Drawing
on powerful computational tools and a state-of-the-art scanning
transmission electron microscope, a team of University of
Wisconsin-Madison and Iowa State University materials science and
engineering researchers has discovered a new nanometer-scale atomic
structure in solid metallic materials known as metallic glasses.
Published
May 11 in the journal Physical Review Letters, the findings fill a gap
in researchers’ understanding of this atomic structure. This
understanding ultimately could help manufacturers fine-tune such
properties of metallic glasses as ductility, the ability to change shape
under force without breaking, and formability, the ability to form a
glass without crystalizing.
Glasses
include all solid materials that have a non-crystalline atomic
structure: They lack a regular geometric arrangement of atoms over long
distances. “The fundamental nature of a glass structure is that the
organization of the atoms is disordered—jumbled up like differently
sized marbles in a jar, rather than eggs in an egg carton,” says Paul
Voyles, a UW-Madison associate professor of materials science and
engineering and principal investigator on the research.
Researchers
widely believe that atoms in metallic glasses are arranged only as
pentagons in an order known as five-fold rotational symmetry. However,
in studies of a zirconium-copper-aluminum metallic glass, Voyles’ team
found there are clusters of squares and hexagons—in addition to clusters
of pentagons, some of which form chains—all located within the space of
just a few nanometers.
“One
or two nanometers is a group of about 50 atoms—and it’s how those 50
atoms are arranged with respect to one another that’s the new and
interesting part,” he says.
Measuring
the atomic structure of glass at this scale has been extremely
difficult. Researchers know that, at a few tenths of a nanometer, atoms
in metallic glasses have the same distances between them as they do in
crystals. They also know that at long distances-hundreds of
nanometers-there’s no order left. “But what happens in between, at this
‘magic’ length of one to three nanometers, is very hard to measure
experimentally and is essentially unexplored in experiments and
simulations,” says Voyles.
An
expert in electron microscopy, Voyles used a powerful, state-of-the-art
scanning transmission electron microscope at UW-Madison as his window
into this nanometer-scale atomic structure. The microscope can generate
an electron probe beam two nanometers in diameter-the ideal size for
examining atoms on a length scale of one to three nanometers.
“And
that, fundamentally, is what makes the experiments work and gives us
access to this information that’s otherwise very difficult to obtain,”
he says. “We can match our experimental probe in size right to the size
of what we want to measure.”
Voyles
and his team coupled the experimental data from the microscope with
state-of-the-art computational methods to conduct simulations that
accurately reflect the experiments. “It’s the combination of those two
things that gives us this new insight,” he says. “We can look at the
results and abstract general principles about rotational symmetry and
nanoscale clustering.”
There
were several clues in the properties of some metallic glasses that
these competing geometric structures might exist. Those arise from the
interrelationships of structure, processing and properties, says Voyles.
“If we understand how the structure controls, for example,
glass-forming ability or the ability to change shape on bending or
pulling, and we understand how different elements participate in these
different kinds of structures, that gives us a handle on controlling
properties by adjusting the composition or adjusting the rate at which
the material was cooled or heated to change the structure in some useful
way,” he says.
One
of the unique characteristics of glasses is their ability to transition
continuously from a solid to a liquid state. While other materials,
when heated, are partly melted and partly solid, glasses as a whole
become increasingly malleable.
While
manufacturers now apply metallic glasses primarily in electrical
transformer cores, their special forming capabilities may enable
manufacturers to make very small, intricate parts. “Unlike conventional
metallic alloys, metallic glasses can be molded like plastic-so they can
be pushed or sucked or blown into very complicated shapes without any
loss of material or machining,” says Voyles.
Those
manufacturing methods hold true even at the micro or nanoscale, so it’s
possible to make, for example, forests of nanowires or the world’s
smallest geared motor. “Five or 10 years from now, there may be more
commercial applications driven by those kinds of things than there are
now,” he says.
For
Voyles and his team, the next step will be to calculate the properties
of the most realistic structural models of metallic glass they have
developed to learn how those properties relate to the structure.
Other
authors on the Physical Review Letters paper include lead author Jinwoo
Hwang, Z.H. Melgarejo and Don Stone of UW-Madison, and Y.E. Kalay, I.
Kalay and M.J. Kramer of Iowa State University.
The
National Science Foundation funded Voyles’ research and an NSF grant
enabled him and other UW-Madison collaborators to purchase the scanning
transmission electron microscope. Installed in 2010, the microscope can
be operated remotely and provides UW-Madison researchers a level of
instrumentation on par with the world-leading federal laboratories and
research universities.
Source: University of Wisconsin-Madison
