Nanocrystals with a surprise effect

Finely structured metals show no signs of deformation

Deformation experiment with nanocrystals PSI
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Specially finely structured metals have a remarkable property: unlike conventional materials, deformations leave no signs of deformation here. This amazing behavior of nanocrystalline materials was revealed by a research team at the Paul Scherrer Institute (PSI) in Switzerland.

With a newly developed tensile tester and a special measuring system, the scientists followed the events in the interior of such metals in real time for the first time. Based on the results, the mechanical properties of materials can be improved. Benefit from this, for example, microelectronics with increasingly sophisticated materials for computer chips, airbag sensors and miniaturized machines.

Done at the nanoscale

Those who plastically deform metals leave their mark: in the small crystalline grains that make up these materials, microscopic irregularities are created by external forces. However, this only applies as long as the grain size is above 50 nanometers, ie larger than 50 millionths of a millimeter. For a long time, nobody knew exactly what happened under this border. "The theory predicts that such irregularities will not occur in nanocrystals, " explains Helena Van Swygenhoven, materials scientist at PSI. However, nobody has ever been able to prove this assumption - the samples were too small and the potential changes too small.

Now, for the first time, Van Swygenhoven's research group has been able to observe changes in nanocrystals in real time. Contrary to theoretical predictions, as previously demonstrated by computer simulations, the scientists encountered processes similar to those of larger structures - but with one decisive difference: the resulting line-shaped disorder traverses the entire grain and disappears again. The crystals are the plastic, ie irreversible deformation subsequently no longer to look at. This surprising finding from the PSI team has attracted much attention in the art world and recently led to two publications in the prestigious science journals "Science" and "Nature".

Dogbone made of nanomaterial

To test material deformation under standardized conditions, material scientists first built a tensile testing machine where they can test samples that are only a few millimeters in size. To be able to clamp the test samples into the device, they must have the shape of a dog bone. The scientists get the actual insight into the small structures with the high-intensity X-ray radiation from the Synchrotron Light Source Switzerland (SLS), a particle accelerator for electrons. The highly concentrated radiation is deflected by the crystal structure of the grains and then collected by a special detector. Due to the deflection of the radiation, changes in the structure of the samples can be detected. "The detector is unique in the world and allows us to measure in real time, " says Van Swygenhoven. display

On the way to MEMS

The work of the materials research group at PSI represents an important step for the next generation of microelectronics: they provide the fundamentals for understanding the material properties of metallic components in microelectromechanical systems. Such so-called MEMS are tiny sensors, valves or motors that are embedded in semiconductor chips. They are used for example in airbags as motion detectors that are not thicker than a human hair. Even developers of novel materials with improved mechanical properties will benefit from the PSI measurements in the future. So hard and simultaneously deformable materials are sought in the aircraft industry.

The research at PSI is supported by the Swiss National Science Foundation.

(Paul Scherrer Institute (PSI), 08.09.2004 - NPO)