A shape-memory alloy (SMA, smart metal, memory metal, memory alloy, muscle wire, smart alloy) is an alloy that “remembers” its original shape and that when deformed returns to its pre-deformed shape when heated. This material is a lightweight, solid-state alternative to conventional actuators such as hydraulic, pneumatic, and motor-based systems. Shape-memory alloys have applications in robotics and automotive, aerospace and biomedical industries.
The two most prevalent shape-memory alloys are copper-aluminium-nickel, and nickel-titanium (NiTi) alloys but SMAs can also be created by alloying zinc, copper, gold and iron. Although iron-based and copper-based SMAs, such as Fe-Mn-Si, Cu-Zn-Al and Cu-Al-Ni, are commercially available and cheaper than NiTi, NiTi based SMAs are preferable for most applications due to their stability, practicability and superior thermo-mechanic performance. SMAs can exist in two different phases, with three different crystal structures (i.e. twinned martensite, detwinned martensite and austenite) and six possible transformations.
NiTi alloys change from austenite to martensite upon cooling; Mf is the temperature at which the transition to martensite completes upon 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). The maximum temperature at which SMAs can no longer be stress induced is called Md, where the SMAs are permanently deformed.
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.
ξ(T) 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 alloying. and work hardening.
Shape memory alloys can transmit very large forces without noticeable fatigue to several 100,000 cycles of motion. In comparison to other actuator materials, shape memory alloys have by far the largest specific working capacity (ratio of work done to material volume). Shape memory elements can operate for several million cycles. However, as the number of cycles increases, the properties of shape memory elements, e.g. may remain a residual strain after conversion.
In principle, all shape memory alloys can all perform shape memory effects. The respective desired effect is the task of the manufacturing and materials technology and must be trained by tuning of operating temperatures and optimization of the effect sizes.
One-way vs. two-way shape memory
Shape-memory alloys have different shape-memory effects. Two common effects are one-way and two-way shape memory. A schematic of the effects is shown below.
The procedures are very similar: starting from martensite, adding a reversible deformation for the one-way effect or severe deformation with an irreversible amount for the two-way, heating the sample and cooling it again.
One-way memory effect
When a shape-memory alloy is in its cold state (below As), 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 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 and can vary 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 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. There are several ways of doing this. A shaped, trained object heated beyond a certain point will lose the two-way memory effect.
Pseudoelastic behavior (“superelasticity”)
In shape memory alloys, in addition to the ordinary elastic deformation, a reversible change in shape caused by external force can be observed. This “elastic” deformation can exceed the elasticity of conventional metals up to twenty times. The cause of this behavior, however, is not the binding force of the atoms, but a phase transformation within the material. The material must be present in the high-temperature phase with austenitic structure. Under external stresses, the face -centered cubic formsAustenite in the tetragonal distorted (body-centered or cubic-body-centered, tetragonal distorted lattice) martensite (stress-induced martensite) around. When discharged, the martensite changes back to austenite. Since each atom retains its neighboring atom during the transformation, it is also called a diffusionless phase transformation. Therefore, the property is called pseudoelastic behavior. The material returns when relieved by its internal tension back to its original form. No temperature changes are required for this.
SMAs also display superelasticity, which is characterized by the recovery of relatively large strains with some, however, dissipation. In addition to temperature-induced phase transformations, martensite and austenite phases can be induced in response to mechanical stress. When SMAs are loaded in the austenite phase (i.e. above a certain temperature), the material will begin to transform into the (twinned) martensite phase when a critical stress is reached. Upon continued loading and assuming isothermal conditions, the (twinned) martensite will begin to detwin, allowing the material to undergo plastic deformation. If the unloading happens before plasticity, the martensite transforms back to austenite, and the material recovers its original shape by developing a hysteresis. For example, these materials can reversibly deform to very high strains – up to 7 percent. A more thorough discussion of the pseudoelastic behavior is presented by the experimental work of Shaw & Kyriakides, and more recently by Ma et al.
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).
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.
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.
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.
Magnetic shape memory alloys
In addition to the above-described thermally excited magnetic alloys, shape memory alloys exist (engl. Magnetic shape memory alloy, MSMA) showing a magnetically excited change in shape. In this case, move through the application of an external magnetic field, the twin boundaries and there is a change in shape and length. The achievable change in length of such alloys is currently in the range up to 10% at relatively small (in contrast to magnetostrictive materials) small transferable forces.
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 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 recrystallizes. They are heated to between 400 °C and 500 °C for 30 minutes, shaped while hot, and then are cooled rapidly by quenching in water or by cooling with air.
The copper-based and NiTi-based shape-memory alloys are considered to be engineering materials. These compositions can be manufactured to almost any shape and size.
The yield strength of shape-memory alloys is lower than that of conventional steel, but some compositions have a higher yield strength than plastic or aluminum. The yield stress for Ni Ti can reach 500 MPa. The high cost of the metal itself and the processing requirements make it difficult and expensive to implement SMAs into a design. As a result, these materials are used in applications where the super elastic properties or the shape-memory effect can be exploited. The most common application is in actuation.
One of the advantages to using shape-memory alloys is the high level of recoverable plastic strain that can be induced. The maximum recoverable strain these materials can hold without permanent damage is up to 8% for some alloys. This compares with a maximum strain 0.5% for conventional steels.
SMA have many advantages over traditional actuators, but do suffer from a series of limitations that may impede practical application. In numerous studies, it was emphasised that only a few of patented shape memory alloy applications are commercially successful due to material limitations combined with a lack of material and design knowledge and associated tools, such as improper design approaches and techniques used. The challenges in designing SMA applications are to overcome their limitations, which include a relatively small usable strain, low actuation frequency, low controllability, low accuracy and low energy efficiency.
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, 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 “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.
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. Design of SMA actuators may also influence both structural and functional fatigue of SMA, such as the pulley configurations in SMA-Pulley system.
SMA actuators are typically actuated electrically by Joule heating. If the SMA is used in an environment where the ambient temperature is uncontrolled, unintentional actuation by ambient heating may occur.
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.
SMAs are being explored as vibration dampers for launch vehicles and commercial jet engines. The large amount of hysteresis observed during the superelastic effect allow SMAs to dissipate energy and dampen vibrations. These materials show promise for reducing the high vibration loads on payloads during launch as well as on fan blades in commercial jet engines, allowing for more lightweight and efficient designs. SMAs also exhibit potential for other high shock applications such as ball bearings and landing gear.
There is also strong interest in using SMAs for a variety of actuator applications in commercial jet engines, which would significantly reduce their weight and boost efficiency. Further research needs to be conducted in this area, however, to increase the transformation temperatures and improve the mechanical properties of these materials before they can be successfully implemented. A review of recent advances in high-temperature shape-memory alloys (HTSMAs) is presented by Ma et al.
A variety of wing-morphing technologies are also being explored.
The first high-volume product (> 5Mio actuators / year) is an automotive valve used to control low pressure pneumatic bladders in a car seat that adjust the contour of the lumbar support / bolsters. The overall benefits of SMA over traditionally-used solenoids in this application (lower noise/EMC/weight/form factor/power consumption) were the crucial factor in the decision to replace the old standard technology with SMA.
The 2014 Chevrolet Corvette became the first vehicle to incorporate SMA actuators, which replaced heavier motorized actuators to open and close the hatch vent that releases air from the trunk, making it easier to close. A variety of other applications are also being targeted, including electric generators to generate electricity from exhaust heat and on-demand air dams to optimize aerodynamics at various speeds.
There have also been limited studies on using these materials in robotics, for example the hobbyist robot Stiquito (and “Roboterfrau Lara”), as they make it possible to create very lightweight robots. Recently, a prosthetic hand was introduced by Loh et al. that can almost replicate the motions of a human hand [Loh2005]. Other biomimetic applications are also being explored. Weak points of the technology are energy inefficiency, slow response times, and large hysteresis.
Bio-engineered robotic hand
There is some SMA-based prototypes of robotic hand that using shape memory effect (SME) to move fingers.
SMAs find a variety of applications in civil structures such as bridges and buildings. One such application is Intelligent Reinforced Concrete (IRC), which incorporates SMA wires embedded within the concrete. These wires can sense cracks and contract to heal macro-sized cracks. Another application is active tuning of structural natural frequency using SMA wires to dampen vibrations.
The first consumer commercial application was 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 second high volume application was an autofocus (AF) actuator for a smart phone. There are currently several companies working on an optical image stabilisation (OIS) module driven by wires made from SMAs
Shape-memory alloys are applied in medicine, for example, as fixation devices for osteotomies in orthopaedic surgery, in dental braces to exert constant tooth-moving forces on the teeth, and 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 grafts where it gives the ability to adapt to the shape of certain blood vessels when exposed to body temperature. On average, 50% of all peripheral vascular stents currently available on the worldwide market are manufactured with Nitinol.
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.
Memory metal has been utilized in orthopedic surgery as a fixation-compression device for 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.
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 orthodontist George Andreasen. 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.
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. One recent example of such device is the Liftware spoon, developed by Verily Life Sciences subsidiary Lift Labs.
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.
Sold in small round lengths for use in affixment-free bracelets.
The materials mainly used as shape memory alloys, which are also called cryogenic materials, are NiTi (nickel – titanium, nitinol) and, with even better properties, NiTiCu (nickel-titanium- copper). Both are most likely to be used as actuator materials. From an exact stoichiometry (quantitative ratio), the transformation temperatures are dependent. At less than 50 atomic percent nickel content, it is about 100 ° C. If the nickel content of the alloy is varied, it is possible to produce pseudoelastic or pseudoplastic behavior as austenite or martensite at room temperature.
Other copper-based materials are CuZn (copper – zinc), CuZnAl (copper-zinc- aluminum) and CuAlNi (copper-aluminum-nickel). Although they are cheaper, they both have higher transformation temperatures and poor shape memory. They are used in particular in medical technology. Less common are FeNiAl (iron-nickel-aluminum), FeMnSi (iron-manganese-silicon) and ZnAuCu (zinc- gold- copper).
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):
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
Ni-Ti approx. 55–60 wt% Ni
Source from Wikipedia