- Shape memory alloy
A shape memory alloy (SMA, also known as a smart alloy, memory metal, or muscle wire) is an
alloythat "remembers" its shape, and can be returned to that shape after being deformed, by applying heat to the alloy. When the shape memory effect is correctly harnessed, this material becomes a lightweight, solid-state alternative to conventional actuators such as hydraulic, pneumatic, and motor-based systems. Shape memory alloys have numerous applications in the medical and aerospace industries.
The three main types of SMA are the
copper- zinc- aluminum- nickel, copper-aluminum-nickel, and nickel- titanium(NiTi) alloys. NiTi alloys are generally more expensive and change from austenite to martensite upon cooling; "Mf" is the temperature at which the transition to Martensite is finished during cooling. Accordingly, during heating "As" and "Af" are the temperatures at which the transformation from Martensite to Austenite starts and finishes. 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).SMA's can also be used by alloying zinc, copper, gold, and iron.
The transition from the
martensitephase to the austenitephase is only dependent on temperature and stress, not time, as most phase changes are, as there is no diffusion involved. Similarly, the austenitestructure gets its name from steel alloys of a similar structure. It is the reversible diffusionless transition between these two phases that allow the special properties to arise. While martensitecan be formed from austeniteby rapidly cooling carbon- steel, this process is not reversible, so steel does not have shape memory properties.
In this figure, ξ (T) represents the martensite fraction. The difference between the heating transition and the cooling transition give rise to the shape of the curve depends on the material properties of the shape memory alloy, such as the
alloying and work hardening.
One-way vs. two-way Shape Memory
Shape memory alloys may have different kinds of shape memory effect. The two most common memory effects are the one-way and two-way shape memory. A schematic view of the two effects is given in the figure below.
In the figure above, the procedures are very similar: starting from martensite (a), adding a reversible deformation for the one-way effect or severe deformation with an irreversible amount for the two-way (b), heating the sample (c) and cooling it again (d).
One Way Memory Effect
When a shape memory alloy is in its cold state (below "As"), the metal can be bent or stretched into a variety of new shapes and will hold that shape until it is heated above the transition temperature. Upon heating, the shape changes back to its original shape, regardless of the shape it was when cold. When the metal cools again it will remain in the hot 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 "As" and is completed at "Af" (typically 2 to 20 °C or hotter, depending on the alloy or the loading conditions). "As" is determined by the alloy type and composition. It can be varied between −150 °C and 200 °C.
Two Way Memory Effect
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 shape. A material that shows a shape memory effect during both heating and cooling is called 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 high-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. There are several ways of doing this.
If a shape memory alloy is heated up to very high temperatures (after it has been trained) then it may lose the two way memory effect, this process is known as "amnesia".
One of the commercial uses of shape memory alloy involves using the pseudo-elastic properties of the metal during the high temperature (austenitic) phase. The frames of reading glasses have been made of shape memory alloy as they can undergo large deformations in their high temperature state and then instantly revert back to their original shape when the stress is removed. This is the result of
pseudo-elasticity; the martensiticphase is generated by stressing the metal in the austeniticstate and this martensite phase is capable of large strains. With the removal of the load, the martensite transforms back into the austenite phase and resumes its original shape.
This allows the metal to be bent, twisted and pulled, before reforming its shape when released. This means the frames of shape memory alloy glasses are claimed to be "nearly indestructable" because it appears no amount of bending will result in permanent plastic deformation.
The first reported steps towards the discovery of the shape memory effect were taken in the 1930s. According to Otsuka and Wayman (1998), A. Ölander discovered the pseudoelastic behavior of the Au-Cd alloy in 1932. Greninger & 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 & Khandros (1949) and also by Chang & Read (1951).
The nickel-titanium alloys were first developed in
1962– 1963by the Naval Ordnance Laboratoryand 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. [Kauffman, George, and Isaac Mayo. "Memory Metal." Chem Matters Oct. 1993: 4-7.]
There is another type of S.M.A., called a 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 polymers have also been developed, and became commercially available in the late 1990s.
Why does a metal or an alloy observe these qualities of memory? Dr. Frederick E. Wang, [ [http://www.stanford.edu/~richlin1/sma/sma.html Shape Memory Alloys] ] an expert in crystalline structures, was the one who came with the early answers to the phenomenon. According to him, Nitinol undergoes phase changes while remaining a solid. Normally these phase changes occur in an alloy when heated to its melting point. Different phase changes occur at different temperatures. [ [http://www.uwgb.edu/dutchs/petrolgy/BSOLSOL.HTM Binary Solid Solution] ] These phase changes, known as martensite and austenite, "involve the rearrangement of the position of particles within the crystal structure of the solid." In shape memory alloys, these phase transformations occur below its melting point. Thus, the alloys can retain their shape without melting. Some alloys change shape within a small difference in temperature. Under the transition temperature, Nitinol is in the martensite phase and can be bent into various shapes. To set the "parent shape" the metal must be held in position and heated to about 500 °C. (varies for different SMAs). The high temperature causes the atoms to arrange themselves into a high symmetry, often cubic arrangement known as the austenite phase. In the low temperature martensite each atom "remembers" which should be its neighbors in austenite, even when the metal is deformed by
crystal twinning deformation.
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
ingotis then hot rolledinto 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 the material recrystallises.They are heated to between 400 °C and 500 °C for 30 minutes. Typical variables for some alloys are 500 °C and for more than 5 minutes.
They are then shaped while hot and are cooled rapidly by quenching in water or by cooling with air.
The copper-based and NiTi (nickel and titanium)-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 aluminium. The yield stress for NiTi 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 superelastic 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 8% for some alloys. This compares with a maximum strain 0.5% for conventional steels.
Boeing, General Electric Aircraft Engines, Goodrich Corporation, NASA, and All Nippon Airwaysdeveloped the Variable Geometry Chevronusing shape memory alloy that reduces aircraft's engine noise. Boeing's upcoming aircraft, the 787 and the 747-8 will be equipped with this new technology. [http://adsabs.harvard.edu/abs/2006SPIE.6171..199C]
The first consumer commercial application for the material was as a
shape memory couplingfor piping, e.g. oil line pipes for industrial applications, water pipes and similar types of piping for consumer/commercial applications. The late 1980s saw the commercial introduction of Nitinol as an enabling technology in a number of mininally 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. On average, 50% of all peripheral vascular stents currently available on the worldwide market are manufactured with Nitinol.
There have also been limited studies on using these materials in robotics (such as "Roboterfrau Lara" [http://www.lararobot.de] ), as they make it possible to create very light robots. Weak points of the technology are energy inefficiency, slow response times, and large
Nitinol wire is also used in
robotics(e.g. the hobbyist robot Stiquito) and in a few magic tricks, particularly those involving heat and shapeshifting.
Eyeglass frames made from titanium-containing SMAs are marketed under the trademarks
The range of applications for SMAs has been increasing in recent years, with one major area of expansion being medicine: for example, the development of
dental bracesthat exert a constant pressure on the teeth. Patented by George Andreasen[US Patent|5018969] who changed the formula and then formally introduced the use of Nitinol on July 26, 1979, U.S. Pat. No. 4,037,324 an American orthodontist and inventorin 1972, for use in arch wires to straighten teeth it revolutionized orthodontal medicine as well as fiber optic development because it returns to its original shape after being bent. The alloy has a patterned shape memory which expands and contracts to given temperatures because of its geometric programming. Harmeet D. Walialater utilized the alloy in the manufacture of root canal files in endodontics.
Materials having the memory effect at different temperatures and at different percentages of its solid solution contents.
* Ag-Cd 44/49 at.% Cd
* Au-Cd 46.5/50 at.% Cd
* Cu-Al-Ni 14/14.5 wt.% Al and 3/4.5 wt.% Ni
* Cu-Sn approx. 15 at.% Sn
* Cu-Zn 38.5/41.5 wt.% Zn
* Cu-Zn-X (X = Si, Al, Sn)
* Fe-Pt approx. 25 at.% Pt
* Mn-Cu 5/35 at.% Cu
* Pt alloys
* Ti-Pd in various concentrations
* Ni-Ti (~55% Ni)
* Duerig, TW, KN Melton, D Stöckel and CM Wayman. "Engineering Aspects of Shape Memory alloys". ISBN 0-7506-1009-3. London: Butterworth Heinemann, 1990.
* K. Shimizu and T. Tadaki, Shape Memory Alloys, H. Funakubo, Ed., Gordon and Breach Science Publishers, 1987
* [http://www.stanford.edu/~richlin1/sma/sma.html Shape Memory Alloys and Their Applications] - Introductory information on Shape Memory Alloys
* [http://jmmedical.com/resources.html Nitinol Technical Information] (from Johnson Matthey, Inc.)
* [http://news.bbc.co.uk/1/hi/health/755819.stm BBC report on medical applications of Nitinol]
* [http://www.ruhr-uni-bochum.de/sfb459/english/ SFB 459: A German Research Center for Shape Memory Alloys]
* [http://www.smaterial.com SMAterial.com - phenomena, crystallography, model, simulation and applications of SMA] - Has .gifanimations demonstrating the effect.
* [http://smart.tamu.edu Texas A&M University's Shape Memory Alloy Research Team] - SMA overview, publications, etc.
* [http://www.fzu.cz/departments/metals/sma/ Institute of Physics, ASCR] - Research Projects, publications, events & conferences, functional materials
* [http://webcast.berkeley.edu/courses/stream.php?type=real&webcastid=14232 Berkeley - Bioeng/ME C117 Structural Aspects of Biomaterials] - Video Lecture: Dr. Scott Robertson,
Stents: Fatigue and Fracture, LBL - Material properties of Nitinol stents
* [http://webcast.berkeley.edu/courses/stream.php?type=real&webcastid=14233 Berkeley - Bioeng/ME C117 Structural Aspects of Biomaterials] - Video Lecture: Dr. Alan Pelton, Stent Design, Nitinol Device Company (NDC) - Material properties of Nitinol stents
* [http://web.archive.org/web/20030810230928/www.sma-inc.com/education.html SMST Proceedings]
* [http://www.materialforschungsservice-dr-prieb.de/Publikationen.htm some patents and publications to memory alloys] de icon
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