The coins in your wallet, the silverware on your table, and even the steel beams holding up the buildings around you are all made up of small metal grains. Under a microscope, these metals resemble interlocking and interwoven crystals.
It's well-known among material scientists that metal becomes stronger as the metal grains that make it up become smaller — but only up to a certain point, 10 nanometers in diameter.
Now, a team of scientists has proven that metal can, in fact, become ultra-strong, even when smaller than 10 nanometers.
Atoms interacting with each other
Experiments conducted by an international team from the U.S. and China have enabled scientists to realize that samples of nickels that are three nanometers in diameter, and under high pressures, became stronger as the grains minimized in size.
Former University of Utah postdoctoral scholar, Zhou Xiaoling, and associate professor of geology, Lowell Miyagi, explain that their results help to show how individual atoms of metal grains interact with each other. They also open up a new method of creating ultra-strong metals.
Zhou said "Our results suggest a possible strategy for making ultrastrong metals. In the past, researchers believed the strongest grain size was around 10-15 nanometers. But now we found that we could make stronger metals at below 10 nanometers."
How did the scientists carry out their tests?
In order to reach this new conclusion, the team tested samples of nickel. They placed various grain size samples under intense pressure in a diamond anvil cell and used X-ray diffraction to observe the nanoscale changes.
"If you’ve ever played around with a spring, you’ve probably pulled on it hard enough to ruin it so that it doesn’t do what it’s supposed to do," Miyagi explained. "That’s basically what we’re measuring here; how hard we can push on this nickel until we would deform it past the point of it being able to recover."
The researchers noticed that the strength continued to rise, all the way down to the smallest grain size available: three nanometers.
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The three nanometer grain was able to withstand a force of 4.2 gigapascals — the equivalent of ten 10,000 lbs. elephants sitting on a single high heel — before it became deformed. That makes it 10 times stronger than commercial-grade nickel grain.
It's an exciting realization, however, Miyagi did mention "We don’t have many applications, industrially, of things where the pressures are as high as in these experiments, but by showing pressure is one way of suppressing grain boundary deformation we can think about other strategies to suppress it, maybe using complicated microstructures where you have grain shapes that inhibit sliding of grains past each other."
Their findings were published in Nature on Monday.