Metals with Memory Could Fix Dents

Engineers have concocted metals that remember their original shapes and with a little heat can snap back to new after being crumpled or dented.

"We showed for the first time that metal can snap back after deformation," lead author Taher Saif of the University of Illinois told LiveScience.

Normally, if you bend a hanger or even a paperclip, it's nearly impossible to restore the metal to a 100 percent unkinked state. Physical properties like this one are determined by the metal's crystalline and chemical structure. The crystalline structure, or microstructure, is the result of tiny groups of atoms that take on different sizes depending on how the atoms within each group are packed together.

Saif said that when the lab method gets scaled up, the memory metal could be used for any metal object that could get dented, ranging from cars and aircraft fuselages to everyday objects, such as garden tools and the metal frames on suitcases.

How it works

Saif, a mechanical engineer, and his graduate students tried to mess with those grain sizes. They examined microstructures within thin films of aluminum and gold. By controlling the temperature during production, the team created metal films with very fine grains, under 100 nanometers. For comparison, the width of a human hair is about 100,000 nanometers.

"We found that the type of metal doesn't matter," Saif said. "What matters is the size of the grains in the metal's crystalline microstructure, and a distribution in the size."

The atom grains had to be small, but not too small, they reported Friday in the journal Science, to store a "memory" of their original state. Grains that are too tiny make a metal brittle and likely to snap when bent, while grains that are too large make a super malleable metal that bends and stays in a droopy position.

Just right

The key to making metals that snap back to their original shapes, the scientists found, is a balance between brittleness and bendiness, or a balance between teensy and relatively giant grains.

With a mix of big and little grains, a sort of tug-of-war ensues: When the bigger grains bend, they push and pull on the smaller grains, which get scrunched like a spring.

Here's one way the metal might be put to practical use: After a fender bender, the springy grains in the modified metal could get sprung and release all their stored energy and force the big grains back to their initial positions.

The scientists found that by applying heat they could speed up the energy release and thus the metal's spring back to its original shape.

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