Researchers have learned how to design the nanoscale features
of materials to make them four times stronger without making them brittle. The new
insight is the result of a significant improvement to an existing computer model
that allowed researchers to, for the first time, simulate the complex mechanical
behavior of nanostructures in metals.
The work, described in the Proceedings
of the National Academy of Sciences by researchers at MIT, OhioState, and
the Georgia Institute of Technology, could lead to more-durable materials for gears
in microscale machines. It could also lead to coatings that dramatically improve
the performance of larger-scale structures, such as metal plating on artificial
joints, says Subra
Suresh, professor of materials science and engineering at MIT.
The advance is part of a larger ongoing effort to use software
to discover new materials that would be impossible or impractical to discover using
experiments alone. Computers make it possible to fine-tune and quickly compare various
material parameters, such as the ratios and locations of the elements used and their
crystal structures. Such techniques have already suggested new combinations of elements
for high-performance battery electrodes, for example.
Simulations of the electrical and optical properties, however,
have been more successful than models of materials’ mechanical behavior. That’s
because the computations are so involved that even the fastest supercomputers can
only simulate the stretching, compressing, and cracking of materials for very short
time periods–on the order of a billionth of a second. That’s not nearly long enough
to test the models against measurements gathered in real-world experiments, which
take place over several seconds, minutes, or even hours.
The MIT researchers based their new model, which can work on
these longer time scales, on one invented in 2000 in which the occurrence of rare
events is used to extrapolate from short time scales to seconds or minutes. All
that’s needed is a good idea of how often these events take place, as well as how
long they take. The method, however, would not work when, as is the case with some
nanostructured materials, the “landscape is very complicated,” says Efthimios
Kaxiras, professor of physics and applied physics at Harvard.
To overcome this problem, the researchers modified the method
so that it can be used to model materials in which the duration of these rare events
isn’t known, or in which the events take place over long time scales. “It’s
a nice modification that gives you additional freedom if you don’t know exactly
where you’re going to end up,” Kaxiras says.
“It gives us the ability to link nanoscale features to the
mechanical properties of materials,” Suresh says.
The researchers used the model to understand for the first time
the way that nanostructures in copper can make the material three to four times
stronger without the usual trade-off of making the material more brittle. What they
found could help them optimize the properties of copper, Suresh says.
The answer lies in the way that copper atoms interact with interfaces
between and within the microscopic crystals that make up copper. Within each of
these crystals, called grains, atoms are free to slide past one another in certain
directions but not others when a force is applied to the material. But when the
atoms reach the edge of a grain, they meet atoms in the neighboring grain that aren’t
free to move in that same direction. As a result, the atoms stop slipping. For decades,
researchers have known that making the grains smaller–say, 10 nanometers across
instead of a few micrometers–makes the material stronger. That’s because there
are more grains and, therefore, more boundaries that prevent the atoms from shifting.
If too much force is applied, however, the dislocated atoms at
these boundaries can cause the materials to break apart. This makes nanostructured
materials more brittle.
The new version of copper contains another set of boundaries,
called twins. These occur inside a grain when atoms on either side of an imaginary
line are mirror images of each other. In the new copper, the grains are much larger
than 10 nanometers, but they’re divided into twins with boundaries about 10 nanometers
apart. These boundaries are much more orderly than grain boundaries are.
The researchers found that the twins resist the movement of the
atoms, making the material stronger. But even if the force
is too great for the twin boundary to stop the atoms from moving, the material doesn’t
break apart because the atoms are lined up in an orderly way.
displaced atoms are absorbed on the other side of the boundary.
Although the model has only been tested against data about nanostructured
copper, Suresh says it can be applied to other metals as well, making it a general
tool that could advance nanodesign in many areas.
Kaxiras says the new model is a significant improvement and that
this first application is “quite interesting.” He adds that “the
results are very insightful.”