Shape memory alloy

A shape memory alloy (SMA, also known as a smart alloy, memory metal, or muscle wire) is an alloy that "remembers" its shape. After a sample of SMA has been deformed from its original crystallographic configuration, it regains its original geometry by itself during heating (one-way effect) or, at higher ambient temperatures, simply during unloading (pseudo-elasticity or superelasticity). These extraordinary properties are due to a temperature-dependent martensitic phase transformation from a low-symmetry to a highly symmetric crystallographic structure. Those crystal structures are known as martensite (at lower temperatures) and austenite (at higher temperatures).

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Overview

The three main types of SMA are the copper-zinc-aluminium-nickel, copper-aluminium-nickel, and nickel-titanium (NiTi) alloys. NiTi alloys are generally more expensive and possess superior mechanical properties when compared to copper-based SMAs. The temperatures at which the SMA changes its crystallographic structure, called transformation temperature, is characteristic of the alloy, and can be tuned by varying the elemental ratios in the alloy. Typically, M_s denotes the temperature at which the structure starts to change from austenite to martensite upon cooling; M_f is the temperature at which the transition is finished. Accordingly, A_s and A_f are the temperatures at which the reverse transformation from martensite to austenite starts and finishes, respectively. It is important to note that 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).

Martensite, named after the German metallurgist Adolf Martens (1850–1914), is any crystal structure that is formed by displacive transformation, as opposed to much slower diffusive transformations. It includes a class of hard minerals occurring as lathe- or plate-shaped crystal grains. When viewed in cross-section, the lenticular (lens-shaped) crystal grains appear acicular (needle-shaped), which is how they are sometimes incorrectly described. "Martensite" most commonly refers to a very hard constituent of steel (the alloy of iron and carbon) important in some tool steels. The martensite is formed by rapid cooling (quenching) of austenite which traps carbon atoms that do not have time to diffuse out of the crystal structure.

In the 1890s, Martens studied samples of different steels under a microscope, and found that the hardest steels had a regular crystalline structure. He was the first to explain the cause of the widely differing mechanical properties of steels. Martensitic structures have since been found in many other practical materials, including shape memory alloys and transformation-toughened ceramics.

Martensite has a different crystalline structure (tetragonal) than the face-centered-cubic austenite from which it is formed, but identical chemical or alloy composition. The transition between these two structures requires very little thermal activation energy because it occurs displacively or martensiticly by the subtle but rapid rearrangement of atomic positions, and has been known to occur even at cryogenic temperatures. Martensite has a lower density than austenite, so that the martensitic transformation results in a relative change of volume:[1] this can be seen vividly in the Japanese katana, which is straight before quenching. Differential quenching causes martensite to form predominantly in the edge of the blade rather than the back; as the edge expands, the blade takes on a gently curved shape.

Martensite is not shown in the equilibrium phase diagram of the iron-carbon system because it is a metastable phase, the kinetic product of rapid cooling of steel containing sufficient carbon. Since chemical processes (the attainment of equilibrium) accelerate at higher temperature, martensite is easily destroyed by the application of heat. This process is called tempering. In some alloys, the effect is reduced by adding elements such as tungsten that interfere with cementite nucleation, but, more often than not, the phenomenon is exploited instead. Since quenching can be difficult to control, many steels are quenched to produce an overabundance of martensite, then tempered to gradually reduce its concentration until the right structure for the intended application is achieved. Too much martensite leaves steel brittle, too little leaves it soft.

Austenite (or gamma phase iron) is a metallic non-magnetic solid solution of iron and an alloying element. In plain-carbon steel, austenite exists above the critical eutectoid temperature of 1000 K (about 727 °C); other alloys of steel have different eutectoid temperatures. It is named after Sir William Chandler Roberts-Austen (1843-1902).]]

In this figure, ξ (T) represents the martensite fraction.

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).

With the one-way effect, cooling from high temperatures does not cause a macroscopic shape change. A deformation is necessary to create the low-temperature shape. On heating, transformation starts at A_s and is completed at A_f (typically 2 to 20 °C or hotter, depending on the alloy or the loading conditions). A_s is determined by the alloy type and composition. It can be varied between −150 °C and maximum 200 °C.

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. 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.

History

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 1962–1963 by the 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.^[1]

There is another type of SMA, called a ferromagnetic shape memory alloy (FSMA), that changes shape under strong magnetic fields. These materials are of particular interest as 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.

Crystal structures

Why does a metal or an alloy observe these qualities of memory? Dr. Frederick E. Wang,^[2] 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.^[3] 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.

Manufacture

The way in which the alloys are made depends on the properties wanted. They are heated to between 400 °C and 500 °C for 1-5 minutes.

Typical variables: 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.

Properties

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 relatively low compared to 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 SMA's 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.

Applications

The first consumer commercial application for the material was as a shape memory coupling for 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.

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 braces that exert a constant pressure on the teeth. Patented by George Andreasen [1] 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 inventor in 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. Walia later utilized the alloy in the manufacture of root canal files in endodontics. There have also been limited studies on using these materials in robotics (such as "Roboterfrau Lara" [2]), as they make it possible to create very light robots. Weak points of the technology are energy inefficiency, slow response times, and large hysteresis.

An SMA of titanium is used to make eyeglass frames under the trademark Flexon.

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.

Boeing, General Electric Aircraft Engines, Goodrich Corporation, NASA, and All Nippon Airways developed the Variable Geometry Chevron using 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. [3]

Materials

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
• Fe-Mn-Si
• Pt alloys
• Co-Ni-Al
• Co-Ni-Ga
• Ni-Fe-Ga
• Ti-Pd in various concentrations
• Ni-Ti (~55% Ni)

References

1. ^ Kauffman, George, and Isaac Mayo. "Memory Metal." Chem Matters Oct. 1993: 4-7.
2. ^ http://www.stanford.edu/~richlin1/sma/sma.html#TOP
3. ^ http://www.uwgb.edu/dutchs/petrolgy/BSOLSOL.HTM

• 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