Going super small to get super strong metals

Under a powerful enough microscope, you can see interlocking crystals that look like a granite countertop.

It's long been known by materials scientists that metals get stronger as the size of the grains making up the metal get smaller -- up to a point.

The strength of metals had a limit.

But experiments led by former University of Utah postdoctoral scholar Xiaoling Zhou, now at Princeton University, associate professor of geology Lowell Miyagi, and Bin Chen at the Center for High Pressure Science and Technology Advanced Research in Shanghai, China, show that that's not always the case -- in samples of nickel with grain diameters as small as 3 nanometers, and under high pressures, the strength of the samples continued to increase with smaller grain sizes.

The result, Zhou and Miyagi say, is a new understanding of how individual atoms of metal grains interact with each other, as well as a way to use those physics to achieve super-strong metals.

But now we found that we could make stronger metals at below 10 nanometers."

Pushing past Hall-Petch

For most metallic objects, Miyagi says, the sizes of the metal grains are on the order of a few to a few hundred micrometers -- about the diameter of a human hair.

But at small grain sizes, it was thought, the grains would simply slide past each other under strain, leading to a weak metal.

Technical limitations previously prevented direct experiments on nanograins, though, limiting understanding of how nanoscale grains behaved and whether there may yet be untapped strength below the Hall-Petch limit.

The high pressure likely overcame the grain sliding effects.

"If you push two grains together really hard," he says, "it's hard for them to slide past each other because the friction between grains becomes large, and you can suppress these grain boundary sliding mechanisms that turns out are responsible for this weakening."

When grain boundary sliding was suppressed at grain sizes below 20nm, the researchers observed a new atomic-scale deformation mechanism which resulted in extreme strengthening in the finest grained samples.

Ultrastrong possibilities

Zhou says that one of the advances of this study is in their method to measure the strength of materials at the nanoscale in a way that hasn't been done before.

Miyagi says another advance is a new way to think about strengthening metals -- by engineering their grain surfaces to suppress grain sliding.

"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."

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Materials provided by University of Utah.