Strong copper that retains ductility

COMBINING OLD-FASHIONED metal-working techniques with modern nanotechnology, engineers at the Johns Hopkins University have produced a form of pure copper metal that is six times stronger than normal, with no significant loss of ductility says a report in the journal Nature.

Strength, on the other hand, refers to how much stress a metal can tolerate before its shape is permanently deformed. ``We were able to get the strength of the pure copper up to and beyond that of copper alloys without adding any other metals to it and without sacrificing ductility," said En Ma, a professor in the Department of Materials Science and Engineering and a co-author of the paper.

To make the pure copper stronger, the Johns Hopkins engineers had to employ extreme cold and mechanical manipulation, followed by a carefully designed heat treatment. ``A real significance of this project was that we showed what traditional metallurgical processing can do in the new era of nanotechnology," said Yinmin Wang lead author of the paper.

The researchers started with a 1-inch cube of pure commercial copper and dipped it into liquid nitrogen for three to five minutes at a temperature of -196 degrees C (-321 degrees F). After removing it, the researchers rolled the copper flat, cooling the sample between rolling passes, to a final thickness of about 1 mm.

This affected the metal's microscopic crystals, each consisting of atoms arranged in a lattice pattern. The severe rolling deformation created a high density of dislocations, meaning atomic planes had been moved out of their proper position within the lattice.

The cold temperatures kept these defects from quickly moving back into their original alignment. Next, the copper was placed in an oven for three minutes to bake at 200 degrees C (392 degree F).

``As they heated up, the dislocations began to disappear in a process called 're-crystallisation,'' Wang explained. "New, ultra fine crystal grains formed that were almost dislocation-free. The higher the stored dislocations'density after rolling, the finer the re-crystallized grains during heating.

In our copper, these new grains were only a couple of hundred nanometres in size, several hundred times smaller than the original crystals, making the copper much stronger than it was in its original form." This change in strength occurred because of the reduction in grain size to a level similar to that of nanocrystalline materials, which are defined as materials with grain sizes less than about 100 nanometres.

When the grains are smaller, Ma explained, more grain boundaries exist to block the moving dislocations, and the metal's strength is increased.

By carefully controlling the temperature and the timing when the researchers heated the metal, however, the Johns Hopkins engineers allowed about 20-25 per cent of the copper's crystals to grow to a larger size in a process called `abnormal grain growth,' meaning non-uniform grain growth.

According to the researchers, this final mix of ultra fine grains and larger ones, described as a `bimodal distribution,' was what gave the new copper its coexisting high strength and ductility.

"By manipulating the grain size distribution starting from a nanometre-scaled grain structure, we reached an inhomogeneous microstructure that is stable during stretching," Ma said.

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