[Editors] MIT model could aid design of nanomaterials

[Elizabeth Thomson]

MIT News Office
Massachusetts Institute of Technology
Room 11-400
77 Massachusetts Avenue
Cambridge, MA  02139-4307
Phone: 617-253-2700
http://web.mit.edu/newsoffice/www

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MIT model could aid design of nanomaterials
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For Immediate Release
THURSDAY, MAR. 1, 2007
Contact: Elizabeth A. Thomson, MIT News Office
Phone: 617-258-5402
Email: thomson at mit.edu

IMAGE AVAILABLE

CAMBRIDGE, Mass.--Researchers from MIT, Georgia Institute of 
Technology and Ohio State University have developed a new computer 
modeling approach to study how materials behave under stress at the 
atomic level, offering insights that could help engineers design 
materials with an ideal balance between strength and resistance to 
failure.

When designing materials, there is often a tradeoff between strength 
and ductility (resistance to breaking)-properties that are critically 
important to the performance of materials.

Recent advances in nanotechnology have allowed researchers to 
manipulate a material's nanostructure to make it both strong and 
ductile. Now, the MIT-related team has figured out why some 
nano-designed metals behave with that desirable compromise between 
strength and ductility.

The team, led by Subra Suresh, the Ford Professor of Engineering in 
the Department of Materials Science and Engineering, developed a 
simulation method derived from experimental data that allows them to 
visualize the deformation of materials on a timescale of minutes. 
Previous methods allowed for only a nanosecond-scale glimpse at the 
atomic-level processes.

"It's a method to look at mechanical properties at the atomic scale 
of real experiments without being bogged down by limitations of 
nanosecond timescales of the simulation methods such as molecular 
dynamics," said Suresh, the senior author of a paper on the work that 
appears as the cover story in the Feb. 27 issue of the Proceedings of 
the National Academy of Sciences.

Using the new method, the researchers found that the ductility and 
strength of materials are greatly influenced by a special kind of 
interface known as the twin boundary-an abrupt internal interface 
each side of which is a precise mirror reflection of atoms of the 
other side. Twin boundaries can be introduced in various densities, 
in a controlled manner, inside a nanocrystalline metal.

For many years, engineers have been able to tinker with the structure 
of metals to make them stronger. Most commonly used metals, including 
copper, silver, gold and aluminum, are traditionally made from 
micrometer-scale "building blocks" called grains, which each contain 
many millions of atoms.

About two decades ago, materials engineers discovered that when they 
made those grains smaller, typically tens of nanometers in average 
size, metals become stronger. Known as nanocrystalline metals, they 
are several times stronger than conventional microcrystalline metals.

However, as nanocrystalline metals become stronger, they also become 
more brittle (less ductile). For example, copper with a grain size of 
10 micrometers may have a ductility of about 50 percent (depending on 
exact composition), but at a 10 nanometer grain size, the ductility 
is below 5 percent, according to Suresh.

"In most applications, you need optimum combinations of strength and 
ductility," Suresh said.

A few years ago, researchers at the Shenyang National Laboratory for 
Materials Science in China synthesized a novel form of nanostructured 
metal, nano-twinned copper. The material was created by introducing 
controlled concentrations of twin boundaries within very small grains 
of the metal using a technique known as pulsed electrodeposition.

The Shenyang group, working in collaboration with Suresh's group at 
MIT, demonstrated in the last two years that nano-twinned copper has 
many of the same desirable characteristics as nano-grained copper, 
and in addition resulted in a good combination of strength and 
ductility.  By controlling the thickness and spacing of twin 
boundaries inside small grains to nanometer-level precision, they 
were able to produce copper with different "tunable" combinations of 
strength and ductility.

Internal interfaces such as grain boundaries (which occur between 
grains) and twin boundaries play a critical role in the strength and 
ductility of metals.

When there are smaller grains in the metal structure, and hence more 
grain boundaries for a given volume, there is more interaction 
between the boundaries and dislocations, or string-like defects in 
the material that move inside and between grains during mechanical 
deformation. The larger proportion of these boundaries contributes to 
the brittleness of the metal.

Adding nano-scale twin boundaries, which effectively subdivide the 
grains, has a similar strengthening effect, but the twin boundaries 
do not promote the same level of brittleness as grain boundaries do.

"You can trick the material and optimize both strength and ductility 
by modifying the interactions between dislocations and these 
nano-scale twin boundaries inside the grain," said Suresh.

The new study reveals that the ductility of nano-twinned copper can 
be attributed to changes in the atomic structure of the twin 
boundaries as the material is deformed.

Metals with more twin boundaries also maintain their electrical 
conductivity better than metals with more grain boundaries, making 
them potentially more useful for applications such as computer chip 
components. Nanocrystalline metals that are both strong and ductile 
could also be useful for many wear-resistant thin-film coating 
applications and MEMS (micro-electro-mechanical systems) devices, 
Suresh said.

The researchers plan to use their new approach to look at such things 
as the structures of other materials and other types of boundaries.

Suresh's collaborators on the paper include two former MIT graduate 
students, Ting Zhu, now an assistant professor at Georgia Tech, and 
Ju Li, now an assistant professor at Ohio State. Other authors are 
Amit Samanta and Hyoung Gyu Kim, both of Ohio State.

The research was funded by the National Science Foundation, the 
Office of Naval Research, the Air Force Office of Scientific 
Research, the Department of Energy, the Ohio Supercomputer Center and 
the Defense University Research Initiative in NanoTechnology.

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Written by Anne Trafton, MIT News Office