World's strongest silver breaks long-standing theoretical limit

Normally when it comes to metals, there’s a trade-off to be made between strength and electrical conductivity, but a new class of materials could change that. A team of researchers has managed to take advantage of defects to make silver much stronger than usual, while still being conductive. Not only that, but it actually breaks through a theoretical limit that’s stood for decades.

Defects are an inescapable part of metals, and oftentimes they contribute to problems like brittleness or softening over time. Combining different metals into alloys helps overcome some of these problems, but usually that comes at the cost of electrical conductivity. Finding the best of both worlds was the aim of this study.

“We asked ourselves, how can we make a material with defects but overcome the softening while retaining the electroconductivity,” says Morris Wang, co-author of the study.

The solution sounds surprisingly simple: they mixed a trace amount of copper into the silver. The end result is 42 percent stronger than the previous strongest silver, while still being conductive. But the most impressive thing about the new alloy is that it exceeds what’s known as the “Hall-Petch limit.”

A hallmark of materials science for over 70 years, the Hall-Petch relation says that as the grains of a metal become smaller, the material itself gets stronger. But there’s a limit. When the grains become too small – a few nanometers wide – their boundaries become unstable and the material softens again.

The researchers managed to push past that limit by creating what they call a “nanocrystalline-nanotwinned metal.” Because copper atoms are slightly smaller than silver’s, they tend to fall into the defects in the boundaries of the grains. That keeps the defects from moving, which is what ultimately causes the material to soften. At the same time, the copper doesn’t get in the way of the electrons moving through the silver, allowing it to retain its conductivity.

The team says that this approach could be applied to many other metals besides silver. This technique could eventually be used to make more efficient solar panels, lighter aircraft or safer nuclear power plants.

“This is a new class of materials and we’re just beginning to understand how they work,” says Frederic Sansoz, co-lead author of the study.

The research was published in the journal Nature Materials.

Source: University of Vermont