Using a powerful electron microscope to view
atomic-level details, Johns Hopkins researchers have
discovered a "twinning" phenomenon in a nanocrystalline
form of aluminum that was plastically deformed during lab
experiments. The finding will help scientists better
predict the mechanical behavior and reliability of new
types of specially fabricated metals. The research results,
an important advance in the understanding of metallic
nanomaterials, were published in a recent issue of the
journal Science.
At the microscopic level, most metals are made up of
tiny crystallites, or grains. Through careful lab
processing, however, scientists in recent years have begun
to produce nanocrystalline forms of metals in which the
individual grains are much smaller. These nanocrystalline
forms are prized because they are much stronger and harder
than their commercial-grade counterparts. Although they are
costly to produce in large quantities, these nanomaterials
can be used to make critical components for tiny machines
called microelectromechanical systems, often referred to as
MEMS, or even smaller nanoelectromechanical systems,
NEMS.
But before they build devices with nanomaterials,
engineers need a better idea of how the metals will behave.
For example, under what conditions will they bend or break?
To find out what happens to these new metals under stress
at the atomic level, Johns Hopkins researchers, led by
Mingwei Chen, conducted experiments on a thin film of
nanocrystalline aluminum. Grains in this form of aluminum
are 1,000th the size of the grains in commercial
aluminum.
Chen and his colleagues employed two methods to deform
the nanomaterial or cause it to change shape. The
researchers used a diamond-tipped indenter to punch a tiny
hole in one piece of film and subjected another piece to
grinding in a mortar. The ultra-thin edge of the punched
hole and tiny fragments from the grinding were then
examined under a transmission electron microscope, which
allowed the researchers to study what had happened to the
material at the atomic level. The researchers saw that some
rows of atoms had shifted into a zig-zag pattern,
resembling the bellows of an accordion. This type of
realignment, called deformation twinning, helps explain how
the nanomaterial, which is stronger and harder than
conventional materials, deforms when subjected to high
loads.
"This was an important finding because deformation
twinning does not occur in traditional coarse-grain forms
of aluminum," said Chen, an associate research scientist in
the Department of
Mechanical Engineering in the university's Whiting
School of Engineering. "Using computer simulations, other
researchers had predicted that deformation twinning would
be seen in nanocrystalline aluminum. We were the first to
confirm this through laboratory experiments."
By seeing how the nanomaterial deforms at the atomic
level, researchers are gaining a better understanding of
why these metals do not bend or break as easily as
commercial metals do.
"This discovery will help us build new models to
predict how reliably new nanoscale materials will perform
when subjected to mechanical forces in real-world devices,"
said Kevin J. Hemker, a professor of mechanical engineering
and a co-author of the Science paper. "Before we can
construct these models, we need to improve our fundamental
understanding of what happens to nanomaterials at the
atomic level. This is a key piece of the puzzle."
The nanocrystalline aluminum used in the experiments
was fabricated in the laboratory of En Ma, a professor in
the Department of Materials Science and
Engineering and another co-author of the research
paper. "This discovery nails down one deformation process
that occurs in nanocrystalline metals," Ma said. "This is
the first time a new mechanism, which is unique to
nanostructures and improbable in normal aluminum, has been
conclusively demonstrated."
Other co-authors of the paper were Hongwei Sheng, an
associate research scientist in the Department of Materials
Science and Engineering; Yinmin Wang, a graduate student in
the Department of Materials Science and Engineering; and
Xuemei Cheng, a graduate student in the Department of Physics and
Astronomy.
The research was funded in part by the National
Science Foundation. The electron microscope laboratory used
in the study was funded by the W. M. Keck Foundation and
the National Science Foundation.
Related Web sites
JHU Department of Mechanical Engineering
JHU Department of Materials Science and
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