Nickel-Titanium, also known as Nitinol, is a metal alloy of nickel and titanium, where the two elements are present in roughly equal amounts.
Nitinol alloys exhibit two closely related and unique properties: shape memory and superelasticity (also called pseudoelasticity). Shape memory refers to the ability of Nitinol to undergo deformation at one temperature, then recover its original, undeformed shape upon heating above its "transformation temperature". Superelasticity occurs at a narrow temperature range just above its transformation temperature; in this case, no heating is necessary to cause the undeformed shape to recover, and the material exhibits enormous elasticity, some 10-30 times that of ordinary metal.
Nitinol's extraordinary ability to accommodate large strains, coupled with its physiological and chemical compatibility with the human body have made it one of the most commonly used materials in medical device engineering and design.
HISTORY
The term Nitinol is derived from its composition and its place of discovery: (Nickel Titanium Naval Ordnance Laboratory). William Buehler along with Frederick Wang, discovered its properties during research at the Naval Ordnance Laboratory in 1962.
While the potential applications for Nitinol were realized immediately, practical efforts to commercialize the alloy didn't take place until a decade later. This delay was largely because of the extraordinary difficulty of melting, processing and machining the alloy. Even these efforts encountered financial challenges that weren't really overcome until the 1990s, when these practical difficulties finally began to be resolved.
The discovery of the shape-memory effect in general dates back to 1932 when Swedish researcher Arne Olander first observed the property in gold-cadmium alloys. The same effect was observed in Cu-Zn in the early 1950s.
HOW IT WORKS
Nitinol's unusual properties are derived from a reversible, solid state phase transformation known as a martensitic transformation.
At high temperatures, Nitinol assumes an interpenetrating simple cubic crystal structure referred to as austenite (also known as the parent phase). At low temperatures, Nitinol spontaneously transforms to a more complicated “monoclinic” crystal structure known as martensite. The temperature at which austenite transforms to martensite is generally referred to as the transformation temperature.
Crucial to Nitinol’s properties are two key aspects of this phase transformation:
First is that the transformation is “reversible,” meaning that heating above the transformation temperature will revert the crystal structure to the simpler austenite phase.
The second key point is that the transformation in both directions is instantaneous.
Martensite's crystal structure (known as a monoclinic, or B19' structure) has the unique ability to undergo limited deformation substantially without breaking atomic bonds. It is able to undergo about 6-8% strain in this manner. When martensite is reverted to Austenite by heating, the original austenitic structure is returned, regardless of whether the martensite phase was deformed. Thus the name "shape memory" refers to the fact that the shape of the high temperature austenite phase is "remembered," even though the alloy is severely deformed at a lower temperature.
A great deal of force can be produced by preventing the reversion of deformed martensite to austenite, in many cases, more than 100,000 psi. One of the reasons that Nitinol works so hard to return its original shape is that it is not just an ordinary metal alloy, but what is known as an intermetallic compound.
In an ordinary alloy, the constituents are randomly positioned on the crystal lattice; in an intermetallic compound, the atoms (in this case, nickel and titanium) have very specific locations in the lattice. The fact that Nitinol is an intermetallic is largely responsible for the difficulty in fabricating devices made from the alloy.
The scenario described above (cooling austenite to form martensite, deforming the martensite, then heating to revert to austenite, thus returning the original, undeformed shape) is known as the thermal shape memory effect.
A second effect, called superelasticity or pseudoelasticity is also observed in Nitinol. This effect is the direct result of the fact that martensite can be formed by applying a stress as well as by cooling. Thus in a certain temperature range, one can apply a stress to austenite, causing martensite to form while at the same time changing shape. In this case, as soon as the stress is removed, the Nitinol will spontaneously return to its original shape. In this mode of use, Nitinol behaves like a super spring, possessing an elastic range some 10 to 30 times greater than that of a normal spring material.
Nitinol is typically composed of approximately 50 to 51% nickel by atomic percent (55 to 56% weight percent). Making small changes in the composition can change the transition temperature of the alloy significantly. One can control the Af temperature in Nitinol to some extent, but convenient superelastic temperature ranges are from about -20 degrees to +60 degrees C.
One often-encountered complication regarding Nitinol is the so-called R-Phase. The R-Phase is another martensitic phase that competes with the martensite phase mentioned above. Because it does not offer the large memory effects of the martensite phase, it is, more often than not, an annoyance.
APPLICATIONS
There are four commonly used types of applications for Nitinol.
Free Recovery: Nitinol is deformed at a low temperature, and heated to recover its original shape.
Constrained Recovery: The same, except that recovery is rigidly prevented, and thus a stress is generated.
Work Production: Here the alloy is allowed to recover, but to do so it must act against a force (thus doing work).
Superelasticity: As discussed above, here the Nitinol acts as a super spring.
In 1989 a survey was conducted in the United States and Canada that involved seven organizations. The survey focused on predicting the future technology, market, and applications of SMA's. The companies predicted the following uses of Nitinol in a decreasing order of importance: (1) Couplings, (2) Biomedical and medical, (3) Toys, demonstration, novelty items, (4) Actuators, (5) Heat Engines, (6) Sensors, (7) Cryogenically activated die and bubble memory sockets, and finally (8) lifting devices.
In colorectal surgery, the material is used in devices for reconnecting the intestine after removing the pathology.
In dentistry, the material is used in orthodontics for brackets and wires connecting the teeth. Once the SMA is placed in the mouth its temperature rises to ambient body temperature. This causes the Nitinol to contract back to its original shape applying a constant force to move the teeth. These SMA wires don't need to be retightened as often as they can contract as the teeth move unlike conventional stainless steel wires. Additionally, Nitinol can be used in endodontics, where Nitinol files are used to clean and shape the root canals during the root canal procedure.
Due to the fact it can change shapes it is also used as a golf club insert.
Due to the property of remembering the shape at different temperature, it is used in space antenna to save the space in space craft.
Another significant application of Nitinol in medicine is in stents: A collapsed stent can be inserted into a vein and heated (returning to its original expanded shape) helping to improve blood flow. Also, as a replacement for sutures where Nitinol wire can be weaved through two structures then allowed to transform into its preformed shape which should hold the structures in place.
Nitinol is highly biocompatible and has properties suitable for use in orthopaedic implants.
Nitinol is also popular in extremely resilient glasses frames. It is also used in some mechanical watch springs.
It can be used as a temperature control system; as it changes shape, it can activate a switch or a variable resistor to control the temperature.
It is used in cell-phone technology as a retractible antenna, or microphone boom, due to its highly flexible & mechanical memory nature.
It is used in some novelty products, such as self-bending spoons which can be used by amateur and stage magicians to demonstrate "psychic" powers or as a practical joke, as the spoon will bend itself when used to stir tea, coffee, or any other warm liquid.
It can also be used as wires which are used to locate and mark breast tumours so that following surgery can be more exact.
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