Researchers at the University of Pennsylvania and Germany’s Max Planck Institute for Intelligent Systems have shown how defects in the atom patterns in crystalline materials first form and can eventually lead to failure.

Crystalline materials have atoms that are lined up in a repeating pattern. However, when that pattern breaks failure tends to start at a defect or where the pattern is disrupted. Prior to this discovery, finding defects in the atom patterns was largely theoretical.

The researchers stretched defect-free palladium nanowires, each a thousand times thinner than a human hair, under tightly controlled conditions to conduct the study.

Contrary to conventional wisdom, the researchers found that the stretching force at which these wires failed was unpredictable. It occurred in a range of values that were more strongly influenced by the ambient temperature than was previously believed.

The researchers say this thermal uncertainty in the failure limit suggests that the point where a failure-inducing defect first appears is on the nanowire’s surface, where atoms behave in a more liquid-like way. Their increased mobility makes it more likely they will rearrange themselves into the beginnings of a “line defect,” which cuts across the nanowire, causing it to break.

“Just pulling it until it fails doesn’t tell you exactly where and how that failure began,” says Associate Professor Daniel Gianola of the Department of Materials Science and Engineering at Pennsylvania's School of Engineering and Applied Science. “Our goal was to deduce the point where the first of the nanowire’s atoms begin to shift out of their original positions and form a mobile defect.”

The palladium nanowires were grown through a vapor deposition method at high temperature. That gave each atom the time and energy to move around until it found its preferred spot in the metal’s crystalline structure. The team also used microscopic robotic manipulator to pluck the wires and attach them to their testing platform inside an electron microscope.

This platform was developed in conjunction with U.S. Energy Department's Sandia National Laboratory and is capable of functioning like an industrial mechanical testing machine at the nanoscale. The research was published in the journal Nature Materials.