The new form of metal is part of a discovery that could launch technological advances from lighter airplanes to better solar panels



The foundational idea behind their work was the Hall-Petch relation, which implies that as the size of a crystal—or grain—of material gets smaller, it gets stronger. But there is a ‘but’ in this principle.

Eventually, when grains of metal reach a size under tens of nanometers wide, the boundaries between the grains become unstable and begin to move. To overcome this and strengthen metals like silver, researchers use an approach called “coherent twin boundaries,” which are a special type of grain boundary. These structures of paired atoms form a symmetrical mirror-like crystalline interface and are exceedingly strong to deformation.

But there is another ‘but.’ Imperfections make twin boundaries soft too, particularly when their interspacing falls under a critical size of a few nanometers.

In a media statement, the American research group explained that nanocrystals are like patches of cloth and nanotwins are like strong but tiny threads in the cloth. Their work, thus, focused on combining both approaches to make a “nanocrystalline-nanotwinned metal” that has “unprecedented mechanical and physical properties.”

What happens is that the copper atoms, slightly smaller than the atoms of silver, move into defects in both the grain boundaries and the twin boundaries. The tiny copper impurities within the silver inhibit the defects from moving but are such a small amount of metal—less than 1% of the total—that the rich electrical conductivity of silver is retained.

“The copper atom impurities go along each interface and not in between. So they don’t disrupt the electrons that are propagating through,” said Frederic Sansoz , a mechanical engineering professor at the University of Vermont who co-led the new discovery.

According to Sansoz, not only does this metal overcome the softening previously observed as grains and twin boundaries get too small, it even exceeds the long-standing theoretical Hall-Petch limit.

“We’ve broken the world record, and the Hall-Petch limit too, not just once but several times in the course of this study, with very controlled experiments,” the researcher said.