Supersonic defects have the right stuff.

You don't need to be in a jet or rocket to break the sound barrier. Striking a hard blow on a piece of metal can do the same thing.

Using computer simulations, researchers have found that a structural defect can spread through a material faster than the speed of sound, which is several thousand meters per second in a typical metal. These defects, known as dislocations, arise when deformation of a material forces perfectly aligned planes of atoms to slip past each other. Hitting the family car with a shopping cart, for example, sends dislocations running through the fender, forming a broad dent.

Previously, theorists thought that these dislocations couldn't break the sound barrier, says Huajian Gao of Stanford University. However, he and Peter Gumbsch of the Max Planck Institute for Metallurgy in Stuttgart, Germany, found that under certain conditions, materials can violate this rule.

The researchers simulated the behavior of atoms in a thin strip of tungsten at temperatures between 10 and 70 kelvins. A sharp blow to the virtual metal initiates a supersonic dislocation, indicated by telltale shock waves left behind. The waves are produced when atoms trailing the dislocation get squeezed together, much as air gets compressed when a jet produces a sonic boom. Gumbsch and Gao report their findings in the Feb. 12 SCIENCE.

"People have done simulations before to see if dislocations can propagate supersonically, but they failed," says Gao. The reason the new one succeeds is that the blow was hard enough to set up a fast initial dislocation. "If you start with a very slow dislocation, you cannot accelerate to a supersonic speed," he explains.

Supersonic movement of dislocations may play an important role in distortions of steel at low temperatures, Gumbsch says. It may also explain seismic shocks that have been observed to travel faster than the speed of sound.

"I believe all of the work, and I like it," says Michael Marder, a physicist at the University of Texas at Austin who models cracking in materials. Other computer simulations "assume that dislocations basically creep along and diffuse. It's significant to show that they can move at speeds comparable to the speed of sound."

Atomic-scale simulations give researchers an invaluable tool with which to study these phenomena, Marder says. It's difficult to image micrometer-size defects zipping along in a real crystal, he points out. "Small things that move fast are really a problem."