Tag Archives: coherent twin boundaries

When your kinks and your defects are your strength: the truth about copper’s coherent twin boundaries

There’s perfection and then there’s imperfection in this story about the nanoscale. From the May 19, 2013 news release on EurekAlert,

One of the basic principles of nanotechnology is that when you make things extremely small—one nanometer is about five atoms wide, 100,000 times smaller than the diameter of a human hair—they are going to become more perfect.

“Perfect in the sense that their arrangement of atoms in the real world will become more like an idealized model,” says University of Vermont engineer Frederic Sansoz, “with smaller crystals—in for example, gold or copper—it’s easier to have fewer defects in them.”

And eliminating the defects at the interface separating two crystals, or grains, has been shown by nanotechnology experts to be a powerful strategy for making materials stronger, more easily molded, and less electrically resistant—or a host of other qualities sought by designers and manufacturers.

Scientists thought they’d found perfection in 2004 (from the news release),

Since 2004, when a seminal paper came out in Science, materials scientists have been excited about one special of arrangement of atoms in metals and other materials called a “coherent twin boundary” or CTB.

Based on theory and experiment, these coherent twin boundaries are often described as “perfect,” appearing like a perfectly flat, one-atom-thick plane in computer models and electron microscope images.

Over the last decade, a body of literature has shown these coherent twin boundaries—found at the nanoscale within the crystalline structure of common metals like gold, silver and copper—are highly effective at making materials much stronger while maintaining their ability to undergo permanent change in shape without breaking and still allowing easy transmission of electrons—an important fact for computer manufacturing and other electronics applications.

It turns out that not all coherent twin boundaries are ‘perfect’ (from the news release),

A team of scientists, including Sansoz, a professor in UVM’s College of Engineering and Mathematical Sciences, and colleagues from the Lawrence Livermore National Laboratory and elsewhere, write in the May 19 edition of Nature Materials that coherent twin boundaries found in copper “are inherently defective.”

With a high-resolution electron microscope, using a more powerful technique than has ever been used to examine these boundaries, they found tiny kink-like steps and curvatures in what had previously been observed as perfect.

Even more surprising, these kinks and other defects appear to be the cause of the coherent twin boundary’s strength and other desirable qualities.

“Everything we have learned on these materials in the past 10 years will have to be revisited with this new information,” Sansoz says

The work was performed at the Lawrence Livermore National Laboratory (from the news release),

The experiment, led by Morris Wang at the Lawrence Livermore Lab, applied a newly developed mapping technique to study the crystal orientation of CTBs in so-called nanotwinned copper and “boom—it revealed these defects,” says Sansoz.

This real-world discovery conformed to earlier intriguing theoretical findings that Sansoz had been making with “atomistic simulations” on a computer. The lab results sent Sansoz back to his computer models where he introduced the newly discovered “kink” defects into his calculations. Using UVM’s Vermont Advanced Computing Center, he theoretically confirmed that the kink defects observed by the Livermore team lead to “rather rich deformation processes at the atomic scale,” he says, that do not exist with perfect twin boundaries.

With the computer model, “we found a series of completely new mechanisms,” he says, for explaining why coherent twin boundaries simultaneously add strength and yet also allow stretching (what scientists call “tensile ductility”)— properties that are usually mutually exclusive in conventional materials.

It seems to me that scientists keep discovering that it’s the imperfections and defects which give rise to strength and, often, beauty. I hope this time they remember what they’ve discovered.

For those who need to know more, here’s a citation for and link to the paper,

Defective twin boundaries in nanotwinned metals by Y. Morris Wang, Frederic Sansoz, Thomas LaGrange, Ryan T. Ott, Jaime Marian, Troy W. Barbee Jr, & Alex V. Hamza. Nature Materials (2013) doi:10.1038/nmat3646 Published online 19 May 2013

This paper is behind a paywall.