NiTiNOL superelasticity

In 1967, on April 3 and 4, under the sponsorship of ONR (Office of Naval Research) I organized the first International Conference on Nitinol called Symposium on TiNi and Associated Compounds . The conference was held at Naval Ordnance Laboratory, the birthplace of Nitinol. As the chairman of the conference I assisted in selecting the papers from this conference that were later published in block form in the Journal of Applied Physics [16]. Despite these efforts the Nitinol transition remained elusive for sometime. In fact, after more than 30 years since the discovery of memory effect and with more than 139 papers have appeared in various journals on this subject, the investigators still do not agree with one another. At the same time more than 4,000 patents worldwide have been filed on the use of the memory effect or superelasticity in Nitinol. Out of all this, the actual application of Nitinol remains only a handful. In sharp contrast other conventional alloys with martensitic transition has no controversy and in fact they are so well understood that a Crystallographic theory of martensitic transformation was formulated [27],... 

Superelasticity refers to the ability of certain materials to undergo unusually large elastic deformations over a certain temperature range. In the most well-known superelastic material, nitinol (NiTi 51% Ni), up to 11 percent recoverable strain is observed (as much as 25 percent in specific directions in single crystals ). As a comparison, only... 

The most well-known SMA, which exhibits superelasticity above its transformation temperature, is NiTi ( 51% Ni by weight), also known by its trade name, nitinol, for nickel-titanium-naval ordnance laboratory. This alloy was developed in 1959 by William J. Buehler (b. 1923) at the United States Naval Ordnance Laboratory, now the Naval Surface Warfare Center, in White Oak, Maryland. Buehler was looking for a new impact-resistant alloy to serve as the nose cone for a new missile. The alloy s superelastic properties were discovered in 1962 by Frederick E. Wang (b. 1932), whom Buehler hired. The very first observation of superelasticity was by the University of Stockholm chemistry professor Arne Olander, who discovered a rubber-hke effect in an Au-Cd alloy (47.5% Au) in 1932 (Olander, 1932). 

The nickel-titanium orthodontic wire (Nitinol, 3M Unitek, Monrovia, CA, USA) that was first marketed [6] had a heavily cold-worked stable martensite structure [25], but superelastic wires [21] were subsequently introduced and followed [34] by wires having shape memory in the oral environment [7]. For the most efficient treatment, the archwire that is bent by the clinician for treating malpositioned teeth should return completely to the initial undeformed state during the process of tooth movement. Complete recovery will not occur for superelastic wires if there is permanent deformation [33], whereas full recovery will take place for nickel-titanium wires with in vivo shape memory. 

Three nickel-titanium wires studied by TMDSC had been previously investigated by conventional DSC [25] superelastic Nitinol SE (3M Unitek) Neo Sentalloy, which has in vivo shape memory (GAC International, Bohemia, NY, USA) and nonsuperelastic Nitinol. The Nitinol SE and Neo Sentalloy archwires had 0.016 inch x 0.022 inch cross-sections, and the Nitinol archwires were 0.019 x 0.025 inch, also a clinically popular size. In addition, the effect of permanent deformation on transformations in Nitinol SE and Neo Sentalloy was studied after bending the archwires to 135° with orthodontic pliers. 

The TMDSC plots for the heating cycle of superelastic Nitinol SE are shown in Figure 7 [37], where two peaks involving the R-phase are evident... 

Cobalt chrome alloys, gold alloys, mercury amalgams, nickel-chrome alloys, nitinol alloys (shape memory and superelastic), stainless steels, tantalum, titanium, and titanium alloys... 

Recently, the Maitland group has returned to a thermosetting polyurethane system, developed in-house [48], and combined the technology with superelastic Nitinol wires with electro-resistive heating [49, 50]. In the most recent embodiment, the SMP is cast around a Nitinol wire in its predetermined coiled shape (Fig. 4). The... 

SMP-Nitinol hybrid wire is programmed to a straightened state, in which the glassy regime of the polymer inhibits superelastic recovery of the NiTi. Upon electro-resistive heating of a thin copper wire wound around the NiTi, the SMP softens and activates, allowing both the polymer and NiTi to recover [49], The device was tested with rabbit blood clots in an in vitro setup, and thermal simulations were performed to calculate the maximum heating of the device. This device was tested in a rabbit acute arterial occlusion model in which four out of five treated clots led to complete or partial restoration of blood flow [50]. 

When the material is in its martensite form, it is soft and ductile and can be easily deformed like tin pewter. Superelastic NiTiNOL is highly elastic, while austenitic NiTiNOL is quite strong and hard, similar in that way to titanium metal. The NiTi material has all these properties, their specific expression depending on the temperature at which it is used. 

Saiidi, M.S., O Brein, M., and Sadrossadat-Zadeh, M. (2009). Cyclic response of concrete bridge columns using superelastic nitinol and bendable concrete . ACI Structural Journal, 106(1) 69-77. 

A device with shape memory technology and osmotic delivery has been developed to treat patients who suffer from a painful bladder condition known as interstitial cystitis/bladder pain syndrome (TARIS Biomedical Inc., MA). The device was made of a dual-lumen silicone catheter with one lumen loaded with lidocaine, which continuously leached out as contacted with urine. The second lumen of the device contains superelastic nitinol wire in a predefined form, which provides the... 

Figure 10. Stress-strain-temperature diagram for a Ni-Ti (Nitinol) shape-memory alloy showing shape-memory and superelastic characteristics and the deformation behavior of the parent phase above the temperature (above which no martensite can form regardless of the magnitude of the stress). Temperature increases from upper right to lower left...

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