Nickel titanium

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Nickel Titanium
Nitinol draht.jpg
Nitinol wires
Material properties
Melting point 1,310 °C (2,390 °F)
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 (10.9×10^6–12.0×10^6 psi)
(martensite)28–40 GPa (4.1×10^6–5.8×10^6 psi)
Yield strength (austenite)195–690 MPa (28.3–100.1 ksi)
(martensite)70–140 MPa (10–20 ksi)
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, 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.

Contents

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 warmer than it.

History

The word "nitinol" is derived from its composition and its place of discovery, Nickel Titanium - Naval Ordnance Laboratory. William J. Buehler [1] along with Frederick E. Wang, [2] discovered its properties during research at the Naval Ordnance Laboratory in 1959. [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. 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. [5]

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 [6] first observed the property in gold–cadmium alloys. The same effect was observed in Cu-Zn (brass) in the early 1950s. [7]

Mechanism

3D view of austenite and martensite structures of the NiTi compound. Nitinol Austenite and martensite.jpg
3D view of austenite and martensite structures of the NiTi compound.

Nitinol's unusual properties are derived from 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 a more complicated monoclinic crystal structure known as martensite (daughter phase). [8] 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. [9]

Thermal hysteresis of nitinol's phase transformation Nitinol transformation hysterisis.svg
Thermal hysteresis of nitinol's phase transformation

The 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 20–50 °C (36–90 °F) but it can be reduced or amplified by alloying [10] and processing. [11]

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 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. [12]

2D view of nitinol's crystalline structure during cooling/heating cycle NiTi structure transformation.jpg
2D view of nitinol's crystalline structure during cooling/heating cycle

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 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. [13] The fact that nitinol is an intermetallic is largely responsible for the complexity in fabricating devices made from the alloy.[ why? ]

The effect of nitinol composition on the Ms temperature. Nitinol Ms vs Ni content.jpg
The effect of nitinol composition on the Ms temperature.

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. [14] 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 40 °C (72 °F) above the Af temperature. This upper limit is referred to as Md, [15] 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). [13] [16] 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 −20 to +110 °C (−4 to 230 °F). 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 (37 °C or 99 °F).

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 non practical use.

Manufacturing

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 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) 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. [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. [18] 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 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. [19]

Challenges

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 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 a great deal of concern in the medical industry regarding the release of nickel, a known allergen and possible carcinogen. [19] (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 TiO2 layer that acts as an effective and self-healing barrier against ion exchange; repeatedly showing that nitinol releases nickel at a slower pace than stainless steel, for example. Early Nitinol medical devices were made without electropolishing, and corrosion was observed.[ citation needed ] 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.[ citation needed ]

There are constant and long-running discussions[ by whom? ] 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, [20] [21] while novel studies demonstrate a dependence of fatigue resistance on the typical inclusion size in an alloy. [17] [18] [22] [23] [24]

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. [25] Laser and tungsten inert gas (TIG) welds have been made between NiTi tubes and stainless steel tubes. [26] [27] 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, [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] ). 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. [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. [37] [38] Research on multi-memory technology is on-going and may deliver enhanced shape memory devices in the near future, [39] [40] and new materials and material structures, such as hybrid shape memory materials (SMMs) and shape memory composites (SMCs). [41]

Applications

Nitinol bueroklammer verbogen.jpg
Nitinol bueroklammer heiss.jpg
A nitinol paperclip bent and recovered after being placed in hot water

There are four commonly used types of applications for nitinol:

Free recovery
Nitinol is deformed at a low temperature, remains deformed, and then is heated to recover its original shape through the shape memory effect.
Constrained recovery
Similar to free recovery, except that recovery is rigidly prevented and thus a stress is generated.
Work production
The alloy is allowed to recover, but to do so it must act against a force (thus doing work).
Superelasticity
Nitinol acts as a super spring through the superelastic effect.

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. [42]

Thermal and electrical actuators

Biocompatible and biomedical applications

Damping systems in structural engineering

Other applications and prototypes

Related Research Articles

<span class="mw-page-title-main">Heat treating</span> Process of heating something to alter it

Heat treating is a group of industrial, thermal and metalworking processes used to alter the physical, and sometimes chemical, properties of a material. The most common application is metallurgical. Heat treatments are also used in the manufacture of many other materials, such as glass. Heat treatment involves the use of heating or chilling, normally to extreme temperatures, to achieve the desired result such as hardening or softening of a material. Heat treatment techniques include annealing, case hardening, precipitation strengthening, tempering, carburizing, normalizing and quenching. Although the term heat treatment applies only to processes where the heating and cooling are done for the specific purpose of altering properties intentionally, heating and cooling often occur incidentally during other manufacturing processes such as hot forming or welding.

<span class="mw-page-title-main">Martensite</span> Type of steel crystalline structure

Martensite is a very hard form of steel crystalline structure. It is named after German metallurgist Adolf Martens. By analogy the term can also refer to any crystal structure that is formed by diffusionless transformation.

<span class="mw-page-title-main">Austenite</span> Metallic, non-magnetic allotrope of iron or a solid solution of iron, with an alloying element

Austenite, also known as gamma-phase iron (γ-Fe), is a metallic, non-magnetic allotrope of iron or a solid solution of iron with an alloying element. In plain-carbon steel, austenite exists above the critical eutectoid temperature of 1000 K (727 °C); other alloys of steel have different eutectoid temperatures. The austenite allotrope is named after Sir William Chandler Roberts-Austen (1843–1902). It exists at room temperature in some stainless steels due to the presence of nickel stabilizing the austenite at lower temperatures.

<span class="mw-page-title-main">Bainite</span> Plate-like microstructure in steels

Bainite is a plate-like microstructure that forms in steels at temperatures of 125–550 °C. First described by E. S. Davenport and Edgar Bain, it is one of the products that may form when austenite is cooled past a temperature where it is no longer thermodynamically stable with respect to ferrite, cementite, or ferrite and cementite. Davenport and Bain originally described the microstructure as being similar in appearance to tempered martensite.

<span class="mw-page-title-main">High-strength low-alloy steel</span> Type of alloy steel

High-strength low-alloy steel (HSLA) is a type of alloy steel that provides better mechanical properties or greater resistance to corrosion than carbon steel. HSLA steels vary from other steels in that they are not made to meet a specific chemical composition but rather specific mechanical properties. They have a carbon content between 0.05 and 0.25% to retain formability and weldability. Other alloying elements include up to 2.0% manganese and small quantities of copper, nickel, niobium, nitrogen, vanadium, chromium, molybdenum, titanium, calcium, rare-earth elements, or zirconium. Copper, titanium, vanadium, and niobium are added for strengthening purposes. These elements are intended to alter the microstructure of carbon steels, which is usually a ferrite-pearlite aggregate, to produce a very fine dispersion of alloy carbides in an almost pure ferrite matrix. This eliminates the toughness-reducing effect of a pearlitic volume fraction yet maintains and increases the material's strength by refining the grain size, which in the case of ferrite increases yield strength by 50% for every halving of the mean grain diameter. Precipitation strengthening plays a minor role, too. Their yield strengths can be anywhere between 250–590 megapascals (36,000–86,000 psi). Because of their higher strength and toughness HSLA steels usually require 25 to 30% more power to form, as compared to carbon steels.

In metallurgy, 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 the shape of a coil spring.

A magnetic shape-memory alloy (MSMA) is a type of smart material that can undergo significant and reversible changes in shape in response to a magnetic field. This behavior arises due to a combination of magnetic and shape-memory properties within the alloy, allowing it to produce mechanical motion or force under magnetic actuation. MSMAs are commonly made from ferromagnetic materials, particularly nickel-manganese-gallium (Ni-Mn-Ga), and are useful in applications requiring rapid, controllable, and repeatable movement.

<span class="mw-page-title-main">Intermetallic</span> Type of metallic alloy

An intermetallic 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.

<span class="mw-page-title-main">Maraging steel</span> Steel known for strength and toughness

Maraging steels are steels that are known for possessing superior strength and toughness without losing ductility. Aging refers to the extended heat-treatment process. These steels are a special class of very-low-carbon ultra-high-strength steels that derive their strength not from carbon, but from precipitation of intermetallic compounds. The principal alloying element is 15 to 25 wt% nickel. Secondary alloying elements, which include cobalt, molybdenum and titanium, are added to produce intermetallic precipitates.

<span class="mw-page-title-main">Titanium alloys</span> Metal alloys made by combining titanium with other elements

Titanium alloys are alloys that contain a mixture of titanium and other chemical elements. Such alloys have very high tensile strength and toughness. They are light in weight, have extraordinary corrosion resistance and the ability to withstand extreme temperatures. However, the high cost of processing limits their use to military applications, aircraft, spacecraft, bicycles, medical devices, jewelry, highly stressed components such as connecting rods on expensive sports cars and some premium sports equipment and consumer electronics.

<span class="mw-page-title-main">Orthodontic archwire</span> Wire used in dental braces

An archwire in orthodontics is a wire conforming to the alveolar or dental arch that can be used with dental braces as a source of force in correcting irregularities in the position of the teeth. An archwire can also be used to maintain existing dental positions; in this case it has a retentive purpose.

<span class="mw-page-title-main">Alloy steel</span> Steel alloyed with a variety of elements

Alloy steel is steel that is alloyed with a variety of elements in amounts between 1.0% and 50% by weight, typically to improve its mechanical properties.

In materials science, pseudoelasticity, sometimes called superelasticity, is an elastic (reversible) response to an applied stress, caused by a phase transformation between the austenitic and martensitic phases of a crystal. It is exhibited in shape-memory alloys.

<span class="mw-page-title-main">Ferroelasticity</span>

Ferroelasticity is a phenomenon in which a material may exhibit a spontaneous strain, and is the mechanical equivalent of ferroelectricity and ferromagnetism in the field of ferroics. A ferroelastic crystal has two or more stable orientational states in the absence of mechanical stress or electric field, i.e. remanent states, and can be reproducibly switched between the states by applying a stress or an electric field greater than some critical value. The application of opposite fields leads to Hysteresis as the system crosses back and forth across an energy barrier. This transition dissipates an energy equal to the area enclosed by the hysteresis loop.

<span class="mw-page-title-main">Austempering</span>

Austempering is heat treatment that is applied to ferrous metals, most notably steel and ductile iron. In steel it produces a bainite microstructure whereas in cast irons it produces a structure of acicular ferrite and high carbon, stabilized austenite known as ausferrite. It is primarily used to improve mechanical properties or reduce / eliminate distortion. Austempering is defined by both the process and the resultant microstructure. Typical austempering process parameters applied to an unsuitable material will not result in the formation of bainite or ausferrite and thus the final product will not be called austempered. Both microstructures may also be produced via other methods. For example, they may be produced as-cast or air cooled with the proper alloy content. These materials are also not referred to as austempered.

Nitinol biocompatibility is an important factor in biomedical applications. Nitinol (NiTi), which is formed by alloying nickel and titanium, is a shape-memory alloy with superelastic properties more similar to that of bone, when compared to stainless steel, another commonly used biomaterial. Biomedical applications that utilize nitinol include stents, heart valve tools, bone anchors, staples, septal defect devices and implants. It is a commonly used biomaterial especially in the development of stent technology.

The R-phase is a phase found in nitinol, a shape-memory alloy. It is a martensitic phase in nature, but is not the martensite that is responsible for the shape memory and superelastic effect.

NiTiNOL 60, or 60 NiTiNOL, is a Nickel Titanium alloy discovered in the late 1950s by the U. S. Naval Ordnance Laboratory. Depending upon the heat treat history, 60 NiTiNOL has the ability to exhibit either superelastic properties in the hardened state or shape memory characteristics in the softened state.

A nickel titanium rotary file is an engine-driven tapered and pointed endodontic instrument made of nickel titanium alloy with cutting edges used to mechanically shape and prepare the root canals during endodontic therapy or to remove the root canal obturating material while performing retreatment. The first nickel titanium rotary file was introduced to the market in 1991. Superelasticity and shape memory are the properties that make nickel titanium rotary files very flexible. The high flexibility makes them superior to stainless steel files for the purpose of rotary root canal preparation. The use of nickel titanium rotary files in dentistry is a common practice.

Elastocaloric materials are a class of advanced materials. These materials show a big change in temperature when mechanical stress is applied and then removed.

References

  1. ↑ Buehler, W. J.; Gilfrich, J. W.; Wiley, R. C. (1963). "Effects of Low-Temperature Phase Changes on the Mechanical Properties of Alloys Near Composition TiNi". Journal of Applied Physics. 34 (5): 1475–1477. Bibcode:1963JAP....34.1475B. doi:10.1063/1.1729603.
  2. ↑ Wang, F. E.; Buehler, W. J.; Pickart, S. J. (1965). "Crystal Structure and a Unique Martensitic Transition of TiNi". Journal of Applied Physics. 36 (10): 3232–3239. Bibcode:1965JAP....36.3232W. doi:10.1063/1.1702955.
  3. ↑ "The Alloy That Remembers", Time, 1968-09-13, archived from the original on November 23, 2008
  4. ↑ Kauffman, G. B.; Mayo, I. (1997). "The Story of Nitinol: The Serendipitous Discovery of the Memory Metal and Its Applications". The Chemical Educator. 2 (2): 1–21. doi:10.1007/s00897970111a. S2CID   98306580.
  5. ↑ Withers, Neil. "Nitinol". Chemistry World. Royal Society of Chemistry. Retrieved 29 January 2018.
  6. ↑ Ölander, A. (1932). "An Electrochemical Investigation of Solid Cadmium-Gold Alloys". Journal of the American Chemical Society. 54 (10): 3819–3833. doi:10.1021/ja01349a004.
  7. ↑ Hornbogen, E.; Wassermann, G. (1956). "Über den Einfluβ von Spannungen und das Auftreten von Umwandlungsplastizität bei β1-β-Umwandlung des Messings". Zeitschrift für Metallkunde. 47: 427–433.
  8. ↑ Otsuka, K.; Ren, X. (2005). "Physical Metallurgy of Ti-Ni-based Shape Memory Alloys". Progress in Materials Science. 50 (5): 511–678. CiteSeerX   10.1.1.455.1300 . doi:10.1016/j.pmatsci.2004.10.001.
  9. ↑ "Nitinol facts". Nitinol.com. 2013. Archived from the original on 2013-08-18. Retrieved 2010-12-04.
  10. ↑ Chluba, Christoph; Ge, Wenwei; Miranda, Rodrigo Lima de; Strobel, Julian; Kienle, Lorenz; Quandt, Eckhard; Wuttig, Manfred (2015-05-29). "Ultralow-fatigue shape memory alloy films". Science. 348 (6238): 1004–1007. Bibcode:2015Sci...348.1004C. doi:10.1126/science.1261164. ISSN   0036-8075. PMID   26023135. S2CID   2563331.
  11. ↑ Spini, Tatiana Sobottka; Valarelli, Fabrício Pinelli; Cançado, Rodrigo Hermont; Freitas, Karina Maria Salvatore de; Villarinho, Denis Jardim; Spini, Tatiana Sobottka; Valarelli, Fabrício Pinelli; Cançado, Rodrigo Hermont; Freitas, Karina Maria Salvatore de (2014-04-01). "Transition temperature range of thermally activated nickel-titanium archwires". Journal of Applied Oral Science. 22 (2): 109–117. doi:10.1590/1678-775720130133. ISSN   1678-7757. PMC   3956402 . PMID   24676581.
  12. ↑ Funakubo, Hiroyasu (1984), Shape memory alloys, University of Tokyo, pp. 7, 176.
  13. 1 2 "Nitinol SM495 Wire" (PDF). 2013. Archived from the original (properties, PDF) on 2011-07-14.
  14. ↑ "Fabrication & Heat Treatment of Nitinol". memry.com. 2011-01-26. Retrieved 2017-03-28.
  15. ↑ R Meling, Torstein; Ødegaard, Jan (August 1998). "The effect of temperature on the elastic responses to longitudinal torsion of rectangular nickel titanium archwires". The Angle Orthodontist. 68 (4): 357–368. PMID   9709837.
  16. ↑ "Nitinol SE508 Wire" (PDF). 2013. Archived from the original (properties, PDF) on 2011-07-14.
  17. 1 2 Urbano, Marco; Coda, Alberto; Beretta, Stefano; Cadelli, Andrea; Sczerzenie, Frank (2013-09-01). The Effect of Inclusions on Fatigue Properties for Nitinol. pp. 18–34. doi:10.1520/STP155920120189. ISBN   978-0-8031-7545-7.{{cite book}}: |journal= ignored (help)
  18. 1 2 Robertson, Scott W.; Launey, Maximilien; Shelley, Oren; Ong, Ich; Vien, Lot; Senthilnathan, Karthike; Saffari, Payman; Schlegel, Scott; Pelton, Alan R. (2015-11-01). "A statistical approach to understand the role of inclusions on the fatigue resistance of superelastic Nitinol wire and tubing". Journal of the Mechanical Behavior of Biomedical Materials. 51: 119–131. doi:10.1016/j.jmbbm.2015.07.003. ISSN   1878-0180. PMID   26241890.
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  20. ↑ Morgan, N.; Wick, A.; DiCello, J.; Graham, R. (2006). "Carbon and Oxygen Levels in Nitinol Alloys and the Implications for Medical Device Manufacture and Durability" (PDF). SMST-2006 Proceedings of the International Conference on Shape Memory and Superelastic Technologies. ASM International. pp. 821–828. doi:10.1361/cp2006smst821 (inactive 1 November 2024). ISBN   978-0-87170-862-5. LCCN   2009499204. Archived from the original (PDF) on 14 July 2011. Retrieved 26 August 2010.{{cite book}}: CS1 maint: DOI inactive as of November 2024 (link)
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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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