SHAPE MEMORY ALLOYS
Shape memory alloys (SMA's) are metals, which exhibit two very unique properties, pseudo-elasticity, and the shape memory effect. Arne Olander first observed these unusual properties in 1938 (Oksuta and Wayman 1998), but not until the 1960's were any serious research advances made in the field of shape memory alloys. The most effective and widely used alloys include NiTi (Nickel - Titanium), CuZnAl, and CuAlNi.
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 10Ã‚Â°C is necessary to initiate this phase change. The two phases, which occur in shape memory alloys, are Martensite, and Austenite.
After alloying and basic processing, SMAs can be formed into a shape(eg , a coil spring) and then set to that shape by a high heat treatment. When cooled , they may be bent, stretched or deformed(with in limits) and than with subsequent moderate heating (well below the heat setting temperature), they can recover some or all of the deformation.
Shape memory alloys have found use in everything from space missions (pathfinder and many more) to floral arrangement (animated butterflies, dragon flies and fairies), from bio_medical applications, to actuators for miniature robots and cellphone antennas and even eyeglasses use SMA wires for their extreme flexibility.
Shape 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, 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.
Shape Memory Effect
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:
Â¢ The space shuttle
Â¢ Vascular Stents
Â¢ Hydraulic Fittings (for Airplanes)
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:
Â¢ Eyeglass Frames
Â¢ Medical Tools
Â¢ Cellular Phone Antennae
Since the discovery of Ni-Ti, at least fifteen different binary, ternary and quaternary alloy types have been discovered that exhibit shape changes and unusual elastic properties consequent to deformation. Some of these alloy types and variants are shown in table 1.
Table 1. Shape memory alloy types.
Â¢ Hafnium-titanium-nickel Â¢ Iron-manganese-silicon
The original nickel-titanium alloy has some of the most useful characteristics in terms of its active temperature range, cyclic performance, recoverable strain energy and relatively simple thermal processing. Ni-Ti and other alloys have two generic properties thermally induced shape recovery and super- or pseudo-elasticity. The latter means that an SMA in its elastic form can undergo a deformation approximately ten times greater than that of a spring-steel equivalent, and full elastic recovery to the original geometry may be expected. This may be possible through several million cycles. The energy density of the alloy can be used to good effect to make high-force actuators - a modern DC brushless electric motor has a mass of 5-10 times that of a thermally activated Ni-Ti alloy, to do the same work.
The super elastic Ni-Ti alloys are stressed by simply working the alloy. These stresses can be removed, just as with many other alloys, by an annealing process. The stressed condition is termed stress-induced martensite, which is the equivalent of being cold/hot worked.
SMAs, particularly nickel-titanium, are commercially available from several sources. However, world production is small compared to other metal commodities (about 200 tonnes were produced 1998) owing to difficulties in the melt/forging production process, and so the cost of the material high US$0.30-US$1.50 (UKÃ‚Â£0.20-Ã‚Â£1.00) per gram for wire forms 1999 prices). Fortunately, most current applications require only small amount of the material. As world production increases (as it has done quite dramatically in the 1990s) so prices should decrease. Wires, strip, rod, bar and sheet are all readily available and alloy foams, sintering powders and sputtering targets of high purity are also produced.
The use of the one way shape memory or super elastic property of NiTi for a specific application requires a piece of SMA to be molded into the desired shape . the characteristic heat treatment is then done to set the specimen to its final shape . The heat treatment methods used to set shapes in both the shape memory and the super elastic forms of NiTi are similar. Adequate heat treatment parameters are needed to set the shape and properties of the item
The two way memory training procedure can be made by SME training or SIM training . In SME training the specimen is cooled below Mf and bent the desired shape . It is then heated to a temperature above Af and allowed freely to take its austenite shape . The procedure is repeated 20 â€œ 30 times which completes the training . The sample now assumes its programmed shape upon cooling under Mf and to another shape when heated above Af
In SIM (stress induced martensite ) training the specimen is bent just above Ms to produce the preferred variants of SIM and then cooled below Mf temperature. Upon subsequent heating above the Af temperature the specimen takes its original austenitic shape . This procedure is repeated 20-30 times
Shape Memory Alloys Find a wide variety of uses in Aeronautical as well as Medical fields
Aircraft maneuverability 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.
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 deviceâ„¢s 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
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 arteryâ„¢s 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 Applications
Another 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 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 Ã‚Â°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
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.
Advantages and Disadvantages
Some of the main advantages of shape memory alloys include:
Â¢ Diverse Fields of Application
Â¢ 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.
Current examples of applications of shape memory alloys.
Â¢ Aids for disabled
Â¢ Aircraft flap/slat adjusters
Â¢ Anti-scald devices
Â¢ Arterial clips
Â¢ Automotive thermostats
Â¢ Braille print punch
Â¢ Catheter guide wires
Â¢ Cold start vehicle actuators
Â¢ Contraceptive devices
Â¢ Electrical circuit breakers
Â¢ Fibre-optic coupling
Â¢ Filter struts
Â¢ Fire dampers
Â¢ Fire sprinklers
Â¢ Gas discharge
Â¢ Graft stents
Â¢ Intraocular lens mount
Â¢ Kettle switches
Â¢ Keyhole instruments
Â¢ Key-hole surgery instruments Â¢ Micro-actuators
Â¢ Mobile phone antennas
Â¢ Orthodontic archwires
Â¢ Penile implant
Â¢ Pipe couplings
Â¢ Robot actuators
Â¢ Rock splitting
Â¢ Root canal drills
Â¢ Satellite antenna deployment
Â¢ Scoliosis correction
Â¢ Solar actuators
Â¢ Spectacle frames
Â¢ Steam valves
Â¢ Switch vibration damper
Â¢ Underwired bras
Â¢ Vibration dampers
The many uses and applications of shape memory alloys ensure a bright future for these metals. Research is currently carried out at many robotics departments 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. Future Applications
There are many possible applications for SMAs. Future applications are envisioned to include engines in cars and airplanes and electrical generators utilizing the mechanical energy resulting from the shape transformations. Nitinol with its shape memory property is also envisioned for use as car frames. (Kauffman and Mayo, 7) Other possible automotive applications using SMA springs include engine cooling, carburetor and engine lubrication controls.