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. Rather, the 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.”