X-rays Reveal Signs of Stress

CRYSTAL CITY, VIRGINIA--When metal bends, the stress shoves atoms into long ridges that weaken the material. These stretch marks are notoriously hard to study, for most are hidden deep inside the metal. Now these buried atomic rifts, called dislocations, have been thrust into the light. Physicists reported here yesterday at the annual meeting of the American Crystallographic Association that they can watch dislocations form using a powerful beam of x-rays.

When x-rays shoot through a metal crystal, a few should glance only lightly off large structures such as dislocations and emerge in very nearly the same direction. For nearly 50 years, a number of researchers have tried to spy this small angle scattering, but most sightings have turned out to be x-rays that carom off the regularly spaced atoms in the crystal, says Gabrielle Long, a physicist at the National Institute of Standards and Technology in Gaithersburg, Maryland.

Long and colleague Lyle Levine hoped they might succeed with the high-intensity bursts of x-rays at the National Synchrotron Light Source at Brookhaven National Laboratory on Long Island. They shined the beam on crystal after crystal of aluminum while stretching it to generate dislocations. For a long time, they saw only a ghost of a signal. Then, one afternoon, with a crystal they had nicknamed George, the signal became mysteriously large: The low-angle scattering intensified 10-fold as it was stretched. After detailed modeling and further testing, the group concluded that the crystal had been fortuitously aligned relative to the x-ray beam so that the dislocations lined up and amplified the signal. "It was beautiful," says Long. "We were jumping around the machine."

"This is high-class stuff," says Ali Argon, a physicist at the Massachusetts Institute of Technology. Long says their measurements of the structure and density of deformations may someday be useful to the auto industry, which currently spends a lot of money figuring out how to pound out precision parts sans dislocations. Argon, however, is skeptical. If auto manufacturers can use atomic-scale information to build better car parts, he says, "my hat is off to them."

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