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88-407: 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

176-427: A shape-memory alloy ( SMA ) is an alloy that can be deformed when cold but returns to its pre-deformed ("remembered") shape when heated. It is also known in other names such as memory metal , memory alloy , smart metal , smart alloy , and muscle wire . The "memorized geometry" can be modified by fixating the desired geometry and subjecting it to a thermal treatment, for example a wire can be taught to memorize

264-460: A compromise between ceramic and metallic properties when hardness and/or resistance to high temperatures is important enough to sacrifice some toughness and ease of processing. They can also display desirable magnetic and chemical properties, due to their strong internal order and mixed ( metallic and covalent / ionic ) bonding, respectively. Intermetallics have given rise to various novel materials developments. Some examples include alnico and

352-446: A consequence of the increased heat transfer rate, the required current to achieve a given actuation force is increased. SMA is subject to structural fatigue – a failure mode by which cyclic loading results in the initiation and propagation of a crack that eventually results in catastrophic loss of function by fracture. The physics behind this fatigue mode is accumulation of microstructural damage during cyclic loading. This failure mode

440-425: A dependence of fatigue resistance on the typical inclusion size in an alloy. 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

528-498: A fixed stoichiometry and even a clear decomposition into species . Schulze in 1967 defined intermetallic compounds as solid phases containing two or more metallic elements, with optionally one or more non-metallic elements, whose crystal structure differs from that of the other constituents . Under this definition, the following are included: The definition of a metal is taken to include: Homogeneous and heterogeneous solid solutions of metals, and interstitial compounds such as

616-486: 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.) When treated (via electropolishing or passivation ), nitinol forms a very stable protective TiO 2 layer that acts as an effective and self-healing barrier against ion exchange; repeatedly showing that nitinol releases nickel at

704-504: A higher temperature and is usually engineered so that the martensitic phase is dominant at operating temperature to take advantage of the shape memory effect, SMAs "start" highly twinned. When the martensite is loaded, these self-accommodating twins provide an easy path for deformation. Applied stresses will detwin the martensite, but all of the atoms stay in the same position relative to the nearby atoms—no atomic bonds are broken or reformed (as they would be by dislocation motion). Thus, when

792-437: A mechanical load to the martensite leads to a re-orientation of the crystals, referred to as “de-twinning”, which results in a deformation which is not recovered (remembered) after releasing the mechanical load. De-twinning starts at a certain stress σ s and ends at σ f above which martensite continue exhibiting only elastic behavior (as long as the load is below the yield stress). The memorized deformation from detwinning

880-403: 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 M s , and the temperature at which the transformation

968-433: A normal spring material. There are, however, constraints: the effect is only observed up to about 40 °C (72 °F) above the A f temperature. This upper limit is referred to as M d , which corresponds to the highest temperature in which it is still possible to stress-induce the formation of martensite. Below M d , martensite formation under load allows superelasticity due to twinning. Above M d , since martensite

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1056-405: A particular point on A f, it is possible to choose a point on the M s  line with a higher temperature, as long as that point M d also has a higher stress . The material initially exhibits typical elastic-plastic behavior for metals. However, once the material reaches the martensitic stress, the austenite will transform to martensite and detwin. As previously discussed, this detwinning

1144-429: 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. Heat treating nitinol is delicate and critical. It is a knowledge intensive process to fine-tune the transformation temperatures. Aging time and temperature controls

1232-409: A reversible solid-state phase transformation known as a martensitic transformation , between two different martensite crystal phases, requiring 69–138 MPa (10,000–20,000 psi) of mechanical stress. 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

1320-408: A shape memory can "learn" to behave in a certain way. Under normal circumstances, a shape-memory alloy "remembers" its low-temperature shape, but upon heating to recover the high-temperature shape, immediately "forgets" the low-temperature shape. However, it can be "trained" to "remember" to leave some reminders of the deformed low-temperature condition in the high-temperature phases. One way of training

1408-444: A significant cause of wire bond failures in semiconductor devices and other microelectronics devices. The management of intermetallics is a major issue in the reliability of solder joints between electronic components. Intermetallic particles often form during solidification of metallic alloys, and can be used as a dispersion strengthening mechanism. Examples of intermetallics through history include: German type metal

1496-813: A slower pace than stainless steel, for example. Early Nitinol medical devices were made without electropolishing, and corrosion was observed. Today's nitinol vascular self-expandable metallic stents show no evidence of corrosion or nickel release, and outcomes in patients with and without nickel allergies are indistinguishable. There are constant and long-running discussions regarding inclusions in nitinol, both TiC and Ti 2 NiO x . 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, while novel studies demonstrate

1584-439: A stress-induced phase transformation. The figure on the right exhibits how this process occurs. Here a load is isothermally applied to a SMA above the austenite finish temperature, A f , but below the martensite deformation temperature, M d . The figure above illustrates how this is possible, by relating the pseudoelastic stress-induced phase transformation to the shape memory effect temperature induced phase transformation. For

1672-422: A temperature-induced phase transformation reverses deformation, as shown in the previous hysteresis curve. Typically the martensitic phase is monoclinic or orthorhombic (B19' or B19 ). Since these crystal structures do not have enough slip systems for easy dislocation motion, they deform by twinning —or rather, detwinning. Martensite is thermodynamically favored at lower temperatures, while austenite ( B2 cubic)

1760-474: 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

1848-407: Is a temperature range spanning about 20–50 °C (36–90 °F) 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

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1936-418: Is a two-dimensional defect in which the stacking of atomic planes of the lattice are mirrored across the plane of the boundary. Depending on stress and temperature, these deformation processes will compete with permanent deformation such as slip. σ ms is dependent on parameters such as temperature and the number of nucleation sites for phase nucleation. Interfaces and inclusions will provide general sites for

2024-489: Is a type of metallic alloy that forms an ordered solid-state compound between two or more metallic elements. Intermetallics are generally hard and brittle, with good high-temperature mechanical properties. They can be classified as stoichiometric or nonstoichiometic intermetallic compounds. Although the term "intermetallic compounds", as it applies to solid phases, has been in use for many years, Hume-Rothery has argued that it gives misleading intuition, suggesting

2112-457: Is also strong interest in using SMAs for a variety of actuator applications in commercial jet engines, which would significantly reduce their weight and boost efficiency. Further research needs to be conducted in this area, however, to increase the transformation temperatures and improve the mechanical properties of these materials before they can be successfully implemented. A review of recent advances in high-temperature shape-memory alloys (HTSMAs)

2200-469: Is an atom that is robbed from the NiTi lattice, thus shifting the composition and making the transformation temperature lower. 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)

2288-399: 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 500 °C (930 °F). 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

2376-406: 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 non practical use. Nitinol is exceedingly difficult to make, due to the exceptionally tight compositional control required, and the tremendous reactivity of titanium. Every atom of titanium that combines with oxygen or carbon

2464-523: Is colder than its transformation temperature, and superelastically when it warmer than it. The word "nitinol" is derived from its composition and its place of discovery, Nickel Titanium - Naval Ordnance Laboratory. William J. Buehler along with Frederick E. 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

2552-405: Is common practice to refer to a nitinol formulation as "superelastic" or "austenitic" if A f 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 (37 °C or 99 °F). One often-encountered effect regarding nitinol is the so-called R-phase . The R-phase

2640-418: Is complete is called the martensite finish temperature , or M f . When the alloy is fully martensite and is subjected to heating, austenite starts to form at the austenite start temperature , A s , and finishes at the austenite finish temperature , A f . The cooling/heating cycle shows thermal hysteresis . The hysteresis width depends on the precise nitinol composition and processing. Its typical value

2728-452: 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

Nickel titanium - Misplaced Pages Continue

2816-403: Is important as certain crystallographic orientations will accommodate higher strains compared to other orientations when under an applied stress. Thus it follows that the material will tend to form orientations that maximize the overall strain prior to any increase in applied stress. One mechanism that aids in this process is the twinning of the martensite phase. In crystallography, a twin boundary

2904-479: Is in actuation. One of the advantages to using shape-memory alloys is the high level of recoverable plastic strain that can be induced. The maximum recoverable strain these materials can hold without permanent damage is up to 8% for some alloys. This compares with a maximum strain 0.5% for conventional steels. SMA have many advantages over traditional actuators, but do suffer from a series of limitations that may impede practical application. In numerous studies, it

2992-455: Is known as functional fatigue, as it is closely related with a change of microstructural and functional properties of the material). The maximum temperature at which SMAs can no longer be stress induced is called M d , where the SMAs are permanently deformed. Shape-memory alloys have different shape-memory effects. The two common effects are one-way SMA and two-way SMA. A schematic of the effects

3080-466: Is lower than that of conventional steel, but some compositions have a higher yield strength than plastic or aluminum. The yield stress for Ni Ti can reach 500  MPa . The high cost of the metal itself and the processing requirements make it difficult and expensive to implement SMAs into a design. As a result, these materials are used in applications where the super elastic properties or the shape-memory effect can be exploited. The most common application

3168-403: Is necessarily exposed to much greater fatigue strains compared to other metals. While the strain-controlled fatigue performance of nitinol is superior to all other known metals, fatigue failures have been observed in the most demanding applications; with a great deal of effort underway to better understand and define the durability limits of nitinol. Nitinol is half nickel, and thus there has been

3256-426: Is no diffusion involved. Similarly, the austenite structure receives its name from steel alloys of a similar structure. It is the reversible diffusionless transition between these two phases that results in special properties. While martensite can be formed from austenite by rapidly cooling carbon - steel , this process is not reversible, so steel does not have shape-memory properties. [REDACTED] In this figure

3344-506: 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 A f temperature ranges from about −20 to +110 °C (−4 to 230 °F). Thus, it

3432-469: Is observed in most engineering materials, not just SMAs. SMAs are also subject to functional fatigue, a failure mode not typical of most engineering materials, whereby the SMA does not fail structurally but loses its shape-memory/superelastic characteristics over time. As a result of cyclic loading (both mechanical and thermal), the material loses its ability to undergo a reversible phase transformation. For example,

3520-470: 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). There are four commonly used types of applications for nitinol: 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

3608-561: 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

Nickel titanium - Misplaced Pages Continue

3696-444: Is presented by Ma et al. A variety of wing-morphing technologies are also being explored. The first high-volume product (> 5Mio actuators / year) is an automotive valve used to control low pressure pneumatic bladders in a car seat that adjust the contour of the lumbar support / bolsters. The overall benefits of SMA over traditionally-used solenoids in this application (lower noise/EMC/weight/form factor/power consumption) were

3784-403: Is recovered after heating to austenite. The phase transformation from austenite to martensite can also occur at constant temperature by applying a mechanical load above a certain level. The transformation is reversed when the load is released. The transition from the martensite phase to the austenite phase is only dependent on temperature and stress, not time, as most phase changes are, as there

3872-424: Is reversible when transforming back from martensite to austenite. If large stresses are applied, plastic behavior such as detwinning and slip of the martensite will initiate at sites such as grain boundaries or inclusions. If the material is unloaded before plastic deformation occurs, it will revert to austenite once a critical stress for austenite is reached (σ as ). The material will recover nearly all strain that

3960-491: 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 240 MPa (35,000 psi) to, in many cases, more than 690 MPa (100,000 psi). One of

4048-424: Is shown below. The procedures are very similar: starting from martensite, adding a deformation, heating the sample and cooling it again. [REDACTED] When a shape-memory alloy is in its cold state (below M f ), the metal can be bent or stretched and will hold those shapes until heated above the transition temperature. Upon heating, the shape changes to its original. When the metal cools again, it will retain

4136-453: 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

4224-460: 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 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

4312-403: Is that their crystal transformation is fully reversible. In most crystal transformations, the atoms in the structure will travel through the metal by diffusion, changing the composition locally, even though the metal as a whole is made of the same atoms. A reversible transformation does not involve this diffusion of atoms, instead all the atoms shift at the same time to form a new structure, much in

4400-430: 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

4488-441: Is the effect that the material remembers two different shapes: one at low temperatures, and one at the high temperature. A material that shows a shape-memory effect during both heating and cooling is said to have two-way shape memory. This can also be obtained without the application of an external force (intrinsic two-way effect). The reason the material behaves so differently in these situations lies in training. Training implies that

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4576-445: Is thermodynamically favored at higher temperatures. Since these structures have different lattice sizes and symmetry, cooling austenite into martensite introduces internal strain energy in the martensitic phase. To reduce this energy, the martensitic phase forms many twins—this is called "self-accommodating twinning" and is the twinning version of geometrically necessary dislocations . Since the shape memory alloy will be manufactured from

4664-458: The carbides and nitrides are excluded under this definition. However, interstitial intermetallic compounds are included, as are alloys of intermetallic compounds with a metal. In common use, the research definition, including post-transition metals and metalloids , is extended to include compounds such as cementite , Fe 3 C. These compounds, sometimes termed interstitial compounds , can be stoichiometric , and share similar properties to

4752-601: The hydrogen storage materials in nickel metal hydride batteries. Ni 3 Al , which is the hardening phase in the familiar nickel-base super alloys , and the various titanium aluminides have also attracted interest for turbine blade applications, while the latter is also used in very small quantities for grain refinement of titanium alloys . Silicides , inter-metallic involving silicon, are utilized as barrier and contact layers in microelectronics . (°C) (kg/m ) The formation of intermetallics can cause problems. For example, intermetallics of gold and aluminium can be

4840-404: 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

4928-426: The SMA consists in applying a cyclic thermal load under constant stress field. During this process, internal defects are introduced into the microstructure which generates internal permanent stresses that facilitate the orientation of the martensitic crystals. Therefore, while cooling a trained SMA in austenitic phase under no applied stress, the martensite is formed detwinned due to the internal stresses, which leads to

5016-497: The SMA is used in an environment where the ambient temperature is uncontrolled, unintentional actuation by ambient heating may occur. Boeing , General Electric Aircraft Engines , Goodrich Corporation , NASA , Texas A&M University and All Nippon Airways developed the Variable Geometry Chevron using a NiTi SMA. Such a variable area fan nozzle (VAFN) design would allow for quieter and more efficient jet engines in

5104-450: The SMAs is governed by a phase transformation between the austenite and the martensite. NiTi alloys change from austenite to martensite upon cooling starting from a temperature below M s ; M f is the temperature at which the transition to martensite completes upon cooling. Accordingly, during heating A s and A f are the temperatures at which the transformation from martensite to austenite starts and finishes. Applying

5192-484: 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. Nitinol's unusual properties are derived from

5280-425: The alloy to a minimum and ensure the metals are well mixed. The ingot is then hot rolled into longer sections and then drawn to turn it into wire. The way in which the alloys are "trained" depends on the properties wanted. The "training" dictates the shape that the alloy will remember when it is heated. This occurs by heating the alloy so that the dislocations re-order into stable positions, but not so hot that

5368-408: The ambient environment. Consequently, SMA actuation is typically asymmetric, with a relatively fast actuation time and a slow deactuation time. A number of methods have been proposed to reduce SMA deactivation time, including forced convection, and lagging the SMA with a conductive material in order to manipulate the heat transfer rate. Novel methods to enhance the feasibility of SMA actuators include

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5456-592: The crucial factor in the decision to replace the old standard technology with SMA. The 2014 Chevrolet Corvette became the first vehicle to incorporate SMA actuators, which replaced heavier motorized actuators to open and close the hatch vent that releases air from the trunk, making it easier to close. A variety of other applications are also being targeted, including electric generators to generate electricity from exhaust heat and on-demand air dams to optimize aerodynamics at various speeds. There have also been limited studies on using these materials in robotics , for example

5544-453: 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

5632-530: 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. 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) Shape memory In metallurgy ,

5720-516: 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 contracted and took its previous shape. While potential applications for nitinol were realized immediately, practical efforts to commercialize

5808-625: The future. In 2005 and 2006, Boeing conducted successful flight testing of this technology. SMAs are being explored as vibration dampers for launch vehicles and commercial jet engines. The large amount of hysteresis observed during the superelastic effect allow SMAs to dissipate energy and dampen vibrations. These materials show promise for reducing the high vibration loads on payloads during launch as well as on fan blades in commercial jet engines, allowing for more lightweight and efficient designs. SMAs also exhibit potential for other high shock applications such as ball bearings and landing gear. There

5896-569: The hobbyist robot Stiquito (and "Roboterfrau Lara" ), as they make it possible to create very lightweight robots. Recently, a prosthetic hand was introduced by Loh et al. that can almost replicate the motions of a human hand [Loh2005]. Other biomimetic applications are also being explored. Weak points of the technology are energy inefficiency, slow response times , and large hysteresis . Intermetallic compound An intermetallic (also called intermetallic compound , intermetallic alloy , ordered intermetallic alloy , long-range-ordered alloy )

5984-982: The intermetallic compounds defined above. The term intermetallic is used to describe compounds involving two or more metals such as the cyclopentadienyl complex Cp 6 Ni 2 Zn 4 . A B2 intermetallic compound has equal numbers of atoms of two metals such as aluminium and iron, arranged as two interpenetrating simple cubic lattices of the component metals. Intermetallic compounds are generally brittle at room temperature and have high melting points. Cleavage or intergranular fracture modes are typical of intermetallics due to limited independent slip systems required for plastic deformation. However, there are some examples of intermetallics with ductile fracture modes such as Nb–15Al–40Ti. Other intermetallics can exhibit improved ductility by alloying with other elements to increase grain boundary cohesion. Alloying of other materials such as boron to improve grain boundary cohesion can improve ductility in many intermetallics. They often offer

6072-498: The magnetic response tends to be faster and more efficient than temperature-induced responses. Metal alloys are not the only thermally-responsive materials; shape-memory polymers have also been developed, and became commercially available in the late 1990s. Many metals have several different crystal structures at the same composition, but most metals do not show this shape-memory effect. The special property that allows shape-memory alloys to revert to their original shape after heating

6160-408: The material recrystallizes . They are heated to between 400 °C and 500 °C for 30 minutes, shaped while hot, and then are cooled rapidly by quenching in water or by cooling with air. The copper-based and NiTi-based shape-memory alloys are considered to be engineering materials. These compositions can be manufactured to almost any shape and size. The yield strength of shape-memory alloys

6248-847: The material shape change. And while heating back the SMA into austenite, it recovers its initial shape. There are several ways of doing this. A shaped, trained object heated beyond a certain point will lose the two-way memory effect. SMAs display a phenomenon sometimes called superelasticity, but is more accurately described as pseudoelasticity . “Superelasticity” implies that the atomic bonds between atoms stretch to an extreme length without incurring plastic deformation. Pseudoelasticity still achieves large, recoverable strains with little to no permanent deformation, but it relies on more complex mechanisms. SMAs exhibit at least 3 kinds of pseudoelasticty. The two less-studied kinds of pseudoelasticity are pseudo-twin formation and rubber-like behavior due to short range order. The main pseudoelastic effect comes from

6336-596: 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. 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 (e.g., peripheral stents , heart valves, smart thermomechanical actuators and electromechanical microactuators), it

6424-407: 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

6512-411: The same phenomenon, as shown on the left. The key to the large strain deformations is the difference in crystal structure between the two phases. Austenite generally has a cubic structure while martensite can be monoclinic or another structure different from the parent phase, typically with lower symmetry. For a monoclinic martensitic material such as Nitinol, the monoclinic phase has lower symmetry which

6600-1081: The shape of a coil spring. Parts made of shape-memory alloys can be lightweight, solid-state alternatives to conventional actuators such as hydraulic , pneumatic , and motor-based systems. They can also be used to make hermetic joints in metal tubing, and it can also replace a sensor-actuator closed loop to control water temperature by governing hot and cold water flow ratio. The two most prevalent shape-memory alloys are copper - aluminium - nickel and nickel - titanium ( NiTi ), but SMAs can also be created by alloying zinc , copper , gold and iron . Although iron-based and copper-based SMAs, such as Fe -Mn-Si, Cu-Zn-Al and Cu-Al-Ni, are commercially available and cheaper than NiTi, NiTi-based SMAs are preferable for most applications due to their stability and practicability as well as their superior thermo-mechanical performance. SMAs can exist in two different phases, with three different crystal structures (i.e. twinned martensite, detwinned martensite, and austenite) and six possible transformations. The thermo-mechanic behavior of

6688-526: The shape, until deformed again. With the one-way effect, cooling from high temperatures does not cause a macroscopic shape change. A deformation is necessary to create the low-temperature shape. On heating, transformation starts at A s and is completed at A f (typically 2 to 20 °C or hotter, depending on the alloy or the loading conditions). A s is determined by the alloy type and composition and can vary between −150 °C and 200 °C . [REDACTED] The two-way shape-memory effect

6776-513: The temperature is raised and austenite becomes thermodynamically favored, all of the atoms rearrange to the B2 structure which happens to be the same macroscopic shape as the B19' pre-deformation shape. This phase transformation happens extremely quickly and gives SMAs their distinctive "snap". Repeated use of the shape-memory effect may lead to a shift of the characteristic transformation temperatures (this effect

6864-620: The temperature of a Cu-Zn alloy. The basic phenomenon of the memory effect governed by the thermoelastic behavior of the martensite phase was widely reported a decade later by Kurdjumov and Khandros (1949) and also by Chang and Read (1951). The nickel-titanium alloys were first developed in 1962–1963 by the United States Naval Ordnance Laboratory and commercialized under the trade name Nitinol (an acronym for Nickel Titanium Naval Ordnance Laboratories). Their remarkable properties were discovered by accident. A sample that

6952-414: The transformation to begin, and if these are great in number, it will increase the driving force for nucleation. A smaller σ ms will be needed than for homogeneous nucleation. Likewise, increasing temperature will reduce the driving force for the phase transformation, so a larger σ ms will be necessary. One can see that as you increase the operational temperature of the SMA, σ ms will be greater than

7040-399: The use of a conductive " lagging ". this method uses a thermal paste to rapidly transfer heat from the SMA by conduction. This heat is then more readily transferred to the environment by convection as the outer radii (and heat transfer area) are significantly greater than for the bare wire. This method results in a significant reduction in deactivation time and a symmetric activation profile. As

7128-402: The vertical axis represents the martensite fraction. The difference between the heating transition and the cooling transition gives rise to hysteresis where some of the mechanical energy is lost in the process. The shape of the curve depends on the material properties of the shape-memory alloy, such as the alloy's composition and work hardening . The shape memory effect (SME) occurs because

7216-434: The way a parallelogram can be made out of a square by pushing on two opposing sides. At different temperatures, different structures are preferred and when the structure is cooled through the transition temperature, the martensitic structure forms from the austenitic phase. Shape-memory alloys are typically made by casting, using vacuum arc melting or induction melting. These are specialist techniques used to keep impurities in

7304-568: The working displacement in an actuator decreases with increasing cycle numbers. The physics behind this is gradual change in microstructure—more specifically, the buildup of accommodation slip dislocations . This is often accompanied by a significant change in transformation temperatures. Design of SMA actuators may also influence both structural and functional fatigue of SMA, such as the pulley configurations in SMA-Pulley system. SMA actuators are typically actuated electrically by Joule heating . If

7392-463: The yield strength, σ y , and superelasticity will no longer be observable. The first reported steps towards the discovery of the shape-memory effect were taken in the 1930s. According to Otsuka and Wayman, Arne Ölander discovered the pseudoelastic behavior of the Au-Cd alloy in 1932. Greninger and Mooradian (1938) observed the formation and disappearance of a martensitic phase by decreasing and increasing

7480-509: Was bent out of shape many times was presented at a laboratory management meeting. One of the associate technical directors, Dr. David S. Muzzey, decided to see what would happen if the sample was subjected to heat and held his pipe lighter underneath it. To everyone's amazement the sample stretched back to its original shape. There is another type of SMA, called a ferromagnetic shape-memory alloy (FSMA), that changes shape under strong magnetic fields. These materials are of particular interest as

7568-413: 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. 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

7656-698: Was emphasised that only a few of patented shape memory alloy applications are commercially successful due to material limitations combined with a lack of material and design knowledge and associated tools, such as improper design approaches and techniques used. The challenges in designing SMA applications are to overcome their limitations, which include a relatively small usable strain, low actuation frequency, low controllability, low accuracy and low energy efficiency. SMA actuators are typically actuated electrically, where an electric current results in Joule heating . Deactivation typically occurs by free convective heat transfer to

7744-435: Was induced from the structural change, and for some SMAs this can be strains greater than 10 percent. This hysteresis loop shows the work done for each cycle of the material between states of small and large deformations, which is important for many applications. In a plot of strain versus temperature, the austenite and martensite start and finish lines run parallel. The SME and pseudoelasticity are actually different parts of

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