Metals with memory straighten up

A new technique to make memory metals may lead to crumpled kitchen foil that lays flat for reuse, bent bumpers that straighten overnight and dents in car doors that disappear when heated.

Normally, when a piece of metal, such as a paperclip, is bent, the change in shape becomes permanent. But, when heat is added to bent metal films having the right microstructure, researchers at the University of Illinois found the films return to their original shapes. The higher the temperature, the sooner the metal films revert.

‘It’s as though the metal has a memory of where it came from,’ said Taher Saif, a professor of mechanical science and engineering at Illinois.

In the study, Saif and graduate students Jagannathan Rajagopalan and Jong Han explored aluminium films and gold films. The aluminium films were 200 nanometres thick, 50-60 microns wide and 300-360 microns long. The gold films were 200 nanometres thick, 12-20 microns wide and 185 microns long. The average grain size in the aluminium films was 65 nanometres; in the gold films, 50 nanometres.