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Shape Memory Alloys

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This paper won Gold at Kapl’05,National Technical Symposium

Shape Memory Alloys

Gautam Parab Pranav Bhonsule

Abstract

Shape memory alloys (SMA) are metallic materials which have the capability to return to some previously defined shape or size when subjected to appropriate thermal conditions. This is possible due to a phase change occurring at micro-structural level. Pseudo elasticity and shape memory effect are two important properties of these alloys.

The most commonly used SMA is Ni-Ti alloy called Nitinol. This alloy displays a strain rate of 8% as compared to 0.2% for steel.

SMA’s have found a wide variety of uses especially in aero industry as actuators. Presently cost is the limiting factor but nevertheless with more research on such alloys it will be possible to increase their range of applicability.

Historic Background

The first recorded observation of the shape memory transformation was by Chang and Read in 1932. They noted the reversibility of the transformation in AuCd by metallographic observations and resistivity changes, and in 1951 the shape memory effect (SME) was observed in a bent bar of AuCd. In 1938, the transformation was seen in brass (CuZn). However, it was not until 1962, when Buehler and co-workers discovered the effect in equiatomic nickel-titanium (NiTi), that research into both the metallurgy and potential practical uses began in earnest. Within 10 years, a number of commercial products were on the market, and understanding of the effect was much advanced. Study of shape memory alloys has continued at an increasing pace since then, and more products using these materials are coming to the market each year

The most effective and widely used alloys include NiTi (Nickel – Titanium), CuZnAl, and CuAlNi.

What are Shape memory alloys?

The term Shape Memory Alloys (SMA) is applied to that group of metallic materials that demonstrate the ability to return to some previously defined shape or size when subjected to the appropriate thermal procedure. Generally, these materials can be plastically deformed at some relatively low temperature, and upon exposure to some higher temperature will return to their shape prior to the deformation

Although a relatively wide variety of alloys are know to exhibit the shape memory effect, only those that can recover substantial amounts of strain or that generate significant force upon changing shape are of commercial interest. To date, this has been the nickel-titanium alloys and copper-base alloys such as CuZnAl and CuAlNi..

Materials that exhibit shape memory only upon heating are referred to as having a one-way shape memory. Some materials also undergo a change in shape upon recooling. These materials have a two-way shape memory.

How Shape Memory Alloys Work?

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 10°C is necessary to initiate this phase change. The two phases, which occur in shape memory alloys, are Martensite, and Austenite.

Figure: 1 The Martensite and Austenite phases

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.

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

The temperatures at which each of these phases begin and finish forming are represented by the following variables: Ms, Mf, As, Af (where Ms and Mf are martensite start and martensite finish temperature while As and Af are austenite start and finish temperature. 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

Pseudo-Elasticity

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 4. 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.

Figure 4: Load Diagram of the pseudo-elastic effect Occurring

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.

Figure 5: Microscopic Diagram of the Shape Memory Effect

Advantages 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.

Applications

We take up three of the applications

1. Aircraft Manoeuvrability:

Aircraft 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.

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.

Fig 6: Hinge less shape memory alloy Flap

2. Bone Plates

Fig 7a: Conventional bone plate used to repair jaw fracture

Fig 7b: Typical Osteosynthesis tools

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

Fig 8: Example of how even a badly fractured face can be reconstructed using bone plates

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.

3. 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.

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 9 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 9). 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.

Future Prospects and Conclusion

Although specific products that might use the NiTi alloys in the future cannot be foretold, some directions are obvious. The cost of these alloys has slowly decreased as use has increased, so uses that require lower-cost alloys to be viable are being explored. Alloy development has yielded several ternary compositions with properties improved over those obtained with binary material, and alloys tailored to specific product needs are likely to multiply. The medical industry has developed a number of products using NiTi alloys because of their excellent biocompatibility and large pseudoelasticity, and many more of these applications are likely. Finally, the availability of small wire that is stable, is easily heated by a small electrical current, and gives a large repeatable stroke should lead to a new family of actuator devices. These devices can be inexpensive, are reliable for thousands of cycles, and are expected to move NiTi into the high-volume consumer marketplace.

Recent interest in the development of iron-base shape memory alloys has challenged the concept that long-range order and thermoelastic martensitic transformation are necessary conditions for shape memory effect. Among the alloys, FePt, FePd, and FeNiCoTi can be heat treated to exhibit thermoelastic martensitic transformation, and, therefore, shape memory effect. However, alloys such as FeNiC, FeMnSi, and FeMnSiCrNi are not ordered and undergo non thermoelastic transformation, and yet exhibit good shape memory effect. These alloys are characteristically different from conventional shape memory alloys in that they rely on stress-induced martensite for shape memory effect, exhibit fairly large transformation hysteresis, and, in general, have less than 4% recoverable strain. The commercial potential of these alloys has yet to be determined, but the effort has opened up new classes of alloys for exploration as shape memory alloys. These new classes include Beta-Ti alloys and iron-base alloys.

References

Smart Structures Lab, IIT Madras

http://www.cs.ualberta.ca/~database/MEMS/sma_mems/sma.html

http://www.jmmedical.com/html/_shape_memory_alloys_.html

(source: www.gautamparab.co.nr)

One Comment

  1. this is really awesome thanks for this important paper….

    1. suresh on January 30th, 2007 at 8:42 pm

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