Impurity clouds from all sides now

In crystalline materials such as steel, imperfections at the atomic scale can have a huge effect on gross mechanical properties. For example, line defects known as dislocations—where extra planes of atoms crowd into the regular crystal structure—significantly alter strength and flexibility.

Researchers know that atoms of any additive that may be present tend to collect around a dislocation, pinning it down and making the crystal less flexible. Now, for the first time, scientists have obtained three-dimensional images of these clouds of additive atoms, known as Cottrell atmospheres.

“Such information is extremely important for the understanding of both the mechanical properties and the mechanism of formation of the atmosphere,” says Didier Blavette of the University of Rouen in France. He and his colleagues report their findings in the Dec. 17, 1999 Science.

Blavette and his group prepared a sample of an iron-aluminum alloy containing a low concentration of boron. By putting an electric field across the sample, they evaporated its atoms one layer at a time, and they used mass spectrometry to identify the atoms as they came off. This technique, which they call a three-dimensional atom-probe, allowed the scientists to deduce the location of all the atoms.

The results showed an area shaped like a tube that was rich in boron atoms—a Cottrell atmosphere surrounding a dislocation.

Other techniques can’t pinpoint the location of atoms so precisely, says Blavette. “This type of image could not be obtained before the advent of 3-D atom probes.”