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Nickel titanium, also known as nitinol, is a metal alloy of nickel and titanium, where the two elements are present in roughly equal atomic percentages. Different alloys are named according to the weight percentage of nickel; e.g., nitinol 55 and nitinol 60.
Nitinol alloys exhibit two closely related and unique properties: the shape memory effect and superelasticity (also called pseudoelasticity). Shape memory is the ability of nitinol to undergo deformation at one temperature, stay in its deformed shape when the external force is removed, then recover its original, undeformed shape upon heating above its "transformation temperature." Superelasticity is the ability for the metal to undergo large deformations and immediately return to its undeformed shape upon removal of the external load. Nitinol can undergo elastic deformations 10 to 30 times larger than alternative metals. Whether nitinol behaves with shape memory effect or superelasticity depends on whether it is above its transformation temperature during the action. Nitinol behaves with the shape memory effect when it is colder than its transformation temperature, and superelastically when it is warmer than it.
While potential applications for nitinol were realized immediately, practical efforts to commercialize the alloy did not take place until two decades later in the 1980s, largely due to the extraordinary difficulty of melting, processing and machining the alloy.
The discovery of the shape-memory effect in general dates back to 1932, when Swedish chemist Arne Ölander first observed the property in gold–cadmium alloys. The same effect was observed in Cu-Zn (brass) in the early 1950s.
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). There are four transition temperatures associated to the austenite-to-martensite and martensite-to-austenite transformations. Starting from full austenite, martensite begins to form as the alloy is cooled to the so-called martensite start temperature, or Ms, and the temperature at which the transformation is complete is called the martensite finish temperature, or Mf. When the alloy is fully martensite and is subjected to heating, austenite starts to form at the austenite start temperature, As, and finishes at the austenite finish temperature, Af. cooling/heating cycle shows thermal hysteresis. The hysteresis width depends on the precise nitinol composition and processing. Its typical value is a temperature range spanning about but it can be reduced or amplified by alloying and processing.
Crucial to nitinol 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 in some ways without breaking atomic bonds. This type of deformation is known as Crystal twinning, which consists of the rearrangement of atomic planes without causing slip, or permanent deformation. 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 restored, regardless of whether the martensite phase was deformed. Thus 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 pressure can be produced by preventing the reversion of deformed martensite to austenite—from to, in many cases, more than . 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. In an ordinary alloy, the constituents are randomly positioned in the crystal lattice; in an ordered 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 complexity in fabricating devices made from the alloy.
To fix the original "parent shape," the alloy must be held in position and heated to about . This process is usually called shape setting. 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 10 to 30 times greater than that of a normal spring material. There are, however, constraints: the effect is only observed up to about above the Af temperature. This upper limit is referred to as Md, which corresponds to the highest temperature in which it is still possible to stress-induce the formation of martensite. Below Md, martensite formation under load allows superelasticity due to twinning. 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 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. Transformation temperatures in nitinol can be controlled to some extent, where Af temperature ranges from about . Thus, it is common practice to refer to a nitinol formulation as "superelastic" or "austenitic" if Af is lower than a reference temperature, while as "shape memory" or "martensitic" if higher. The reference temperature is usually defined as the room temperature or the human body temperature ().
One often-encountered effect 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 usually of no practical use.
There are two primary melting methods used today. Vacuum arc remelting (VAR) 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. Vacuum induction melting (VIM) 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. While both methods have advantages, it has been demonstrated that an industrial state-of-the-art VIM melted material has smaller inclusions than an industrial state-of-the-art VAR one, leading to a higher fatigue resistance. 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. Other methods are also used on a boutique scale, including plasma arc melting, induction skull melting, and e-beam melting. Physical vapour deposition is also used on a laboratory scale.
When melting nitinol by induction (or any titanium alloy) it is hard to find adequate material for crucibles. Molten titanium is chemically corrosive, requires extreme temperatures and specific conditions to avoid contamination.For example, if a granite crucible is used, the carbon in granite can bind with titanium resulting in an impure alloy.Another thing to consider is erosion of the crucible walls. Factors that play into the amount of erosion include but are not limited to the movement of the molten nitinol, the crucible's porosity, and the time the molten metal is touching the material. The porosity of a crucible must be balanced so that the crucible has good temperature shock resistance and a short wetting distance, so it doesn't erode.There are few materials for crucibles that work with titanium casting which is a contributing factor to the difficulty in mass production of nitinol. Experiments are being done to use a coating over a crucible to block chemical interactions so that the base material only must be heat shock resistant and avoid erosion. For Example, Y2O3 is being researched as a coating in an Aluminum oxide crucible. The Y2O3 does not chemically interact with the titanium, and the aluminum oxide is heat shock resistant making a suitable combination.
Another method to manufacture nitinol that is less used is reactive sintering. Reactive sintering in a vacuum is used to compress nitinol powder into a shape. The powder is heated up to around about 580 and 650 degrees Celsius via an exothermic chemical reaction and pressed into shape.This method produces a porous surface and because it is in a vacuum the titanium doesn't have the chance to bond to oxygen. However, the result is not necessarily homogeneous. It can form ni3ti and ti3ni due to the slow heating produced from the reaction. Heating it quickly can improve homogeneity and reduce the size of the pores. Spark plasma sintering was invented in 1980 as a method to make the heating as rapid as possible by sending an electric current through the system. This way the nitinol is purer without ni3ti or ti3ni forming.
Heat treating nitinol is delicate and critical. Aging time and temperature control the precipitation of various Ni-rich phases, and thus controls how much nickel resides in 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 products. Nitinol that is made with a focus on shape memory effect is heat treated in a range of 350 degrees Celsius to 450 degrees Celsius. For superelastic Nitinol the temperature is closer to 500 degrees Celsius. If the nickel component is greater than 55.5% then the heat treatment temperature is in the range of 600 to 900 degrees Celsius
Additive manufacturing allows for control over density and porosity. Porosity effects cell ingrowth and nickel ion release in implants. The more porous the material the more surface area for cells to bond to as well as the more surface contact with tissue for nickel to leach.Additionally, Elasticity is affected by how porous the result is, so additive manufacturing gives control over the elasticity of each layer. Different lattice shapes also provide different shape memory behavior. For example, a Schwartz structure provides more strength. While a diamond structure allows for better transformation strain.
Creating slicing software for additive manufacturing of a shape memory material like nitinol must include how it changes. Lexcellant and Boubon modeled the superelasticity of SMAs in one dimension.
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. (Nickel is also present in substantial amounts in stainless steel and cobalt-chrome alloys also used in the medical industry.) During heat treating, nitinol forms a very stable protective TiO2 layer that acts as a barrier against ion exchange; repeatedly showing that nitinol releases nickel at a slower pace than stainless steel. Initially it was believed that a thicker oxide layer would keep more nickel from leaching into tissue, but it has since been proven to not be directly proportional.The thicker the oxide layer the more likely it is to form microcracks. These microcracks can cause the oxide layer to flake off as the nitinol deforms. Eventually these micro cracks can reach down to the pure nitinol layer and begin releasing nickel again.Additionally, when the oxide layer forms, the layer below becomes richer in nickel so if the oxide layer is compromised it will leach more nickel into nearby tissue. Thicker oxide layers are also more susceptible to low ph. When heat treated, the oxide layer that nitinol forms is black. This black oxide is thick, cracked, and prone to flaking. These cracks are made worse by pitting as anodes form along microcracks. Surface processing can make these cracks smaller and reduce pitting. After surface processing the oxide layer will become thinner and change color. The color can vary depending on the method. Examples include, but are not limited to blue, red, gold, and amber oxide layers.Surface processing methods can vary from mechanical to electrical and chemical polishing. Electrical and chemical polishing seem to be the most effective. In an experiment as reviewed by the following source(), Electropolished wire was compared to untreated thick oxidized wire. It was found after 5000 cycles (deformations from shape memory) the thick oxide wire started to fracture, but the electropolished wire lasted 100,000 cycles. Because of this the process for creating an effective oxide layer is to polish (electro or chem) the nitinol before heat treating and then after heat treating until the oxide layer is acceptably thin.
There are constant and long-running discussions regarding inclusions in nitinol, both TiC and Ti2NiOx. As in all other metals and alloys, inclusions can be found in nitinol. The size, distribution and type of inclusions can be controlled to some extent. Theoretically, smaller, rounder, and fewer inclusions should lead to increased fatigue durability. In literature, some early works report to have failed to show measurable differences,
Nitinol is difficult to weld, both to itself and other materials. Laser welding nitinol to itself is a relatively routine process. Strong joints between NiTi wires and stainless steel wires have been made using nickel filler. Laser and tungsten inert gas (TIG) welds have been made between NiTi tubes and stainless steel tubes. More research is ongoing into other processes and other metals to which nitinol can be welded.
Actuation frequency of nitinol is dependent on heat management, especially during the cooling phase. Numerous methods are used to increase the cooling performance, such as forced air, flowing liquids, thermoelectric modules (i.e. Peltier or semiconductor heat pumps), heat sinks, conductive materials and higher surface-to-volume ratio (improvements up to 3.3 Hz with very thin wires and up to 100 Hz with thin films of nitinol). The fastest nitinol actuation recorded was carried by a high voltage capacitor discharge which heated an SMA wire in a manner of microseconds, and resulted in a complete phase transformation (and high velocities) in a few milliseconds.Vollach, Shahaf, and D. Shilo. "The mechanical response of shape memory alloys under a rapid heating pulse." Experimental Mechanics 50.6 (2010): 803-811.
Recent advances have shown that processing of nitinol can expand thermomechanical capabilities, allowing for multiple shape memories to be embedded within a monolithic structure. Research on multi-memory technology is on-going and may deliver enhanced shape memory devices in the near future, and new materials and material structures, such as hybrid shape memory materials (SMMs) and shape memory composites (SMCs).
Superelastic materials undergo stress-induced transformation and are commonly recognized for their "shape-memory" property. Due to its superelasticity, NiTi wires exhibit "elastocaloric" effect, which is stress-triggered heating/cooling. NiTi wires are currently under research as the most promising material for the technology. The process begins with tensile loading on the wire, which causes fluid (within the wire) to flow to HHEX (hot heat exchanger). Simultaneously, heat will be expelled, which can be used to heat the surrounding. In the reverse process, tensile unloading of the wire leads to fluid flowing to CHEX (cold heat exchanger), causing the NiTi wire to absorb heat from the surrounding. Therefore, the temperature of the surrounding can be decreased (cooled).
Elastocaloric devices are often compared with magnetocaloric devices as new methods of efficient heating/cooling. Elastocaloric device made with NiTi wires has an advantage over magnetocaloric device made with gadolinium due to its specific cooling power (at 2 Hz), which is 70X better (7 kWh/kg vs. 0.1 kWh/kg). However, elastocaloric device made with NiTi wires also have limitations, such as its short fatigue life and dependency on large tensile forces (energy consuming).
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 SMAs. 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.
(2025). 9780871708625, ASM International. ISBN 9780871708625 while novel studies demonstrate a dependence of fatigue resistance on the typical inclusion size in an alloy.
Applications
There are four commonly used types of applications for nitinol:
(1989). 9781558651005, Future Technology Surveys. ISBN 9781558651005
Thermal and electrical actuators
Biocompatible and biomedical applications
Damping systems in structural engineering
Other applications and prototypes
(1996). 9780852929261, Prentice Hall. ISBN 9780852929261
(2025). 9781933531052, NSTA Press. ISBN 9781933531052
(2025). 9789811663352, Springer Singapore. ISBN 9789811663352 fasteners, connecting components, electrical connections, and electromechanical actuators.
Further reading
A process of making parts and forms of Type 60 Nitinol having a shape memory effect, comprising: selecting a Type 60 Nitinol. Inventor G, Julien, CEO of Nitinol Technologies, Inc. (Washington State)
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