Photo by Will Kirk

Strong and tough pure metals could have applications in microelectromechanical systems in which alloys may be more difficult to produce and also more prone to corrosion. The new treatment process was first reported in the Oct. 31 issue of Nature.

Until now, 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. However, researchers at the Johns Hopkins University, Baltimore, recently blended old-fashioned metal-working technology with modern nanotechnology to produce a pure copper metal six times stronger than normal — with no significant loss of ductility.

The researchers started with a 1-in. cube of pure commercial copper and dipped it in -196°C liquid nitrogen for three to five minutes. Then the copper was rolled flat, allowing for cooling between rolling passes, until the metal was about 1 mm thick. This affected the metal’s microscopic crystals, each consisting of atoms arranged in a lattice pattern. Severe rolling deformation created a high density of dislocations, meaning atomic planes moved out of position within the lattice. The cold temperatures kept these defects from quickly moving back into their original alignment.

Next, the copper was oven heated, causing the dislocations to disappear in a process that researcher Yinmin Wang refers to as “recrystallization.” The new ultra-fine grains were several hundred times smaller than the original crystals, so more grain boundaries existed to block the moving dislocations and the metal’s strength was in turn increased.

By controlling the heating process, about a quarter of the copper’s crystals were allowed to grow larger than the rest. The final mix of fine and large grains allows high strength and ductility.

The research team plans to test their process with other pure metals and alloys.