Shape memory materials
Introduction
Nitinol is an alloy of nickel (60-70%) and titanium (30-40%), and is the best-known example of the so-called shape memory alloys.
Although scientists knew some properties of this type of materials since 1945[1], the first practical applications did not begin to be developed until 30 years later. In the US Navy laboratories, William Beuhler discovered an alloy of nickel (Ni) and titanium (Ti) that had these properties, in a research program aimed at obtaining an alloy with high resistance to corrosion. The team of researchers who discovered it named the new material NiTiNOL (acronym for Ni-Ti-Naval Ordnance Laboratory). It is an alloy of nickel and titanium in almost equimolar proportions and that it has spectacular shape memory properties. Shape memory manifests itself when, after plastic deformation, the material recovers its shape after gentle heating. The name of this material has become synonymous with this type of alloy, just as Teflon is with polytetrafluoroethylene.
Shape memory alloys owe their properties to a phase transition between an austenite-type structure and a martensite-type structure. Phase transitions in solids can occur by two very different mechanisms. The most common consists of the displacement of atoms from their equilibrium positions, through a process known as diffusion, to adopt a new structure that is more stable under the pressure and temperature conditions at which the material is found. This type of transition generally occurs slowly.
Shape memory alloys also undergo a phase transition that is produced by a cooperative movement of a large number of atoms, which suffer very small displacements from their equilibrium positions. Since there is no diffusion of atoms, this transformation is very fast (it can reach the speed of sound). This type of transformation is called martensitic, because they were first described for the transformation of steel between its austenite (ductile and malleable) and martensite (brittle and hard) phases.
Martensite (low temperature) is a less symmetric phase than austenite (face-centered cubic). Once the martensite phase has been generated by cooling, it can be easily and plastically deformed, but the transformation by heating recovers the ONLY possible austenite-type structure. This effect, on a macroscopic scale, manifests itself in the recovery of the initial shape.
In a typical shape memory transformation process, the part is cooled from the austenite state to transform it into martensite. In this phase the material is malleable and easily deformed, changing shape. Heating to a temperature higher than the transformation temperature returns the object to its original shape.