Nickel Titanium
| Material properties | |
|---|---|
| Density | 6.45 g/cm3 (0.233 lb/cu in) |
| Electrical resistivity (austenite) | 82×10−6 Ω·cm |
| (martensite) | 76×10−6 Ω·cm |
| Thermal conductivity (austenite) | 0.18 W/cm·K |
| (martensite) | 0.086 W/cm·K |
| Coefficient of thermal expansion (austenite) | 11×10−6/°C |
| (martensite) | 6.6×10−6/°C |
| Magnetic permeability | < 1.002 |
| Magnetic susceptibility (austenite) | 3.7×10−6 emu/g |
| (martensite) | 2.4×10−6 emu/g |
| Elastic modulus (austenite) | 75–83 GPa |
| (martensite) | 28–40 GPa |
| Yield strength (austenite) | 195–690 MPa |
| (martensite) | 70–140 MPa |
| Poisson's ratio | 0.33 |
| Nitinol properties are particular to the precise composition of the alloy and its processing. These specifications are typical for commercially available shape memory nitinol alloys. | |
Nickel titanium, also known as Nitinol (part of shape memory alloy), is a metal alloy of nickel and titanium, where the two elements are present in roughly equal atomic percentages e.g. Nitinol 55, Nitinol 60.
Nitinol alloys exhibit two closely related and unique properties: shape memory effect (SME) and superelasticity (SE; also called pseudoelasticity, PE). Shape memory is 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.
The word Nitinol is derived from its composition and its place of discovery: (Nickel Titanium-Naval Ordnance Laboratory). William J. Buehler along with Frederick Wang, discovered its properties during research at the Naval Ordnance Laboratory in 1959. Buehler was attempting to make a better missile nose cone, which could resist fatigue, heat and the force of impact. Having found that a 1:1 alloy of nickel and titanium could do the job, in 1961 he presented a sample at a laboratory management meeting. The sample, folded up like an accordion, was passed around and flexed by the participants. One of them applied heat from his pipe lighter to the sample and, to everyone's surprise, the accordion-shaped strip stretched and took its previous shape.
While the potential applications for nitinol were realized immediately, practical efforts to commercialize the alloy did not 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 were not readily overcome until the 1980s, when these practical difficulties finally began to be resolved.
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