Metals with memory - is there such a thing? At least in science fiction films they exist. As they do in research laboratories at EADS.
In science fiction movies, Hollywood heros are often confronted with almost indestructible fighting machines as their enemies. Due to the “self-healing” smart metals these fighting machines are made of wounds and bugs disappear within seconds.
EADS Innovation Works in Ottobrunn can't quite keep pace with California's dream factories. But this doesn't mean that developments being pursued at the Metallic Structures Department of EADS Innovation Works are simply a vision. They already have a tangible shape. “Films like the ‘Terminator’ are amusing fantasies, nothing more,” says Andreas Schuster with a grin. “However,” the Deputy Research Director is quick to add, “it's exactly what our metals here in the lab are supposed to be able to do, in principle: remember and then adopt their original shape, so that they can again show their muscle after being deformed.”
EADS Innovation Works has been collaborating with partners for many years on the development of so-called Shape Memory Alloys (SMA). These “smart” metals have already made their way into many different applications. The aerospace industry employs SMAs in the manufacture of decoupling mechanisms and pipe connections, while in medical engineering they are used to produce catheter guides for coronary angioplasty procedures. More everyday applications include their use in highly flexible eyeglass frames or in wire inner-supports for brassieres.
Scientifically speaking, the memory effect occurs at temperatures well under the metal's melting point, through a solid state phase change in which the crystal structure of the alloy collapses or shears at the molecular level. Says Schuster: “I like to compare the phenomenon to a falling row of dominoes. What then happens with our alloys, if you like, is that the domino pieces are able to right themselves again.”
Alloys that belong to the group with such built-in “intelligence” include nickel-titanium, copper-zinc-aluminium and gold-cadmium. SMAs demonstrate different effects. These depend on the temperature applied, the composition of the alloys and their thermo-mechanical “training”. Some metal alloys deform at lower temperatures and recover their shape upon heating in the higher temperature ranges only once. This one-way effect is exploited, for example, in satellite propellant pipe connectors developed by EADS Innovation Works together with Astrium. When heated, the expanded connectors shrink onto the pipes to form a strong bond at the point where they are conjoined.
Intelligent alloys that change their shape during heating and re-cooling without outside influence are said to demonstrate a two-way effect. EADS Innovation Works has developed a patented prototype of a thermal Velcro fastener using this effect. This fastener enables unpliant parts to be repeatedly and reversibly joined in several dimensions. The joined parts can be easily separated with minimal force in the vertical position.
Another example that illustrates the diversity of SMA applications can be found orbiting the earth. To safeguard the optical lenses of the Scimachy experiment during launch of the Envisat-1 scientific satellite, these lenses were shielded with protective covers. Once in orbit, a wire made from shape memory alloys automatically opened Scimachy's “eyes”. Schuster collaborated with Astrium on the development and testing of this nickel-titanium-copper based release mechanism.
Intelligent alloys that change their shape during heating and re-cooling without outside influence are said to demonstrate a two-way effect. EADS Innovation Works has developed a patented prototype of a thermal Velcro fastener using this effect. This fastener enables unpliant parts to be repeatedly and reversibly joined in several dimensions. The joined parts can be easily separated with minimal force in the vertical position.
