14 DAY TRIAL // Everybody knows that a piece of metal, once bent, is impossible to be reverted to its original state.
But, a recent study revealed that, when heat is added to bent metal films having the right microstructure, 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 A. Saif, a professor of mechanical science and engineering at Illinois, and senior author of a paper that describes his findings.
A shape memory alloy (SMA, also known as a smart alloy or memory metal) is a metal that "remembers" its geometry. After a sample of SMA has been deformed from its original atomic configuration, it regains its original geometry by itself during heating (one-way effect) or, at higher ambient temperatures, simply during unloading (pseudo-elasticity or superelasticity). These extraordinary properties are due to a temperature-dependent martensitic phase transformation from a low-symmetry to a highly symmetric crystallographic structure. Those crystal structures are known as martensite and austenite.
The study was performed on 200-nanometer-thick aluminum films and gold films, having a grain size of 65 and 50 nanometers, respectively.
"We found that the type of metal doesn't matter, said Saif, who also is "What matters is the size of the grains in the metal's crystalline microstructure, and a distribution in the size."
Raising the temperature by about 50 degrees Celsius causes the grains to grow larger than they typically are, that is about one-third to one-half the thickness of a metal film
If the grains are uniformly too small, the metal will be brittle and break while being bent. If the grains are uniformly too large, the metal will bend, but then stay in that position.
To return to the initial shape, what's needed is a balance between brittleness and malleability, that can be achieved through a combination of small and large grains.
Sarif's study revealed that variations in the microstructure lead to plastic deformation in the larger grains and elastic accommodations in the smaller grains.
The bigger grains bend, but push and pull on the smaller grains, which become elastically deformed like a spring, but If the metal is then left alone, the smaller grains will release this energy and force the bigger grains back to their original shapes over time.
The use of this technology, once refined, in day-to-day applications could vary from crumpled kitchen foil that lays flat for reuse, to bent bumpers that straighten overnight and even dents in car doors that disappear when heated with a hairdryer. It should be a very successful commercial product in a certain European country that starts with "F" and ends with "rance" , where most people don't bother looking in the rear-view mirror when parking, and if there's a shortage of space, no problem, the two cars in front and behind can always be "gently pushed" for a tight fit.