Published online 15 September 2005 | Nature | doi:10.1038/news050912-10

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Computer models show how to give metals more mettle.

A simulation of a shock wave travelling up through nanocrystalline copper, squishing grains together as it goes. © Science

Sudden shocks make you harder. At least, they can do if you're a metal. A team of researchers in the United States and Switzerland hope that this discovery could point the way to ultra-hard metals for engineering in extreme environments, such as nuclear fusion reactors.

The team simulated the atomic-scale structure of a slab of copper, made up of a patchwork of grains about 20 nanometres (millionths of a millimetre) across. Their study showed that the material becomes harder and stronger after a shock wave has passed through. An explosion could produce such a shock, suggest the team, led by Eduardo Bringa at the Lawrence Livermore Laboratory in California.

Ultra-hard metals are needed not just for military armour but for applications such as nuclear fusion. Researchers are looking into initiating fusion reactions using laser blasts, and very strong materials are needed to contain these reactions.

“We can go even further along the hardening path than people thought.” James McNaney

Lawrence Livermore National Laboratory, California

Metals are a patchwork of grains stuck together. These materials bend and deform when misalignments of rows of atoms, called dislocations, slip through a grain. This allows the material to adapt itself to different shapes when under stress, making the metal soft.

Smaller grains make for harder metals, because dislocations tend to get stuck when they reach the edge of a grain. So in grains just a few tens of nanometres across, dislocations can move travel a very small distance, limiting how much the material can change shape.

Slip and slide

But there is a limit to the strengthening effects of shrinking crystalline grains. If stressed far enough, the grains themselves may slip and slide against each other, deforming the material.

Bringa and colleagues sought to stop these slips by investigating what happens when one subjects a metal with nanoscale grains to sharp shocks.

A shock wave creates a very high pressure over a very narrow region as it travels through a material. As the region of stress is on the same size scale as a grain itself, the pressure can't force grains to slide over each other.

Instead, the material can only accommodate the deformation by dislocations appearing within the grains. But this time, that hardens the metal, they report in Science1. The dislocations produce kinks on the grain edges that knit them together, providing extra strength.

"The dislocations hook the grain boundaries a little bit and stop them sliding," says team member James McNaney of the Lawrence Livermore. "That way, we can take nanocrystalline metals even further along the hardening path than people thought we could."

Shocker

The researchers have preliminary evidence that same thing happens in real life, as well as simulations. A piece of nanocrystalline nickel, after being subjected to an explosive shock, becomes peppered with dislocations inside the grains. "Normally you never see that, because the grains slide first," says McNaney.

The team hasn't tested the strength of their shocked nickel, because such measurements are tricky with such tiny amounts of material.

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Julia Weertman, a specialist in nanocrystalline metals at Northwestern University in Evanston, Illinois, says that the results are interesting and could be useful for fusion research. But for large-scale industrial applications such as aerospace engineering, she says, the question is whether one can make enough of the material to be useful.

"The jury is out on that," McNaney admits, although he says that there are already industrial techniques for shocking large volumes of materials. He thinks that these strengthened metals might make ultra-hard coatings for other materials.

Weertman also cautions that hardness comes at the cost of brittleness. "Dislocations serve a purpose in engineering," she says. "They make things more forgiving. That's why we make aircraft out of [bendy] metal instead of [brittle] ceramics." McNaney agrees: "What you'd like is something that is ductile and strong at the same time."

Lawrence Livermore National Laboratory, California