Researchers at the
California Institute of
Technology have developed a way to make some notoriously
brittle materials ductile -- yet stronger than ever -- simply by
reducing their size.
|
| Scanning electron micrograph of a typical
as-fabricated 100-nm-diameter tensile sample. Credit: Dongchan
Jang/Caltech |
The work, by Dongchan Jang, senior postdoctoral scholar, and
Julia R. Greer, assistant professor of materials science and
mechanics at Caltech, could eventually lead to the development of
innovative, superstrong, yet light and damage-tolerant materials.
These new materials could be used as components in structural
applications, such as in lightweight aerospace vehicles that last
longer under extreme environmental conditions and in naval vessels
that are resistant to corrosion and wear.
A paper about the work appears in the February 7 advance online
edition of the journal Nature Materials.
"Historically," says Greer, "structural materials have always
had to rely on their processing conditions, and thereby have been
'slaves' to their properties." For example, ceramics are very
strong, which makes them great for structural applications. At the
same time, these materials are very heavy, which is problematic for
many applications, and they are extremely brittle, which is less
than ideal for supporting heavy loads. In fact, says Greer, "they
fail catastrophically under mechanical loads." Metals and alloys,
on the other hand, are ductile, and therefore unlikely to shatter,
but they lack the strength of ceramics.
Materials scientists have developed an intriguing class of
materials called glassy metallic alloys, which are amorphous and
lack the crystalline structure of traditional metals. The
materials, also known as metallic glasses, are composed of random
arrangements of metallic elements like zirconium, titanium, copper,
and nickel. They are lightweight-a "huge advantage" for their
incorporation into new types of devices, Greer says-and yet are
comparable in strength to ceramics. Unfortunately, their random
structure makes metallic glasses quite brittle. "They also fail
catastrophically under tensile loads," she says.
But now Greer and Jang, the first author on the Nature Materials
paper, have developed a strategy to overcome these obstacles-by
making metallic glasses that are almost vanishingly small.
The scientists devised a process to make zirconium-rich metallic
glass pillars t PI hat are just 100 nanometers in diameter-roughly
400 times narrower than the width of a human hair. At this size,
Greer says, "the metallic glasses become not only even stronger,
but also ductile, which means they can be deformed to a certain
elongation without breaking. Strength plus ductility," she says,
represents "a very lucrative combination for structural
applications."
As yet, there are no immediate applications for the new
materials, although it may be possible to combine the nanopillars
into arrays, which could then form the building blocks of larger
hierarchical structures with the strength and ductility of the
smaller objects.
The work, however, "convincingly shows that 'size' can be
successfully used as a design parameter," Greer says. "We are
entering a new era in materials science, where structural materials
can be created not only by utilizing monolith structures, like
ceramics and metals, but also by introducing 'architectural'
features into them."
For example, Greer is working toward fabricating a
"brick-and-mortar" architecture using tiny plates of a metallic
glass and ultrafine-grained ductile metal with nanoscale dimensions
that could then be used to fabricate new engineering composites
with amplified strength and ductility.
To use this architecture-driven approach to create structural
materials with enhanced properties-that are, for example,
superstrong, yet light and ductile-researchers must understand how
each constituent part deforms during use and under stress.
"Our findings," she says, "provide a powerful foundation for
utilizing nanoscale components, which are capable of sustaining
very high loads without exhibiting catastrophic failure, in
bulk-scale structural applications specifically by incorporating
architectural and microstructural control."
Adds Greer: "The particularly cool aspect of the experiment is
that it is nearly impossible to do! Dongchan, my amazing postdoc,
was able to make individual 100-nanometer-diameter tensile metallic
glass nanopillar samples, which no one had ever done before, and
then used our custom-built in situ mechanical deformation
instrument, SEMentor, to perform the experiments. He fabricated the
samples, tested them, and analyzed the data. Together we were able
to interpret the results and to formulate the phenomenological
theory, but the credit goes all to him."
The work in the Nature Materials paper, "Transition from a
strong-yet-brittle to a stronger-and-ductile state by size
reduction of metallic glasses," was funded by the National Science
Foundation and the Office of Naval Research, and utilized the
fabrication and characterization facilities of the Kavli
Nanoscience Institute at Caltech.
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