Nitinol alloys exhibit two closely related and unique properties: the shape memory effect and superelasticity (also called pseudoelasticity).

Superelasticity is the ability for the metal to undergo large deformations and immediately return to its undeformed shape upon removal of the external load.

The word "nitinol" is derived from its composition and its place of discovery, Nickel Titanium - Naval Ordnance Laboratory.

[3][4] 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.

One of them applied heat from his pipe lighter to the sample and, to everyone's surprise, the accordion-shaped strip contracted and took its previous shape.

The discovery of the shape-memory effect in general dates back to 1932, when Swedish chemist Arne Ölander[6] first observed the property in gold–cadmium alloys.

At high temperatures, nitinol assumes an interpenetrating simple cubic 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 (daughter phase).

One of the reasons that nitinol works so hard to return to its original shape is that it is not just an ordinary metal alloy, but what is known as an intermetallic compound.

Thus in a certain temperature range, one can apply a stress to austenite, causing martensite to form while at the same time changing shape.

Above Md, since martensite is no longer formed, the only response to stress is slip of the austenitic microstructure, and thus permanent deformation.

Nitinol is exceedingly difficult to make, due to the exceptionally tight compositional control required, and the tremendous reactivity of titanium.

Vacuum induction melting (VIM) is done by using alternating magnetic fields to heat the raw materials in a crucible (generally carbon).

[17] Other research report that VAR employing extreme high-purity raw materials may lead to a reduced number of inclusions and thus to an improved fatigue behavior.

Because it is the material of choice for applications requiring enormous flexibility and motion (e.g., peripheral stents, heart valves, smart thermomechanical actuators and electromechanical microactuators), it is necessarily exposed to much greater fatigue strains compared to other metals.

[19] (Nickel is also present in substantial amounts in stainless steel and cobalt-chrome alloys also used in the medical industry.)

In literature, some early works report to have failed to show measurable differences,[20][21] while novel studies demonstrate a dependence of fatigue resistance on the typical inclusion size in an alloy.

Numerous methods are used to increase the cooling performance, such as forced air,[28] flowing liquids,[29] thermoelectric modules (i.e. Peltier or semiconductor heat pumps),[30] heat sinks,[31] conductive materials[32] and higher surface-to-volume ratio[33] (improvements up to 3.3 Hz with very thin wires[34] and up to 100 Hz with thin films of nitinol[35]).

[36] Recent advances have shown that processing of nitinol can expand thermomechanical capabilities, allowing for multiple shape memories to be embedded within a monolithic structure.

However, elastocaloric device made with NiTi wires also have limitations, such as its short fatigue life and dependency on large tensile forces (energy consuming).

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.