View source for Shape-memory alloy

{{Short description|Alloy which returns to a preset shape when heated}}

{{Lead too short|date=May 2023}}
In [[metallurgy]], a '''shape-memory alloy''' ('''SMA''') is an [[alloy]] that can be [[cold forming|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'''.{{citation needed|date=April 2019}} 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.

Parts made of shape-memory alloys can be lightweight, solid-state alternatives to conventional [[actuator]]s such as [[hydraulic]], [[pneumatic]], and motor-based systems. They can also be used to make [[Hermetic seal|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.

== Overview ==
The two most prevalent shape-memory alloys are [[copper]]-[[aluminium]]-[[nickel]] and [[nickel]]-[[titanium]] ([[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 [[Iron|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<ref>{{cite journal |last1=Wilkes |first1=Kenneth E. |last2=Liaw |first2=Peter K. |last3=Wilkes |first3=Kenneth E. |title=The fatigue behavior of shape-memory alloys |journal=JOM |date=October 2000 |volume=52 |issue=10 |pages=45–51 |doi=10.1007/s11837-000-0083-3 |bibcode=2000JOM....52j..45W |s2cid=137826371 }}</ref><ref>{{cite journal |last1=Cederström |first1=J. |last2=Van Humbeeck |first2=J. |title=Relationship Between Shape Memory Material Properties and Applications |journal=Le Journal de Physique IV |date=February 1995 |volume=05 |issue=C2 |pages=C2-335–C2-341 |doi=10.1051/jp4:1995251 |doi-access=free }}</ref><ref>{{cite book |doi=10.31399/asm.hb.v02.a0001100 |chapter=Shape Memory Alloys |title=Properties and Selection: Nonferrous Alloys and Special-Purpose Materials |pages=897–902 |year=1990 |isbn=978-1-62708-162-7 |last1=Hodgson |first1=Darel E. |last2=Wu |first2=Ming H. |last3=Biermann |first3=Robert J. }}</ref> as well as their superior thermo-mechanical performance.<ref>{{cite journal |last1=Huang |first1=W. |title=On the selection of shape memory alloys for actuators |journal=Materials & Design |date=February 2002 |volume=23 |issue=1 |pages=11–19 |doi=10.1016/S0261-3069(01)00039-5 }}</ref> SMAs can exist in two different phases, with three different crystal structures (i.e. twinned martensite, detwinned martensite, and austenite) and six possible transformations.<ref>{{cite journal |last1=Sun |first1=L. |last2=Huang |first2=W. M. |title=Nature of the multistage transformation in shape memory alloys upon heating |journal=Metal Science and Heat Treatment |date=21 May 2010 |volume=51 |issue=11–12 |pages=573–578 |doi=10.1007/s11041-010-9213-x |bibcode=2009MSHT...51..573S |s2cid=135892973 }}</ref><ref>{{cite journal |last1=Mihálcz |first1=István |title=Fundamental characteristics and design method for nickel-titanium shape memory alloy |journal=Periodica Polytechnica Mechanical Engineering |date=2001 |volume=45 |issue=1 |pages=75–86 |url=https://pp.bme.hu/me/article/view/1410 }}</ref> The thermo-mechanic behavior of the SMAs is governed by a phase transformation between the austenite and the martensite.

<!-- Austenite phase is low temperature phase, so by cooling we go from martensitic phase to austenite phase and vice-versa. -->NiTi alloys change from [[austenite]] to [[martensite]] upon cooling starting from a temperature below ''M<sub>s</sub>''; ''M<sub>f</sub>'' is the temperature at which the transition to martensite completes upon cooling. Accordingly, during heating ''A<sub>s</sub>'' and ''A<sub>f</sub>'' are the temperatures at which the transformation from martensite to austenite starts and finishes. 

Applying 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<sub>s</sub>'' and ends at ''σ<sub>f</sub>'' above which martensite continue exhibiting only elastic behavior (as long as the load is below the yield stress). The memorized deformation from detwinning 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 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.

[[File:Martensite to Austenite transition graph.svg]]

In this figure 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<ref>{{cite journal|last1=Wu|first1=S|last2=Wayman|first2=C|title=Martensitic transformations and the shape-memory effect in Ti50Ni10Au40 and Ti50Au50 alloys|journal=Metallography|volume=20|page=359|year=1987|doi=10.1016/0026-0800(87)90045-0|issue=3}}</ref> and [[work hardening]].<ref>{{cite journal |last1=Filip |first1=Peter |last2=Mazanec |first2=Karel |title=Influence of work hardening and heat treatment on the substructure and deformation behaviour of TiNi shape memory alloys |journal=Scripta Metallurgica et Materialia |date=May 1995 |volume=32 |issue=9 |pages=1375–1380 |doi=10.1016/0956-716X(95)00174-T }}</ref>

==Shape memory effect==
[[File:Shape_Memory_Effect_Animation.ogg|right|thumb|upright=1.5|This animation illustrates the full shape memory effect:{{ordered list
|Cooling from austenite to (twinned) martensite, which happens either at beginning of the SMA’s lifetime or at the end of a thermal cycle.
|Applying a stress to detwin the martensite.
|Heating the martensite to reform austenite, restoring the original shape.
|Cooling the austenite back to twinned martensite.}}]]
The shape memory effect (SME)<ref>{{Cite journal|last1=QADER|first1=Ibrahim Nazem|last2=KOK|first2=Mediha |last3=Dağdelen|first3=Fethi|last4=AYDOĞDU|first4=Yıldırım|date=2019-09-30|title="A review of smart materials: researches and applications"|journal=El-Cezeri Fen ve Mühendislik Dergisi|doi=10.31202/ecjse.562177|issn=2148-3736|doi-access=free}}</ref> occurs because a temperature-induced phase transformation reverses deformation, as shown in the previous hysteresis curve. Typically the martensitic phase is monoclinic or orthorhombic (B19' or [https://www.atomic-scale-physics.de/lattice/struk/b19.html B19]). Since these crystal structures do not have enough slip systems for easy dislocation motion, they deform by [[crystal twinning|twinning]]—or rather, detwinning.<ref>{{Cite book |title=Mechanical behavior of materials |last=Courtney |first=Thomas H. |date=2000 |publisher=McGraw Hill |isbn=0070285942 |edition=2nd |location=Boston |oclc=41932585}}</ref>

Martensite is thermodynamically favored at lower temperatures, while austenite ([https://www.atomic-scale-physics.de/lattice/struk/b2.html B2] cubic) 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 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.<ref>{{Cite journal |last1=Otsuka |first1=K. |last2=Ren |first2=X. |date=July 2005 |title=Physical metallurgy of Ti–Ni-based shape memory alloys|journal=Progress in Materials Science |volume=50 |issue=5 |pages=511–678 |doi=10.1016/j.pmatsci.2004.10.001 |issn=0079-6425}}</ref>

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 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.<ref>{{Cite web |url=http://smart.tamu.edu/overview/smaintro/simple/definition.html |title=Definition of a Shape Memory Alloy |website=smart.tamu.edu |access-date=2019-05-24 |archive-date=2018-10-01 |archive-url=https://web.archive.org/web/20181001232324/http://smart.tamu.edu/overview/smaintro/simple/definition.html |url-status=dead }}</ref> 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 is known as functional fatigue, as it is closely related with a change of microstructural and functional properties of the material).<ref name="ot">{{cite book |url=http://catdir.loc.gov/catdir/samples/cam034/97036119.pdf |title=Shape Memory Materials |date=1999 |publisher=Cambridge University Press |isbn=0-521-66384-9 |editor=K. Otsuka |editor2=C.M. Wayman}}{{page needed|date=December 2019}}</ref> The maximum temperature at which SMAs can no longer be stress induced is called ''M<sub>d</sub>'', where the SMAs are permanently deformed.<ref>{{cite book |author=Duerig, T.W. |title=Materials Properties Handbook: Titanium Alloys |author2=Pelton, A.R. |date=1994 |publisher=American Society for Metals |isbn=0-87170-481-1 |editor=Gerhard Welsch |pages=1035–48 |chapter=Ti-Ni shape memory alloys |editor2=Rodney Boyer |editor3=E.W. Collings}}</ref>

== One-way vs. two-way shape memory ==
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 is shown below.
The procedures are very similar: starting from martensite, adding a deformation, heating the sample and cooling it again.

=== One-way memory effect ===
[[File:One way shape memory effect.svg]]

When a shape-memory alloy is in its cold state (below ''M<sub>f</sub>''), 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 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<sub>s</sub>'' and is completed at ''A<sub>f</sub>'' (typically 2 to 20&nbsp;°C or hotter, depending on the alloy or the loading conditions). ''A<sub>s</sub>'' is determined by the alloy type and composition and can vary between {{val|-150|u=°C}} and {{val|200|u=°C}}.

===Two way effect ===
[[File:Two way shape memory effect.svg]]

The two-way shape-memory effect 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 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 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.<ref>{{Cite book |url=https://www.worldcat.org/oclc/272298744 |title=Shape memory alloys : modeling and engineering applications |date=2008 |publisher=Springer |others=Dimitris C. Lagoudas |isbn=978-0-387-47685-8 |location=New York |oclc=272298744}}</ref> 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 the material shape change. And while heating back the SMA into austenite, it recovers its initial shape.

There are several ways of doing this.<ref>[http://www-personal.umich.edu/~btrease/share/SMA-Shape-Training-Tutorial.pdf Shape Memory Alloy Shape Training Tutorial]. (PDF) . Retrieved on 2011-12-04.</ref> A shaped, trained object heated beyond a certain point will lose the two-way memory effect.

== Pseudoelasticity ==
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.
 
[[File:Pseudoelasticity_Animation.ogg|thumb|left|An animation of pseudoelasticity]]
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.<ref>{{Cite journal |last1=Kazuhiro Otsuka |last2=Ren |first2=Xiaobing |date=1997 |title=Origin of rubber-like behaviour in metal alloys |journal=Nature |language=en |volume=389 |issue=6651 |pages=579–582 |doi=10.1038/39277 |issn=1476-4687|bibcode=1997Natur.389..579R |s2cid=4395776 }}</ref>

[[File:Superelastic behavior of the austenitic to martensitic phase transformation.png|thumb|upright=1.5|At stresses above the martensitic stress (A), austenite will transform to martensite and induce large macroscopic strains until no austenite remains (C). Upon unloading, martensite will revert to austenite phase beneath the austenitic stress (D), at which point strain will be recovered until the material is fully austenitic and little to no deformation remains.<ref>{{Cite journal|last1=Qian|first1=Hui|last2=Li|first2=Hongnan|last3=Song|first3=Gangbing|last4=Guo|first4=Wei|date=2013|title=Recentering Shape Memory Alloy Passive Damper for Structural Vibration Control|journal=Mathematical Problems in Engineering|volume=2013|pages=1–13|doi=10.1155/2013/963530|issn=1024-123X|doi-access=free}}</ref>]]
The main pseudoelastic effect comes from 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<sub>f</sub>, but below the martensite deformation temperature, M<sub>d</sub>. 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 a particular point on A<sub>f,</sub> it is possible to choose a point on the M<sub>s</sub> &nbsp;line with a ''higher'' temperature, as long as that point M<sub>d</sub> 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 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.<ref name="Shaw">{{Cite journal|last1=Shaw|first1=J.|last2=Kyriakides|first2=S.|year=1995|title=Thermomechanical aspects of NiTi |journal=Journal of the Mechanics and Physics of Solids|volume=43|issue=8|pages=1243–1281|bibcode=1995JMPSo..43.1243S|doi=10.1016/0022-5096(95)00024-D}}</ref><ref>{{Cite journal |last1=Chowdhury |first1=Piyas |last2=Sehitoglu |first2=Huseyin |date=2017|title=A revisit to atomistic rationale for slip in shape memory alloys|journal=Progress in Materials Science |volume=85 |pages=1–42 |doi=10.1016/j.pmatsci.2016.10.002|issn=0079-6425|doi-access=free }}</ref> If the material is unloaded before plastic deformation occurs, it will revert to austenite once a critical stress for austenite is reached (σ<sub>as</sub>). The material will recover nearly all strain that was induced from the structural change, and for some SMAs this can be strains greater than 10 percent.<ref name="Ma">{{Cite journal |last1=Ma |first1=J. |last2=Karaman |first2=I. |last3=Noebe |first3=R. D.|year=2010|title=High temperature shape memory alloys|journal=International Materials Reviews |volume=55|issue=5|pages=257|doi=10.1179/095066010x12646898728363|bibcode=2010IMRv...55..257M |s2cid=218662109 }}</ref><ref>{{Cite journal |last1=Tanaka |first1=Y. |last2=Himuro|first2=Y.|last3=Kainuma|first3=R.|last4=Sutou|first4=Y.|last5=Omori|first5=T.|last6=Ishida|first6=K.|date=2010-03-18|title=Ferrous Polycrystalline Shape-Memory Alloy Showing Huge Superelasticity |journal=Science|volume=327|issue=5972|pages=1488–1490|doi=10.1126/science.1183169|pmid=20299589 |issn=0036-8075|bibcode=2010Sci...327.1488T |s2cid=9536512 }}</ref> 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.

[[File:Shape memory effect vs pseudoelasticity.png|alt=|left|thumb|Stress-Temperature graph of martensite and austenite lines in a shape memory alloy.]]
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 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 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.<ref>{{Cite journal|last1=Frankel|first1=Dana J.|last2=Olson|first2=Gregory B.|date=2015|title=Design of Heusler Precipitation Strengthened NiTi- and PdTi-Base SMAs for Cyclic Performance|journal=Shape Memory and Superelasticity|volume=1|issue=2|pages=162–179|doi=10.1007/s40830-015-0017-0|issn=2199-384X|bibcode=2015ShMeS...1...17F|doi-access=free}}</ref> One mechanism that aids in this process is the twinning of the martensite phase. In crystallography, a twin boundary 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<sub>ms</sub> is dependent on parameters such as temperature and the number of nucleation sites for phase nucleation. Interfaces and inclusions will provide general sites for the transformation to begin, and if these are great in number, it will increase the driving force for nucleation.<ref>{{Cite journal|last1=San Juan|first1=J.|last2=Nó|first2=M.L.|date=2013|title=Superelasticity and shape memory at nano-scale: Size effects on the martensitic transformation|journal=Journal of Alloys and Compounds|language=en|volume=577|pages=S25–S29|doi=10.1016/j.jallcom.2011.10.110}}</ref> A smaller σ<sub>ms</sub> will be needed than for homogeneous nucleation. Likewise, increasing temperature will reduce the driving force for the phase transformation, so a larger σ<sub>ms</sub> will be necessary. One can see that as you increase the operational temperature of the SMA, σ<sub>ms</sub> will be greater than the yield strength, σ<sub>y</sub>, and superelasticity will no longer be observable.

== History ==
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 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).<ref name=ot />

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 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.<ref>{{cite journal|author1=Kauffman, George |author2=Isaac Mayo |name-list-style=amp |title=Memory Metal|journal=ChemMatters |date=October 1993|pages= 4–7|url=http://wychem.scienceontheweb.net/ChemCD/ChemMatters/931004t.pdf}}</ref><ref>[http://www.wolaa.org/files/Nitinol_Oral_History.pdf Oral history by William J. Buehler] {{Webarchive|url=https://web.archive.org/web/20160303194903/http://www.wolaa.org/files/Nitinol_Oral_History.pdf |date=2016-03-03 }}. wolaa.org.</ref>

There is another type of SMA, called a [[magnetic shape memory|ferromagnetic shape-memory alloy]] (FSMA), that changes shape under strong magnetic fields. These materials are of particular interest as 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 polymer]]s have also been developed, and became commercially available in the late 1990s.

== Crystal structures ==
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 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 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.

== Manufacture ==
Shape-memory alloys are typically made by casting, using vacuum arc melting or induction melting. These are specialist techniques used to keep impurities in the alloy to a minimum and ensure the metals are well mixed. The [[ingot]] is then [[hot rolled]] into longer sections and then [[Wire drawing|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 [[dislocation]]s re-order into stable positions, but not so hot that the material [[Recrystallization (metallurgy)|recrystallizes]]. They are heated to between {{val|400|u=°C}} and {{val|500|u=°C}} for 30 minutes, shaped while hot, and then are cooled rapidly by quenching in water or by cooling with air.

== Properties ==
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 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 {{val|500|ul=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 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 {{val|8|s=%}} for some alloys. This compares with a maximum strain {{val|0.5|s=%}} for conventional steels.

== Practical limitations ==

SMA have many advantages over traditional actuators, but do suffer from a series of limitations that may impede practical application. In numerous studies, it 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.<ref>{{Cite journal | doi =  10.1177/1045389X16679296| title = Designing shape memory alloy linear actuators: A review| journal = Journal of Intelligent Material Systems and Structures| volume = 28| issue = 13| page = 1699| year = 2016| last1 = M. Jani | first1 = J.| last2 = Leary | first2 = M. | last3 = Subic | first3 = A.| s2cid = 138509568}}</ref> 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.<ref>{{Cite journal | doi = 10.1016/j.matdes.2013.11.084| title = A review of shape memory alloy research, applications and opportunities| journal = Materials and Design | volume = 56| issue = 5| pages = 1078–1113| year = 2014| last1 = M. Jani | first1 = J.| last2 = Leary | first2 = M. | last3 = Subic | first3 = A.| last4 = Gibson | first4 = Mark A.}}</ref>

=== Response time and response symmetry ===
SMA actuators are typically actuated electrically, where an electric current results in [[Joule heating]]. Deactivation typically occurs by free convective heat transfer to 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,<ref>{{cite journal|last=Lara-Quintanilla|first=A.|author2=Hulskamp, A. W. |author3=Bersee, H. E. |title=A high-rate shape memory alloy actuator for aerodynamic load control on wind turbines|journal=Journal of Intelligent Material Systems and Structures|date=October 2013|volume=24|issue=15|pages=1834–1845|doi=10.1177/1045389X13478271|s2cid=110098888|url=http://resolver.tudelft.nl/uuid:a7be07c6-81b2-4486-a993-72a2da24644f}}</ref> 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 the use of a conductive "[[Thermal insulation|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 a consequence of the increased [[heat transfer]] rate, the required current to achieve a given actuation force is increased.<ref>{{cite journal |last1=Huang|first1=S|last2=Leary|first2=Martin|last3=Attalla|first3=Tamer|last4=Probst|first4=K| last5=Subic |first5=A |title=Optimisation of Ni–Ti shape memory alloy response time by transient heat transfer analysis |journal=Materials & Design |volume=35|pages=655–663|year=2012|doi=10.1016/j.matdes.2011.09.043}}</ref>

[[File:Comparative force-time response of bare and lagged Ni-Ti shape memory alloy.png|thumb|center|upright=3|Comparative force-time response of bare and lagged Ni-Ti shape memory alloy.<ref>{{cite journal|last1=Leary|first1=M|last2=Schiavone|first2=F|last3=Subic|first3=A|title=Lagging for control of shape memory alloy actuator response time |journal=Materials & Design |volume=31 |pages=2124–2128 |year=2010 |doi=10.1016/j.matdes.2009.10.010 |issue=4}}</ref>]]

=== Structural fatigue and functional fatigue ===
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 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, 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.<ref name=Miyazaki>{{Cite journal | doi = 10.1016/j.msea.2006.02.054| title = Development and characterization of Ni-free Ti-base shape memory and superelastic alloys| journal = Materials Science and Engineering: A| volume = 438–440| pages = 18–24| year = 2006| last1 = Miyazaki | first1 = S.| last2 = Kim | first2 = H. Y. | last3 = Hosoda | first3 = H.}}</ref> Design of SMA actuators may also influence both structural and functional fatigue of SMA, such as the pulley configurations in SMA-Pulley system.<ref name=JMJ>{{Cite journal | doi = 10.1088/0964-1726/25/5/057001| title = Fatigue of NiTi SMA-pulley system using Taguchi and ANOVA| journal = Smart Materials and Structures| volume = 25| issue = 5| pages = 057001| year = 2016| last1 = M. Jani | first1 = J.| last2 = Leary | first2 = M. | last3 = Subic | first3 = A.| bibcode = 2016SMaS...25e7001M| s2cid = 138542543}}</ref>

=== Unintended actuation ===
SMA actuators are typically actuated electrically by [[Joule heating]]. If the SMA is used in an environment where the ambient temperature is uncontrolled, unintentional actuation by ambient heating may occur.

== Applications ==

=== Industrial ===

==== Aircraft and spacecraft ====
[[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 the future. In 2005 and 2006, Boeing conducted successful flight testing of this technology.<ref name=Mabe>{{Cite book | doi = 10.1117/12.776816| chapter = Variable area jet nozzle using shape memory alloy actuators in an antagonistic design| title = Industrial and Commercial Applications of Smart Structures Technologies 2008| volume = 6930| pages = 69300T| year = 2008| last1 = Mabe | first1 = J. H. | last2 = Calkins | first2 = F. T. | last3 = Alkislar | first3 = M. B. | s2cid = 111594060| editor3-first = M. Brett| editor3-last = McMickell| editor2-first = Benjamin K| editor2-last = Henderson| editor1-first = L. Porter| editor1-last = Davis}}</ref>

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.<ref name=Hartl>{{Cite journal | doi = 10.1243/09544100jaero211| title = Aerospace applications of shape memory alloys| journal = Proceedings of the Institution of Mechanical Engineers, Part G: Journal of Aerospace Engineering| volume = 221| issue = 4| pages = 535| year = 2007| last1 = Lagoudas | first1 = D. C. | last2 = Hartl | first2 = D. J. | doi-access = free}}</ref> SMAs also exhibit potential for other high shock applications such as ball bearings and landing gear.<ref name=DellaCorte>DellaCorte, C. (2014) [https://ntrs.nasa.gov/search.jsp?R=20140010477 Novel Super-Elastic Materials for Advanced Bearing Applications].</ref>

There 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.<ref name=Webster>{{Cite book | doi = 10.1117/12.669027| chapter = High integrity adaptive SMA components for gas turbine applications| title = Smart Structures and Materials 2006: Industrial and Commercial Applications of Smart Structures Technologies| volume = 6171| pages = 61710F| year = 2006| last1 = Webster | first1 = J. | s2cid = 108583552| editor1-first = Edward V| editor1-last = White}}</ref> 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) is presented by Ma et al.<ref name=Ma />

A variety of wing-morphing technologies are also being explored.<ref name=Hartl />

==== Automotive ====
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 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.

==== Robotics ====
There have also been limited studies on using these materials in [[robotics]], for example the hobbyist robot [[Stiquito]] (and "Roboterfrau Lara"<ref>[http://www.lararobot.de/ The Lara Project – G1 and G2]. Lararobot.de. Retrieved on 2011-12-04.</ref>), 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, [[#Response time and response symmetry|slow response times]], and large [[hysteresis]].

==== Valves ====
SMAs are also used for actuating [[valves]].<ref>{{cite web |url=https://www.gesundheitsindustrie-bw.de/en/article/news/ultra-compact-valves-shape-memory-actuators |title = Ultra-compact: valves with shape memory actuators - Healthcare industry| date=24 March 2021 }}</ref> The SMA valves are particularly compact in design.

==== Bio-engineered robotic hand ====
There is some SMA-based prototypes of robotic hand that using shape memory effect (SME) to move fingers.<ref>{{Citation|last1=Duerig|first1=T.W.|title=Wide Hysteresis Shape Memory Alloys|date=1990|work=Engineering Aspects of Shape Memory Alloys|pages=130–136|publisher=Elsevier|isbn=9780750610094|last2=Melton|first2=K.N.|last3=Proft|first3=J.L.|doi=10.1016/b978-0-7506-1009-4.50015-9}}</ref>

==== Civil structures ====
SMAs find a variety of applications in civil structures such as bridges and buildings. In the form of rebars or plates, they can be used for flexural, shear and seismic strengthening of concrete and steel structures. Another application is Intelligent Reinforced Concrete (IRC), which incorporates SMA wires embedded within the concrete. These wires can sense cracks and contract to heal micro-sized cracks. Also the active tuning of structural natural frequency using SMA wires to dampen vibrations is possible, as well as the usage of SMA fibers in concrete.<ref name=Song>{{Cite journal | doi = 10.1016/j.engstruct.2005.12.010| title = Applications of shape memory alloys in civil structures| journal = Engineering Structures| volume = 28| issue = 9| pages = 1266| year = 2006| last1 = Song | first1 = G.| last2 = Ma | first2 = N.| last3 = Li | first3 = H. -N. | bibcode = 2006EngSt..28.1266S}}</ref>

==== Piping ====
The first consumer commercial application  was  a [[shape-memory coupling]] for piping, e.g. oil pipe lines, for industrial applications, water pipes and similar types of piping for consumer/commercial applications.

=== Consumer electronics ===
==== Smartphone cameras ====
Several smartphone companies have released handsets with [[Image stabilization|optical image stabilisation]] (OIS) modules incorporating SMA actuators, manufactured under licence from Cambridge Mechatronics.

=== Medicine ===
Shape-memory alloys are applied in medicine, for example, as fixation devices for [[osteotomy|osteotomies]] in [[orthopaedic surgery]], as the [[actuator]] in surgical tools; active steerable surgical needles for minimally invasive [[percutaneous]] cancer interventions in the surgical procedures such as [[biopsy]] and [[brachytherapy]],<ref>{{cite book |doi=10.1115/DMD2019-3307 |chapter=3D Steerable Active Surgical Needle |title=2019 Design of Medical Devices Conference |year=2019 |last1=Karimi |first1=Saeed |last2=Konh |first2=Bardia |isbn=978-0-7918-4103-7 |s2cid=200136206 }}</ref>  in [[dental braces]] to exert constant tooth-moving forces on the teeth, in [[Capsule Endoscopy]] they can be used as a trigger for biopsy action.

The late 1980s saw the commercial introduction of [[Nitinol]] as an enabling technology in a number of minimally invasive endovascular medical applications. While more costly than stainless steel, the self expanding properties of Nitinol alloys manufactured to BTR (Body Temperature Response), have provided an attractive alternative to balloon expandable devices in [[stent graft]]s where it gives the ability to adapt to the shape of certain blood vessels when exposed to body temperature. On average, {{val|50|s=%}} of all peripheral vascular [[stent]]s currently available on the worldwide market are manufactured with Nitinol.

==== Optometry ====
[[Glasses|Eyeglass frames]] made from titanium-containing SMAs are marketed under the trademarks [[Flexon]] and TITANflex. These frames are usually made out of shape-memory alloys that have their transition temperature set below the expected room temperature. This allows the frames to undergo large deformation under stress, yet regain their intended shape once the metal is unloaded again. The very large apparently elastic strains are due to the stress-induced martensitic effect, where the crystal structure can transform under loading, allowing the shape to change temporarily under load. This means that eyeglasses made of shape-memory alloys are more robust against being accidentally damaged.

==== Orthopedic surgery ====

Memory metal has been utilized in [[orthopedic surgery]] as a fixation-compression device for [[osteotomy|osteotomies]], typically for lower extremity procedures. The device, usually in the form of a large staple, is stored in a refrigerator in its malleable form and is implanted into pre-drilled holes in the bone across an osteotomy. As the staple warms it returns to its non-malleable state and compresses the bony surfaces together to promote bone union.<ref>{{cite journal |last1=Mereau |first1=Trinity M. |last2=Ford |first2=Timothy C. |title=Nitinol Compression Staples for Bone Fixation in Foot Surgery |journal=Journal of the American Podiatric Medical Association |date=March 2006 |volume=96 |issue=2 |pages=102–106 |doi=10.7547/0960102 |pmid=16546946 |s2cid=29604863 }}</ref>

==== Dentistry ====
The range of applications for SMAs has grown over the years, a major area of development being dentistry. One example is the prevalence of [[dental braces]] using SMA technology to exert constant tooth-moving forces on the teeth; the nitinol [[archwire]] was developed in 1972 by [[Orthodontics|orthodontist]] [[George Andreasen]].<ref>[https://www.nytimes.com/1989/08/15/obituaries/george-andreasen-55-orthodontics-inventor.html Obituary of Dr. Andreasen]. New York Times (1989-08-15). Retrieved in 2016.</ref> This revolutionized clinical orthodontics. Andreasen's alloy has a patterned shape memory, expanding and contracting within given temperature ranges because of its geometric programming.

[[Harmeet D. Walia]] later utilized the alloy in the manufacture of root canal files for [[endodontics]].

==== Essential tremor ====
Traditional active cancellation techniques for tremor reduction use electrical, hydraulic, or pneumatic systems to actuate an object in the direction opposite to the disturbance. However, these systems are limited due to the large infrastructure required to produce large amplitudes of power at human tremor frequencies. SMAs have proven to be an effective method of actuation in hand-held applications, and have enabled a new class active tremor cancellation devices.<ref>{{cite thesis |last1=Pathak |first1=Anupam |year=2010 |title=The Development of an Antagonistic SMA Actuation Technology for the Active Cancellation of Human Tremor |hdl=2027.42/76010 }}</ref> One recent example of such device is the [[Liftware]] spoon, developed by [[Verily Life Sciences]] subsidiary [[Lift Labs]].

=== Engines ===
Experimental solid state heat engines, operating from the relatively small temperature differences in cold and hot water reservoirs, have been developed since the 1970s, including the Banks Engine, developed by [[Ridgway Banks]].

=== Crafts ===
Sold in small round lengths for use in affixment-free bracelets.

=== Heating and cooling ===

German scientists at [[Saarland University]] have produced a prototype machine that transfers heat using a nickel-titanium ("nitinol") alloy wire wrapped around a rotating cylinder.  As the cylinder rotates, heat is absorbed on one side and released on the other, as the wire changes from its "superelastic" state to its unloaded state.  According to a 2019 article released by Saarland University, the efficiency by which the heat is transferred appears to be higher than that of a typical heat pump or air conditioner.<ref>{{cite web |author=Saarland University |date=March 13, 2019 |title=Research team uses artificial muscles to develop an air conditioner for the future |url=https://phys.org/news/2019-03-team-artificial-muscles-air-conditioner.html |website=phys.org }}</ref>

Almost all air conditioners and [[heat pumps]] in use today employ vapor-compression of [[refrigerants]].  Over time, some of the refrigerants used in these systems leak into the atmosphere and contribute to [[global warming]].  If the new technology, which uses no refrigerants, proves economical and practical, it might offer a significant breakthrough in the effort to reduce climate change.{{citation needed|date=December 2019}}

=== Clamping Systems ===
Shape memory alloys (SMAs), such as [[Nickel titanium|nickel-titanium (Nitinol)]], are used in clamping systems due to their unique thermo-responsive behavior.<ref>{{cite web |url=https://www.refractorymetal.org/nitinol-spring/ |title=NT0404 Nitinol Spring (Nickel Titanium) |website=Advanced Refractory Metals |access-date=Sep 7, 2024}}</ref> The clamps made from SMA are used in the dentofacial surgery to heal [[mandibular fractures]].<ref>{{cite journal |last1=Drugacz |first1=J |last2=Lekston |first2=Z |year=1995 |title=Use of TiNiCo shape-memory clamps in the surgical treatment of [[mandibular fractures]] |journal=J Oral Maxillofac Surg |volume=53 |issue=6 |pages=665–71 |doi=10.1016/0278-2391(95)90166-3 |pmid=7776049}}</ref>

== Materials ==
A variety of alloys exhibit the shape-memory effect. Alloying constituents can be adjusted to control the transformation temperatures of the SMA. Some common systems include the following (by no means an exhaustive list):
<!-- Please sort. Consider using full names of elements and adding links to pages about them -->
{{div col |colwidth=18em}}
* Ag-Cd 44/49 at.% Cd<!-- at.%??? please explain briefly and link to page explaining it if possible -->
* Au-Cd 46.5/50 at.% Cd
* Co-Ni-Al<ref name=CoNiAl>{{Cite journal | doi = 10.1016/j.msea.2010.12.056|url=http://nickel-titanium.com/wp-content/uploads/Savas-Dilibal-02-Co-Ni-Al.pdf| title = On the volume change in Co–Ni–Al during pseudoelasticity| journal = Materials Science and Engineering: A| volume = 528| issue = 6| pages = 2875| year = 2011| last1 = Dilibal | first1 = S.| last2 = Sehitoglu | first2 = H.| last3 = Hamilton | first3 = R. F. | last4 = Maier | first4 = H. J. | last5 = Chumlyakov | first5 = Y.}}</ref>
* Co-Ni-Ga
* Cu-Al-Be-X(X:Zr, B, Cr, Gd)
* Cu-Al-Ni 14/14.5 wt.% Al, 3/4.5 wt.% Ni
* Cu-Al-Ni-Hf
* Cu-Sn approx. 15 at.% Sn
* Cu-Zn 38.5/41.5 wt.% Zn
* Cu-Zn-X (X = Si, Al, Sn)
* Fe-Mn-Si
* Fe-Pt approx. 25 at.% Pt
* Mn-Cu 5/35 at.% Cu
* Ni-Fe-Ga
* [[Nickel titanium|Ni-Ti approx. 55–60 wt.% Ni]]
* Ni-Ti-Hf
* Ni-Ti-Pd
* Ni-Mn-Ga<ref name=NiMnGa>{{Cite journal | doi = 10.1016/j.msea.2010.10.042| title = Underlying mechanism of dual hysteresis in NiMnGa single crystals| journal = Materials Science and Engineering: A| volume = 528| issue = 3| pages = 1877| year = 2011| last1 = Hamilton | first1 = R. F. | last2 = Dilibal | first2 = S.| last3 = Sehitoglu | first3 = H.| last4 = Maier | first4 = H. J. }}</ref>
* Ni-Mn-Ga-Cu
* Ni-Mn-Ga-Co
* Ti-Nb
{{div col end}}

==References==
{{Reflist}}

== External links ==
{{Commons category-inline|Shape-memory materials}}

''[https://www.youtube.com/watch?v=vSNtifE0Z2Q Veritasium] - How NASA Reinvented The Wheel''{{Authority control}}

[[Category:Alloys]]
[[Category:Smart materials]]
[[Category:Metallurgy]]
[[Category:Nickel–titanium alloys]]