Improved by defects. Shock waves create defects that strengthen a sample of nanocrystalline copper, as seen in this simulation.

A Shocking Way to Toughen Metals

Blacksmiths have toughened metals for centuries by working them, a process that hinders the ability of crystalline grains from slipping past one another. Now a team of researchers has figured out a way to do much the same thing to metals at the nanoscale, toughening the materials by as much as 20%. The discovery could open the door for new research tools and perhaps clues to future ultrahard materials.

Metals are made up of a myriad of tiny crystalline grains. As a metal deforms, atomic-scale defects can travel through individual grains. When the defect hits the edge of grain, it usually stops. In recent years, researchers have shown that metals can be toughened by making the grains smaller and smaller, thus shortening the defects' movements. The trouble is that when grains get down to just a couple of dozen nanometers, the way a metal deforms changes as the grains now mainly slide past each other--and when metal weakens this way, there's little to be done.

In hopes of toughening such small grained metals, researchers in the United States and Switzerland led by physicist Eduardo Bringa at Lawrence Livermore National Laboratory in California carried out both supercomputer simulations and experiments on nanocrystalline copper that was exposed to an intense shock wave from a high powered laser. In both the simulation and the experiment, the shock wave caused numerous defects to occur within and between grains, and these defects prevented neighboring grains from slipping past one another under intense pressure. The result, the authors report 16 September in Science, is that the hardness of the material went up between 10% and 20% each time they fired the laser.

"It's a very significant paper" because it shows a new way to toughen materials, says Julia Weertman, a materials scientist at Northwestern University in Evanston, Illinois. One potential product, the researchers say, would be ultrahard metal capsules to confine reacting elements in laser-driven fusion experiments. But because the current process only creates about a cubic millimeter of hardened material, Bringa says, it's hard to imagine how it could be scaled up to make tougher airplane parts or military armor, for example. But Weertman notes that by understanding what causes the materials to harden, researchers may ultimately be able to reproduce the effects by growing the desired particles in the first place.

Related site
Eduardo Bringa's home page