Nickel titanium

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Nickel titanium is the common name for alloys of nickel and titanium in which the two elements are present in equal (or nearly equal) amounts. Thus rather than being simply a mixture of the two, the name really refers to the equiatomic compound, NiTi.

Nitinol alloys exhibit two closely related and very 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.

Contents

[edit] 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.[1]

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 due to the extraordinary difficultly in melting, processing and machining the alloy. Even these efforts encountered financial challenges that weren't really overcome until the 1990's, when finally these practical difficulties began to be resolved.

The discovery of the shape-memory effect in general dates back to 1932 when Swedish researcher Arne Olander [2] first observed the property in gold-cadmium alloys. The same effect was observed in Cu-Zn in the early 1950's[3].

[edit] How it works

Austenite and Martensite structures of the NiTi compound.

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—more specifically, martensite begins to form at the so-called Ms temperature, and the temperature at which it is complete is called the Mf 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. Upon heating, however, there is a slight upward shift in the temperatures, now beginning at the As temperature, and finishing at the Af temperature. 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.[4]

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 effect of Nitinol composition on the Ms temperature.

The scenario described above (cooling austenite to form martensite, defoming 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. There are, however, constraints: the effect is only observed some 0-40 degrees C above the Af temperature.

Nitinol is typically composed of approximately 50 to 51% nickel by atomic percent (55 to 56% weight percent).[5][6]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.

[edit] Making Nitinol and Nitinol devices

Nitinol is exceedingly difficult to melt due to the exceptionally tight compositional control required, and the tremendous reactivity of titanium. Every atom of titanium that combines with oxygen or carbon is an atom that is robbed from the NiTi lattice, thus shifting the composition and making the transformation temperature that much colder. There are two primary melting methods used today:

  • Vacuum Arc Remelting: This is done by striking an electrical arc between the raw material and a water-cooled copper strike plate. Melting is done in a high vacuum, and the mold itself is water cooled copper, so no carbon is introduced during melting.
  • Vacuum Induction Melting: This is done by using alternating magnetic fields to heat the raw materials in a crucible (generally carbon). This is also done in a high vacuum, but carbon is introduced during the process.

While both methods have advantages, there are no substantive data showing that material from one process is better than the other. Other methods are also used on a boutique scale, including plasma arc melting, induction skull melting, and e-beam melting. Physical vapor deposition is also used on a laboratory scale.

Hot working of Nitinol is relatively easy, but cold working is difficult because the enormous elasticity of the alloy increases die or roll contact, leading to tremendous frictional resistance and tool wear. For similar reasons, machining is extremely difficult--to make things worse, the thermal conductivity of Nitinol is poor, so heat is difficult to remove. Grinding (abrasive cutting), Electrical discharge machining (EDM) and laser cutting are all relatively easy.

Heat treating Nitinol is delicate and critical. It is the essential tool in fine-tuning the transformation temperature. Aging time and temperature controls the precipitation of various Ni-rich phases, and thus controls how much nickel resides on the NiTi lattice; by depleting the matrix of nickel, aging increases the transformation temperature. The combination of heat treatment and cold working is essential in controlling the properties of Nitinol.[7]

[edit] Hot Topics

Fatigue failures of Nitinol devices are a constant subject of discussion. Because it is the material of choice for applications requiring enormous flexibility and motion, it is necessarily exposed to much greater fatigue strains than are other metals (i.e., peripheral stents and heart valves). While the strain controlled fatigue performance of Nitinol is superior to all other know metals, fatigue failures have been observed in the most demanding applications. There is a great deal of effort underway trying to better understand and define the durability limits of Nitinol.

Nitinol is half nickel, and thus there has been a great deal of concern in the medical industry regarding the release of nickel, a known allergen and possible carcinogen.[8] (Nickel is also present in substantial amounts in stainless steel and cobalt-chrome alloys.) When properly treated (via electropolishing and/or passivation), Nitinol forms a very stable protective TiO2 layer that acts as a very effective and self-healing barrier against ion exchange. It has been repeatedly shown that Nitinol releases nickel at a slower pace then stainless steel, for example. Having said that, very early medical devices were made without electropolishing, and corrosion was observed. Today's nitinol vascular self-expandable metallic stents, for example, show no evidence of corrosion or nickel release, and the outcomes in patients with and without nickel allergies are indistinguishable.

There are constant and long-running discussions regarding inclusions in Nitinol, both TiC and Ti2NiOx. All metals contain inclusions, and Nitinol cannot be melted without inclusions--they are omnipresent. The size, distribution and type of inclusions can be controlled to some extent. Theoretically, smaller, rounder and few inclusions should lead to increased fatigue durability. All studies done to date, however, have failed to show measurable differences.[9][10]

[edit] 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.[11]

  • In colorectal surgery[1], 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 and in endodontics, where Nitinol files are used to clean and shape the root canals during the root canal procedure. 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.
  • Due to the fact it can change shapes it is also used as a golf club insert.
  • 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.
  • Nickel titanium can be used to make the underwires for underwire bras.[12][13][14]

[edit] See also

[edit] References

  1. ^ "The Alloy That Remembers", Time, 1968-09-13, http://www.time.com/time/magazine/article/0,9171,838687,00.html .
  2. ^ Olander, A (1932), J. Amer. Chem. Soc. 54: 3819 
  3. ^ Hornbogen, E. (1956), Z. Metallkunde 47: 47 
  4. ^ Funakubo, Hiroyasu (1984), Shape memory alloys, University of Tokyo, pp. 7, 176 .
  5. ^ http://www.nitinol.com/media/files/material-properties-pdfs/sm495_wire_data%20%5BConverted%5D_v2.pdf
  6. ^ http://www.nitinol.com/media/files/material-properties-pdfs/se508_wire_data%20%5BConverted%5D_v2.pdf
  7. ^ Pelton, A.; Russell, S. and DiCello, J. (2003), "The physical metallurgy of nitinol for medical applications", JOM Journal of the Minerals, Metals and Materials Society 55 (5) 
  8. ^ http://ntp.niehs.nih.gov/ntp/roc/eleventh/profiles/s118nick.pdf
  9. ^ Morgan, N. (2006), Carbon and Oxygen Levels in Nitinol Alloys and the Implications for Medical Device Manufacture and Durability, ASM International, p. 821 
  10. ^ Miyazaki, S. (1989), 9, Materials Research Society, p. 257 
  11. ^ Miller, Richard K. and Terri Walker (1989), Survey on shape memory alloys, p. 17 .
  12. ^ Brady, George Stuart; Henry R. Clauser, John A. Vaccari (2002). Materials Handbook (15 ed.). McGraw-Hill Professional. p. 633. ISBN 9780071360760. http://books.google.com/books?id=vIhvSQLhhMEC&pg=PA633&dq=%22nickel+underwire%22&lr=&num=100&as_brr=3&as_pt=ALLTYPES. Retrieved 2009-05-09. 
  13. ^ Sang, David; Peter Ellis, Lawrie Ryan, Jane Taylor, Derek McMonagle, Louise Petheram, Phil Godding (2005). Scientifica (Illustrated ed.). Nelson Thornes. p. 80. ISBN 0748779965. http://books.google.com/books?id=_iLwLRjoxQIC&pg=PA80&dq=%22nickel+underwire%22&lr=&num=100&as_brr=3&as_pt=ALLTYPES#PPA80,M1. Retrieved 2009-05-09. 
  14. ^ Jones, Gail; Michael R. Falvo, Amy R. Taylor, Bethany P. Broadwell (2007). "Nanomaterials: Memory Wire". Nanoscale science (Illustrated ed.). NSTA Press. p. 109. ISBN 1933531053. http://books.google.com/books?id=pVWw-ZcEaDIC&pg=PT92&dq=%22nickel+underwire%22&lr=&num=100&as_brr=3&as_pt=ALLTYPES#PPT92,M1. Retrieved 2009-05-09. 

[edit] External links

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