IntroductionMaterials that sense the physical, chemical and biological environment by significantly changing their properties discontinuously or in a controlled fashion of increasing interest a great variety of uses, from the electronics industry, to medicine to space structures is one a characteristic for Innovative and advanced new material. They are the materials which have the capability to respond to changes in their condition or the environment to which they are exposed, in a useful and usually repetitivemanner. They are called by a Smart materials.

Shape memory alloys (SMAs) are excellent candidates for control systems and are commonly referred to as "smart" materials due to their ability to change shape with temperature. These alloys can also generate significant amounts of strain (and stress) and are ideal for use in actuators. This type of material is particularly appropriate for use in switches for electrical actuation

INTRODUCTION OF SHAPE MEMORY ALLOYSShape memory alloys (SMAs) are metals that "remember" their original shapes. SMAs are useful for such things as actuators which are materials that "change shape, stiffness, position, natural frequency, and other mechanical characteristics in response to temperature or electromagnetic fields" (Rogers, 155). When an SMA is cold, or below its transformation temperature, it has a very low yield strength and can be deformed quite easily into any new shape--which it will retain. However, when the material is heated above its transformation temperature it undergoes a change in crystal structure which causes it to return to its original shape. If the SMA encounters any resistance during this transformation, it can generate extremely large forces. This phenomenon provides a unique mechanism for remote actuation. The potential uses for SMAs especially as actuators have broadened the spectrum of many scientific fields. The study of the history and development of SMAs can provide an insight into a material involved in cutting-edge technology.

History of the Shape Memory Alloy

Shape Memory Alloy (SMA) is materials which have the ability to remember its shape even after large deformations. When the SMA is heated above its characteristic transition temperature, it will return to its original shape. Below transformation temperature SMA has very low yield strength and can be deformed easily into any new shape, which it can retain. This special phenomenon makes the SMA often called as a smart material. Shape memory effect firstly discovered in the 1930s, by A. Olander when he discovered the pseudoelastic behavior of the Au-Cd alloy in 1932. Then, Greninger and Mooradian in 1938 observed the formation and disappearance of a martensitic phase by increasing and decreasing the temperature of a Cu-Zn alloy. A decade later, the basic phenomenon of the shape memory effect governed by thermoplastic behaviour of the martensite phase was widely reported by Kurdjumov and Khandros in 1949 and also by Chang and Read (1951). In the early 1960s, Buehler and his co-workers at the U.S. Naval Ordnance Laboratory discovered the shape memory effect in an equiatomic alloy of nickel and titanium, which can be considered a break through in the field of shape memory materials. This alloy then was named as Nitinol (Nickel-Titanium Naval Ordnance Laboratory). Since that time, intensive investigations have been made to reveal the mechanics of its basic behaviour. The use of Nitinol as is fascinating because of its special functional behaviour, which is completely new compared to the conventional metal alloys. (A. Falvo, 2005)

Characteristic General principlesShape memory metal alloy can exist in two different temperature dependent crystal structures (phases) called martensite (lower temperature) and austenite (higher temperature or parent phase). Several properties of austenite and martensite are notably different.

Martensite - Austenite phase transformationExactly what made these metals "remember" their original shapes was in question after the discovery of the shape-memory effect. Dr. Frederick E. Wang, an expert in crystal physics, pinpointed the structural changes at the atomic level which contributed to the unique properties these metals have. (Kauffman and Mayo, 4) He found that NiTi had phase changes while still a solid. These phase changes, known as martensite and austeniteNiTi shape memory alloys can exist in a two different temperature-dependent crystal structures that are called martensite (which is lower temperature) and austenite (which is higher temperature or parent phase). The process is crystallographic reversible and these two phases have the same chemical composition but different crystallographic structures, and thus, different thermal, mechanical and electrical properties. (S.M. Mahfuzur Rahman, K. Kwang Ahn, 2008)

Under the transition temperature, NiTi is in the martensite phase. The transition temperature varies for different compositions from about -50 C to 166 C (Jackson, Wagner, and Wasilewski, 1). In the martensite phase, NiTi can be bent into various shapes. To fix the "parent shape" (as it is called), the metal must be held in position and heated to about 500 C. The high temperature "causes the atoms to arrange themselves into the most compact and regular pattern possible" resulting in a rigid cubic arrangement known as the austenite phase (Kauffman and Mayo, 5-6). Above the transition temperature, NiTi reverts from the martensite to the austenite phase which changes it back into its parent shape. This cyclecan be repeated millions of times (Jackson, Wagner, and Wasilewski, 1).

(a) Austenite (b) Martensite lattice structures(Source: A. Falvo, (2005))Figure 2.2(1)

Figure 2.2(1) (a) and (b) shows the molecule structure of the austenite and martensite. The martensite will transforms into austenite through either increasing in the temperature or by removing the applied stress. This shows that mechanical loading and thermal loading have opposite effects on NiTi alloys. When austenite is cooled, it begins to change into martensite. The temperature at which this phenomenon starts is called martensite start temperature (Ms), while the temperature at which martensite is again completely reverted is called martensite finish temperature (Mf). When martensite is heated, it begins to change into austenite. The temperature at which this phenomenon starts is called austenite start temperature (As), while the temperature at which this phenomenon is complete is called austenite finish temperature (Af). (J. R. Santiago,2002). Figure 2.2(2) below shows the graph of the relationship between changes in length of the SMA with the temperature.

Figure 2.2(2) SMA Length vs. Temperature Schematic.(Source: J.R. Santiago, (2002))

Figure 2.2 Stress-Strain Characteristics of NiTi at Various Temperatures

The most common shape memory material is an alloy of nickel and titanium called NiTi. This particular alloy has very good electrical and mechanical properties, long fatigue life, and high corrosion resistance. As an actuator, it is capable of up to 5% strain recovery and 50,000 psi restoration stress with many cycles. By example, a NiTi wire 0.020 inches in diameter can lift as much as 16 pounds. NiTi also has the resistance properties which enable it to be actuated electrically by joule heating. When an electric current is passed directly through the wire, it can generate enough heat to cause the phase transformation. In most cases, the transition temperature of the SMA is chosen such that room temperature is well below the transformation point of the material. Only with the intentional addition of heat can the SMA exhibit actuation. In essence, NiTi is an actuator, sensor, and heater all in one material.Shape memory alloys, however, are not for all applications. One must take into account the forces, displacements, temperature conditions, and cycle rates required of a particular actuator. The advantages of NiTi become more pronounced as the size of the application decreases. Large mechanisms may find solenoids, motors, and electromagnets more appropriate. But in applications where such actuators cannot be used, shape memory alloys provide an excellent alternative. There are few actuating mechanisms which produce more useful work per unit volume than NiTi.NiTi is available in the form of wire, rod and bar stock, and thin film. Examples of SMA products developed by TiNi Alloy Company include silicon micro-machined gas flow micro valves, non-explosive release devices, tactile feedback device (skin stimulators), and aerospace latching mechanisms. If you are considering an application for shape memory alloys, TiNi Alloy Company can assist you in the design, prototyping, and manufacture of actuators and devices.Physical Properties of NiTi Density: 6.45gms/cc Melting Temperature: 1240-1310 C Resistivity (hi-temp state): 82 uohm-cm Resistivity (lo-temp state): 76 uohm-cm Thermal Conductivity: 0.1 W/cm- C Heat Capacity: 0.077 cal/gm- C Latent Heat: 5.78 cal/gm; 24.2 J/gm Magnetic Susceptibility (hi-temp): 3.8 uemu/gm Magnetic Susceptibility (lo-temp): 2.5 uemu/gmMechanical Properties of NiTi Ultimate Tensile Strength: 754 - 960 MPa or 110 - 140 ksi Typical Elongation to Fracture: 15.5 percent Typical Yield Strength (hi-temp): 560 MPa, 80 ksi Typical Yield Strength (lo-temp): 100 MPa, 15 ksi Approximate Elastic Modulus (hi-tem): 75 GPa, 11 Mpsi Approximate Elastic Modulus (lo-temp): 28 GPa, 4 Mpsi Approximate Poisson's Ratio: 0.3Actuation Energy Conversion Efficiency: 5% Work Output: ~1 Joule/gram Available Transformation Temperatures: -100 to +100 C

ManufactureThere are various ways tomanufactureNiTi. Current techniques of producing nickel-titanium alloys include vacuum melting techniques such as electron-beam melting, vacuum arc melting or vacuum induction melting. "The cast ingot is press-forged and/or rotary forged prior to rod and wire rolling. Hot working to this point is done at temperatures between 700 C and 900 C" (Stoeckel and Yu, 3).There is also a process of cold working of Ni-Ti alloys. The procedure is similar to titanium wire fabrication. Carbide and diamond dies are used in the process to produce wires ranging from 0.075mm to 1.25mm in diameter. (Stoeckel and Yu, 4) Cold working of NiTi causes "marked changes in the mechanical and physical properties of the alloy" (Jackson, Wagner, and Wasilewski, 21). These processes of the production of NiTi are described in greater detail in Jackson, Wagner, and Wasilewski's report (15-22).Properties

Thepropertiesof NiTi are particular to the exact composition of the metal and the way it was processed. The physical properties of NiTi include a melting point around 1240 C to 1310 C, and a density of around 6.5 g/cm (Jackson, Wagner, and Wasilewski, 23). Various other physical properties tested at different temperatures with various compositions of elements include electrical resistivity, thermoelectric power, Hall coefficient, velocity of sound, damping, heat capacity, magnetic susceptibility, and thermal conductivity (Jackson, Wagner, and Wasilewski, 23-55). Mechanical properties tested include hardness, impact toughness, fatigue strength, and machinability (Jackson, Wagner, and Wasilewski, 57-73).The large force generated upon returning to its original shape is a very useful property. Other useful properties of NiTi are its "excellent damping characteristics at temperatures below the transition temperature range, its corrosion resistance, its nonmagnetic nature, its low density and its high fatigue strength" (Jackson, Wagner, and Wasilewski, 77). NiTi is also to an extent impact- and heat-resistant (Kauffman and Mayo, 4). These properties translate into many uses for NiTi.Other miscellaneous applications of shape memory alloys include use in household appliances, in clothing, and in structures. Adeep fryerutilizes the thermal sensitivity by lowering the basket into the oil at the correct temperature. (Falcioni, 114) According to Stoeckel and Yu, "one of the most unique and successful applications is the Ni-Ti underwire brassiere" (11). These bras, which were engineered to be both comfortable and durable, are already extremely successful in Japan (Stoeckel and Yu, 11). NiTi actuators as engine mounts and suspensions can also control vibration. These actuators can helpful prevent the destruction of such structures as buildings and bridges. (Rogers, 156)

Mechanical properties of Nitinol

A relatively large number of researchers have been interested in exploring the mechanical characteristics of NiTi in its two phases. Researchers are interested in studying and specifying the properties of NiTi materials under various types of thermo mechanical loadings. Several experimental studies have been conducted to specify the mechanical properties of SMA. The outcomes of experimental research in the past two decades assisted in developing a range for the mechanical parameters that would be expected from NiTi in its austenite and martensite phases. Table 2.3 presents a summary of the mechanical properties for NiTi. As explained in the following, mechanical properties could be due to several factors such as alloys composition, manufacturing process, strain rate and cyclic loading.

Table 2.3: Summary of NiTinol mechanical properties

Shape memory alloy material

There a numerous alloys that exhibit shape memory but overall there are two which are commercially available due in part for their proven ability to excel in some design aspects like maximum strain achievable, biocompatibility, lifespan and others.Table 2.4 shows the different kinds of alloys and their commercial availability.

Table 2.4: Shape Memory Alloy materials.(Source: J.R. Santiago, (2002)

Applications

Shape Memory Alloys Find a wide variety of uses in Aeronautical as well as Medical fields

Aircraft ManoeuvrabilityAircraft manoeuvrability depends heavily on the movement of flaps found at the rear or trailing edge of the wings. The efficiency and reliability of operating these flaps is of critical importance.

Most aircraft in the air today operate these flaps using extensive hydraulic systems. These hydraulic systems utilize large centralized pumps to maintain pressure, and hydraulic lines to distribute the pressure to the flap actuators. In order to maintain reliability of operation, multiple hydraulic lines must be run to each set of flaps. This complex system of pumps and lines is often relatively difficult and costly to maintain.

Many alternatives to the hydraulic systems are being explored by the aerospace industry. Among the most promising alternatives are piezoelectric fibers, electrostrictive ceramics, and shape memory alloys.The flaps on a wing generally have the same layout shown on the left, with a large hydraulic system attached to it at the point of the actuator connection. "Smart" wings, which incorporate shape memory alloys, are typically like the wing this system is much more compact and efficient, in that the shape memory wires only require an electric current for movement.

Hinge less shape memory alloy Flap

The shape memory wire is used to manipulate a flexible wing surface. The wire on the bottom of the wing is shortened through the shape memory effect, while the top wire is stretched bending the edge downwards, the opposite occurs when the wing must be bent upwards. The shape memory effect is induced in the wires simply by heating them with an electric current, which is easily supplied through electrical wiring, eliminating the need for large hydraulic lines. By removing the hydraulic system, aircraft weight, maintenance costs, and repair time are all reduced.

Medical Applications The variety of forms and the properties of SMAs make them extremely useful for a range of medical applications. For example, a wire that in its deformed shape has a small cross-section can be introduced into a body cavity or an artery with reduced chance of causing trauma. Once in place and after it is released from a constraining catheter the device is triggered by heat from the body and will return to its original memorised shape.Increasing a devices volume by direct contact or remote heat input has allowed the development of new techniques for keyhole or minimally invasive surgery. This includes instruments that have dynamic properties, such as miniature forceps, clamps and manipulators. SMA-based devices that can dilate, constrict, pull together, push apart and so on have enabled difficult or problematic tasks in surgery to become quite feasible 1. Stents The property of thermally induced elastic recovery can be used to change a small volume to a larger one. An example of a device using this is a stent. A stent, either in conjunction with a dilation balloon or simply by self-expansion, can dilate or support a blocked conduit in the human body. Coronary artery disease, which is a major cause of death around the world, is caused by a plaque in-growth developing on and within an arterys inner wall. This reduces the cross-section of the artery and consequently reduces blood flow to the heart muscle. A stent can be introduced in a deformed shape, in other words with a smaller diameter. This is achieved by travelling through the arteries with the stent contained in a catheter. When deployed, the stent expands to the appropriate diameter with sufficient force to open the vessel lumen and reinstate blood flow.2. Vena-cava Filters Vena-cava filters have a relatively long record of successful in-vivo application. The filters are constructed from Ni-Ti wires and are used in one of the outer heart chambers to trap blood clots, which might be the cause of a fatality if allowed to travel freely around the blood circulation system. The specially designed filters trap these small clots, preventing them from entering the pulmonary system and causing a pulmonary embolism. The vena-cava filter is introduced in a compact cylindrical form about 2.0-2.5mm in diameter. When released it forms an umbrella shape. The construction is designed with a wire mesh spacing sufficiently small to trap clots. This is an example of the use of superelastic properties, although there are also some thermally actuated vena cava filters on the market.

3. Dental and Orthodontic ApplicationsAnother commercially important application is the use of superelastic and thermal shape recovery alloys for orthodontic applications. Archwires made of stainless steel have been employed as a corrective measure for misaligned teeth for many years. Owing to the limited stretch and tensile properties of these wires, considerable forces are applied to teeth, which can cause a great deal of discomfort. When the teeth succumb to the corrective forces applied, the stainless steel wire has to be re-tensioned. Visits may be needed to the orthodontist for re-tensioning every three to four weeks in the initial stages of treatment.Superelastic wires are now used for these corrective measures. Owing to their elastic properties and extendibility, the level of discomfort can be reduced significantly as the SMA applies a continuous, gentle pressure over a longer period. Visits to the orthodontist are reduced to perhaps three or four per year. Bone Plates

Bone plates are surgical tools, which are used to assist in the healing of broken and fractured bones. The breaks are first set and then held in place using bone plates in situations where casts cannot be applied to the injured area. Bone plates are often applied to fractures occurring to facial areas such the nose, jaw or eye sockets. Repairs like this fall into an area of medicine known as osteosynthesis.

Currently osteotemy equipment is made primarily of titanium and stainless steel. The broken bones are first surgically reset into their proper position. Then a plate is screwed onto the broken bones to hold them in place, while the bone heals back together. This method has been proven both successful and useful in treating all manner of breaks, however there are still some drawbacks. After initially placing the plate on the break or fracture the bones are compressed together and held under some slight pressure, which helps to speed up the healing process of the bone. Unfortunately, after only a couple of days the tension provided by the steel plate is lost and the break or fracture is no longer under compression, slowing the healing process.

Bone plates can also be fabricated using shape memory alloys, in particular nickel titanium. Using a bone plate made out of NiTi, which has a transformation temperature of around Af much greater than 15 C surgeons follow the same procedure as is used with conventional bone plates. The NiTi plates are first cooled to well below their transformation temperature, then they are placed on the set break just like titanium plates. However, when the body heats the plate up to body temperature the NiTi attempts to contract applying sustained pressure on the break or fracture for far longer than stainless steel or titanium. This steady pressure assists the healing process and reduces recovery time. There are still some problems to consider before NiTi bone plates will become commonplace. Designing plates to apply the appropriate amount of pressure to breaks and fractures is the most important difficulty, which must be overcome.

Example of how even a badly fractured face can be reconstructed using bone plates

Robotic Muscles

There have been many attempts made to re-create human anatomy through mechanical means. The human body however, is so complex that it is very difficult to duplicate even simple functions. Robotics and electronics are making great strides in this field, of particular interest are limbs such hands, arms, and legs.

.Shape memory alloys mimic human muscles and tendons very well. SMA's are strong and compact so that large groups of them can be used for robotic applications, and the motion with which they contract and expand are very smooth creating a life-like movement unavailable in other systems.

Creating human motion using SMA wires is a complex task but a simple explanation is detailed here. For example to create a single direction of movement (like the middle knuckle of your fingers) the setup shown in Figure 1 could be used. The bias spring shown in the upper portion of the finger would hold the finger straight, stretching the SMA wire, then the SMA wire on the bottom portion of the finger can be heated which will cause it to shorten bending the joint downwards (as in Figure 1). The heating takes place by running an electric current through the wire; the timing and magnitude of this current can be controlled through a computer interface used to manipulate the joint.

There are still some challenges that must be overcome before robotic hands can become more commonplace. The first is generating the computer software used to control the artificial muscle systems within the robotic limbs. The second is creating large enough movements to emulate human flexibility (i.e. being able to bend the joints as far as humans can). The third problem is reproducing the speed and accuracy of human reflexes.

How Shape Memory Alloys WorkFigure 1: The Martensite and Austenite phases

Texas A&M SMART Lab - http://smart.tamu.edu/The two unique properties described above are made possible through a solid state phase change, that is a molecular rearrangement, which occurs in the shape memory alloy. Typically when one thinks of a phase change a solid to liquid or liquid to gas change is the first idea that comes to mind. A solid state phase change is similar in that a molecular rearrangement is occurring, but the molecules remain closely packed so that the substance remains a solid. In most shape memory alloys, a temperature change of only about 10C is necessary to initiate this phase change. The two phases, which occur in shape memory alloys, areMartensite, andAustenite.Martensite, is the relatively soft and easily deformed phase of shape memory alloys, which exists at lower temperatures. The molecular structure in this phase is twinned which is the configuration shown in the middle of Figure 2. Upon deformation this phase takes on the second form shown in Figure 2, on the right. Austenite, the stronger phase of shape memory alloys, occurs at higher temperatures. The shape of the Austenite structure is cubic, the structure shown on the left side of Figure 2. The un-deformed Martensite phase is the same size and shape as the cubic Austenite phase on a macroscopic scale, so that no change in size or shape is visible in shape memory alloys until the Martensite is deformed.

Figure 2: Microscopic and Macroscopic Views of the Two Phases of Shape Memory Alloys

Oulu University - http://herkules.oulu.fi/isbn9514252217/html/x317.html

The temperatures at which each of these phases begin and finish forming are represented by the following variables:Ms,Mf,As,Af. The amount of loading placed on a piece of shape memory alloy increases the values of these four variables as shown in Figure 3. The initial values of these four variables are also dramatically affected by the composition of the wire (i.e. what amounts of each element are present).Figure 3: The Dependency of Phase Change Temperature on Loading

Texas A&M SMART Lab - http://smart.tamu.edu/

Shape Memory EffectFigure 4: Microscopic Diagram of the Shape Memory Effect

Oulu University - http://herkules.oulu.fi/isbn9514252217/html/x317.html

The shape memory effect is observed when the temperature of a piece of shape memory alloy is cooled to below the temperature Mf. At this stage the alloy is completely composed of Martensite which can be easily deformed. After distorting the SMA the original shape can be recovered simply by heating the wire above the temperature Af. The heat transferred to the wire is the power driving the molecular rearrangement of the alloy, similar to heat melting ice into water, but the alloy remains solid. The deformed Martensite is now transformed to the cubic Austenite phase, which is configured in the original shape of the wire.The Shape memory effect is currently being implemented in: Coffepots The space shuttle Thermostats Vascular Stents Hydraulic Fittings (for Airplanes)Pseudo-elasticityFigure 5: Load Diagram of the pseudo-elastic effect Occurring

Pseudo-elasticity occurs in shape memory alloys when the alloy is completely composed of Austenite (temperature is greater than Af). Unlike the shape memory effect, pseudo-elasticity occurs without a change in temperature. The load on the shape memory alloy is increased until the Austenite becomes transformed into Martensite simply due to the loading; this process is shown in Figure 5. The loading is absorbed by the softer Martensite, but as soon as the loading is decreased the Martensite begins to transform back to Austenite since the temperature of the wire is still above Af, and the wire springs back to its original shape.Some examples of applications in which pseudo-elasticity is used are: Eyeglass Frames Bra Underwires Medical Tools Cellular Phone Antennae Orthodontic ArchesAdvantages and Disadvantages Some of the main advantages of shape memory alloys include: Bio-compatibility Diverse Fields of Application Good Mechanical Properties (strong, corrosion resistant)There are still some difficulties with shape memory alloys that must be overcome before they can live up to their full potential. These alloys are still relatively expensive to manufacture and machine compared to other materials such as steel and aluminum. Most SMA's have poor fatigue properties; this means that while under the same loading conditions (i.e. twisting, bending, compressing) a steel component may survive for more than one hundred times more cycles than an SMA element.courtesy:cs.ualberta.ca/~database/MEMS/sma_mems/sma.htmlshapememoryalloy(SMA).Examples of this type ofalloyarenickel-titaniumcopper-zinc-aluminiumandcopper-aluminium-nickel.When asmart alloyisbentortwisted(calleddeforming)it keeps its new shape until it isheated.When thetemperatureisraisedabove a certain levelthe alloy returns to itsoriginal shape.

Some of the main advantages of shape memory alloys include: Bio-compatibility Diverse Fields of Application Good Mechanical Properties (strong, corrosion resistant) The use of NiTi as a biomaterial has severable possible advantages.Its shape memory property and super elasticity are unique characteristics and totally new in the medical field. The possibility to make self-locking, self expanding and self- compressing thermally activated implants is fascinating. As far as special properties and good bio compatibility are concerned, it is evident that NiTi has a potential to be a clinical success in several applications in future.There are still some difficulties with shape memory alloys that must be overcome before they can live up to their full potential. These alloys are still relatively expensive to manufacture and machine compared to other materials such as steel and aluminum. Most SMA's have poor fatigue properties; this means that while under the same loading conditions (i.e. twisting, bending, compressing) a steel component may survive for more than one hundred times more cycles than an SMA element.

ConclusionThe many uses and applications of shape memory alloys ensure a bright future for these metals. Research is currently carried out at manyroboticsdepartments and materials science departments. With the innovative ideas for applications of SMAs and the number of products on the market using SMAs continually growing, advances in the field of shape memory alloys for use in many different fields of study seem very promising.