloys made of metals—in this case cerium and
aluminum. They resist wear and corrosion and they have useful magnetic
properties. If you took apart the plastic anti-theft tag on a DVD case, you’d
find a thin piece of metallic glass that looks like aluminum foil. When you
rent or buy a DVD, the checkout clerk rubs it across a pad to demagnetize the
metallic glass so it won’t trigger an alarm when you leave.
Scientists have been
investigating metallic glasses for half a century, and in 1982 turned up the
surprising discovery that these glasses do have some atomic structure, forming
patterns over distances spanning just a few atoms. But no long-range patterns
were apparent.
“The structure of glass is
still mysterious. We know little about it, even though we use glass a lot,”
said Qiaoshi (Charles) Zeng of Zhejiang Univ. in China,
who led a research team of scientists from SLAC, Stanford, the Carnegie
Institution of Washington, George Mason Univ., and China’s
Jilin Univ. “And it’s not easy investigating
the structure of glass by traditional methods.”
Zeng, Mao, and their
colleagues were not looking for order when they squeezed samples of the
metallic glass between the tips of two diamonds at Argonne National
Laboratory’s Advanced Photon Source, applying 250,000 bars of pressure. They
were simply doing a series of experiments on how materials behave in extreme
conditions.
All the samples were taken
from a centimeter-long, extremely thin ribbon of the metallic glass. Under
intense pressure, all of the samples “devitrified,” abruptly switching out of
their glassy state to form a face-centered cubic crystal—one whose atoms are arranged
like ping-pong balls packed into a box.
What’s more, all the atoms
in the crystallized samples lined up in the same direction—an indication, the
researchers wrote, that this underlying structure ran throughout the whole
ribbon of glass, and was put there when the glass formed.
Zeng, who will be joining Mao’s group at Stanford in July, said the
high-pressure technique may offer a new approach for making single-crystal
materials from glasses. In addition, he said, it provides a unified
understanding of the atomic structures of materials by directly linking the two
most extreme examples: highly ordered single crystals and highly disorganized
glass.
