Just like nanotech engineers in The Diamond Age, scientists from MIT, Georgia Institute of Technology, and Ohio State University now have a computer model to study how materials behave at the atomic level, a development that might spur further discoveries in nanoscience:
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.