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Boundaries with a golden twist.

Boundaries with a golden twist

A close look at the etched surface of a chunk of metal often reveals a patchwork of grains jammed together. Each grain boundary marks the meeting place of two crystals, oriented so their atoms are usually aligned in different directions. What happens at such boundaries tells materials scientists a great deal about the properties of polycrystalline materials--from how much they expand when heated to how easily they fracture when put under stress.

Recently, researchers at Cornell University in Ithaca, N.Y., using a novel X-ray technique to probe the internal structure of grain interfaces in gold, obtained the most detailed data yet on the positions of atoms along a boundary between two crystalline regions and preliminary results on how atoms at such a boundary respond to heat. They report some of their findings in the Nov. 7 PHYSICAL REVIEW LETTERS.

Michael R. Fitzsimmons and his collaborators concentrated on crystal interfaces known as twist grain boundaries. They manufactured their samples by welding together two single-crystal gold films, producing roughly the same effect as rotating a sandwich's top slice of bread so that its edges no longer line up with those of the bottom slice.

The researchers, using high-intensity X-rays produced by the Cornell High Energy Synchrotron Source, looked for subtle changes in the intensity and position of X-ray signals reflected from atoms at a boundary. To find atomic positions, they calculated what the diffraction pattern would look like for a number of possible structures, varying the positions of atoms in their models until their theoretical results matched the measured signals.

The Cornell work shows the thermal properties of a twist boundary are quite different from those of the bulk material. Atoms at boundaries seem to vibrate 50 percent more strongly than those in the bulk material. Furthermore, the material expands three times as much at right angles to a boundary as it does along a boundary. In other words, if it were possible to create a gold rod by neatly stacking appropriately oriented crystals, the rod, when heated, would expand more in one direction than in the other.
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Title Annotation:twist grain boundaries
Publication:Science News
Date:Nov 26, 1988
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