Background

Combining old-fashioned metal working techniques with modern technology, 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.

Previous Attempts to Strengthen Metals

The achievement, reported in the 31 October issue of Nature, is important because earlier attempts to strengthen a pure metal such as copper have almost always resulted in a material that is much less ductile, making it more likely to fracture when stretched. Strength refers to how much stress a metal can tolerate before its shape is permanently deformed.

Potential Applications of Stronger Pure Metals

En Ma, a professor in the Department of Materials Science and Engineering at The Johns Hopkins University, and coauthor of the paper said, ‘We were able to get the strength of pure copper up to and beyond that of copper alloys without adding any other metals to it and without sacrificing ductility.’ Ma went on to say that such strong and tough pure metals could have applications in microelectromechanical systems, for which suitable alloys may be more difficult to produce and may be more prone to corrosion, and in biomedical devices, in which a pure metal may be preferable to alloys that could expose the body to toxic metallic or non-metallic elements.

The Strengthening Process

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

Rolling

The researchers started with a 1 inch (25.4mm) cube of pure commercial copper and dipped it into liquid nitrogen at a temperature of -196°C for three to five minutes. After removing it, the researchers rolled the copper flat, cooling the sample between rolling passes until it had reached a thickness of about 1mm 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 that 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.

Heat Treating

Next, the copper was heat treated - three minutes at 200°C. As it heated up, the dislocations began to disappear in a process called ‘recrystallisation,’ said Wang. ‘New, ultrafine crystal grains formed that were almost dislocation-free. The higher the stored dislocations’ density after rolling, the finer the recrystallised grains during heating. In our copper, these new grains were only a couple of nanometers in size, several hundred times smaller than the original crystals, making the copper much stronger than it was in its original form,’ he continued.

Why These Materials Are Stronger Than More Conventional Materials

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

However, by carefully controlling the temperature and the timing when they heated the metal, the Johns Hopkins engineers allowed about 20-25% of the copper’s crystals to grow to a larger size in a process called ‘abnormal grain growth’, i.e. non-uniform grain growth. According to the researchers, this final mix of ultrafine grains and larger ones, described as ‘bimodal distribution’ is what gave the new copper its coexisting high strength and ductility. ‘By manipulating the grain size distribution starting from a nanometer-scaled grain structure, we reached an inhomogeneous microstructure that is stable during stretching,’ said Ma. ‘That reinstated the copper’s ability to stretch uniformly without fracture, a feature very important for the formability of the high strength copper when processing it into different shapes in forming operations.’

Application to Other Metals and Alloys

Next, the researchers plan to test their process with other pure metals as well as with metal alloys to see if it produces the same change in mechanical properties. ‘Materials with uniformly nanocrystalline grains can give you very high strength, but usually not enough ductility’ said Ma. ‘They are also difficult to process, often involving compaction of nanocrystalline powders. If you want a metal that is both strong and ductile, you may want to go down the bulk processing route. Our work demonstrates that extraordinary properties can be derived from a nanostructured material by first creating and then tailoring the ultrafine grain structures.’