...metallic glass?  If it's good enough for cell phone cases...

Using state-of-the-art lab techniques and powerful computer
simulations, Johns Hopkins researchers have discovered how atoms pack
themselves in unusual materials known as metallic glasses. Their
findings should help scientists better understand the atomic scale
structure of this material, which is used to make sports equipment,
cell phone cases, armor-piercing projectiles and other products.

The discovery, marking the culmination of a two-year research project,
was reported in the Jan. 26 issue of the journal Nature. The work
represents a major step forward because the tools used to study
traditional crystalline metals do not work well with metallic glass,
and a better understanding of the material has been sorely needed.

"How the atoms pack themselves in metallic glass has been a mystery,"
said Howard Sheng, an associate research scientist in the Department
of Materials Science and Engineering and lead author of the Nature
paper. "We set out to decipher this packing information, and we were
ultimately able to provide a clear description of how the atoms
arrange themselves in metallic glass."

In conventional metals, atoms crystallize into uniform
three-dimensional patterns known as lattices. But about a half-century
ago, materials scientists learned how to make glassy metals by cooling
a metallic liquid so quickly that the internal atomic configurations
froze before the atoms had a chance to arrange themselves into a
lattice pattern. The new material was described as amorphous, meaning
its atoms seemed to be arranged in an irregular fashion without the
long-range order characteristic of crystalline materials. This
amorphous atomic structure is commonly found in other materials such
as window glass, but it rarely occurs in metals.


Unlike window panes, metallic glasses are not transparent or easy to
shatter. Many traditional metals are easy to bend out of shape because
of defects (dislocations) in their crystal lattice. But metallic
glasses have no crystal lattice and no such dislocations, and their
disorderly arrangement of atoms gives them distinctive mechanical and
magnetic properties. Metallic glasses, which are usually made of two
or more metals, can display great strength, large elastic strain and
toughness. Another advantage is that, like weaker plastic materials,
they can easily be heated, softened and molded into complex shapes.

Despite the great potential of metallic glasses, the researchers who
make them have been hampered by a scarcity of basic science knowledge
about the materials. Powerful transmission electron microscopes can be
used to view rows of atoms lined up in traditional metals. But when
these instruments are used on a metallic glass, the resulting image is
one of a scattered array of atoms, forming no obvious pattern. Because
so little has been known about how atoms are arranged in metallic
glasses, a number of basic materials science problems, such as how a
metallic glass deforms, remain unsolved.

To help fill the knowledge gap, a team supervised by Evan Ma, a
professor of materials science and engineering at Johns Hopkins,
launched a two-pronged approach to solve the mystery of how metallic
glass atoms are arranged. "Our goal was to advance the understanding
of atomic packing in metallic glasses," Ma said. "This is a difficult
task because of the lack of long-range order in these amorphous
structures. Yet it is of fundamental importance because it is the
structure that determines properties."

The researchers made samples of a number of binary metallic glasses,
each composed of two elements, and then subjected them to high-tech
lab tests to gather information about the samples' three-dimensional
atomic configurations. Some of these experiments, conducted at Oak
Ridge and Brookhaven national laboratories, involved X-ray diffraction
and extended X-ray absorption fine structure data taken at synchrotron
X-ray sources. Other analyses, utilizing a method called reverse Monte
Carlo simulations, were conducted with a computer cluster at Johns
Hopkins.

Independent of these lab tests, the researchers used powerful computer
resources provided by the National Energy Research Scientific
Computing Center to run virtual experiments aimed at uncovering the
arrangement of metallic glass atoms. Results from the lab experiments
and the computer trials were used to validate one another, confirming
the researchers' conclusions.

One of their key findings was that metallic glass atoms do not arrange
themselves in a completely random way. Instead, groups of seven to 15
atoms tend to arrange themselves around a central atom, forming
three-dimensional shapes called Kasper polyhedra. Similar shapes are
found in crystalline metals, but in metallic glass, the researchers
said, these polyhedra are distorted and do not align themselves in
long rows. In metallic glass, the polyhedra join together in unique
ways as small nanometer-scale clusters. In the journal article, these
structural features were described as chemical and topological
short-range order and medium-range order.

The Johns Hopkins engineers also made important discoveries about how
low-density spaces form among these clusters in metallic glass. These
"cavities" affect the way the material forms as a glass and the
mechanical properties it will possess.

Sheng, the lead author of the journal article, believes these
discoveries will lead to significant advances in the understanding of
metallic glass. "Our findings," he said, "should allow the people who
make metallic glass to move closer to intelligent design techniques,
developing materials with the precise mechanical characteristics
needed for specific products. The discoveries also advance our
understanding of materials science in general."

Source: Johns Hopkins University
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