What are Shape Memory Alloys?
Shape Memory Alloys are materials that “remember” their original shape. If deformed, they recover their original shape upon heating.
Shape memory alloys (SMAs) are the special materials that have the ability to return to a predetermined shape when heated.
When the alloy is in the transformation temperature it undergoes low yield strength and will deform easily into any new shape which it will retain, if this alloy is heated above its transformation temperature it changes its crystal lattice structure which returns to its real shape.
SMAs are remarkably different from other materials are primarily due to shape memory effect (SME) and pseudoplasticity which are related with the specific way the phase transformation occurs, biocompatibility, high specific strength, high corrosion resistance, high wear resistance and high anti-fatigue property.
They can take large stresses without undergoing permanent deformation.
They can be formed into various shapes like bars, wires, plates and rings thus serving various functions.
Shape memory alloys (SMA’s) are special type of alloys exhibiting unique properties, as the name suggests they are alloys with memory i.e.: they return to their original configuration post deformation. These alloys can be deformed when cold and return to its original shape when heated. Shape memory alloys (SMA’s) possess two very unique properties namely pseudoelasticity and the shape memory effect.
Shape memory alloys (SMA’s) are special type of alloys exhibiting unique properties, as the name suggests they are alloys with memory i.e.: they return to their original configuration post deformation. These alloys can be deformed when cold and return to its original shape when heated. Shape memory alloys (SMA’S) possess two very unique properties namely pseudo-elasticity and the shape memory effect.
Types of Shape Memory Alloys
There are two types of shape memory alloys based on Shape Memory Effect
One-way shape memory alloy: A material which exhibits shape memory effect only upon heating is known as one-way shape memory.
Two-way shape memory alloy: A material which shows a shape memory effect during both heating and cooling is called two-way shape memory.
Examples of Shape Memory Alloys Are
1.Ni –Ti alloy (Nitinol)
2.Cu –Al –Ni alloy
3.Cu –Zn –Al alloy
4.Au –Cd alloy
5.Ni –Mn –Ga and Fe based alloys
The only two alloy systems that have achieved any level of commercial exploitation are,
o Based on Copper (Cu): Cu-Al-Ni and Cu-Zn-Al are used for narrow hysteresis and adaptability to two shape memory.
o Based on Nickel (Ni): Ni-Ti-ternary(x) Now way days Ni-Ti-X are used more than 90% of new SMA applications, NI-Ti alloys are more expensive to melt and produced than copper alloy, but they are preferred for their corrosion resistance, biocompatibility, and higher electrical resisting for resistive heating in actuator application.
Shape memory effect
The change of shape of a material at low temperature by loading and regaining of original shape by heating it, is known as shape memory effect .
The shape memory effect occurs in alloys due to the change in their crystalline structure with the change in temperature and stress.While loading, twinned martensite becomes deformed martensite at low temperature.On heating, deformed martensite becomes austenite (shape recovery) and upon cooling it gets transformed to twinned martensite (fig.).
SMAs exhibit changes in electrical resistance, volume and length during the transformation with temperature.
o The mechanism involved in SMA is reversible (austenite to martensite and vice versa.)
o Stress and temperature have a great influence on martensite transformation.
o Pseudo elasticity : Pseudo –elasticity occurs in shape memory alloys when it is completely in austenite phase (temperature is greater than austenite finish temperature) Unlike the shape memory effect, Pseudo-elasticity occurs due to stress induced phase transformation without a change in temperature. The load on the shape memory alloy changes austenite phase into martensite as soon as the loading decreases the martensite begins to transform to austenite. This phenomenon of deformation of a SMA on application of large stress and regaining of shape on removal of the load is known as pseudo elasticity. This pseudo elasticity is also known as super elasticity.
The temperature range for the martensite to austenite transformation which takes place upon heating is somewhat higher than that for the reverse transformation upon cooling.
The difference between the transition temperature upon heating and cooling is called hysteresis. The difference of temperature is found to be 20°-30°C.
Properties of Shape Memory Alloys.
o The Ni-Ti alloys have greater shape memory strain up to 8.5% tend to be much more thermally stable.
o They have excellent corrosion resistance and susceptibility, and have much higher ductility.
o Machining by turning or milling is very difficult except with special tools.
o Welding, brazing or soldering the alloys is generally difficult.
o The material does respond well to abrasive removal such as grinding, and shearing.
o Punching can be done if thicknesses are kept small.
There are various industries or fields where Shape Memory Alloys used are :
§ Orthodontic application :- The shape memory effect, is exploited in such applications. The wires are in martensitic phase during the insertion into brackets, they try to recover the original undeformed shape whenever the patient ingests hot food or drinks.The wires exert light forces on the teeth for the entire period in which the temperature is above the normal values.
§ Vascular Application :- The first vascular SMA application was the venous “Simon filter used to prevent emboli in patients unable to tolerate anticoagulants. A saline solution flows in the catheter to keep the temperature low during its insertion into the body. When the catheter is in position, the filter is released, the saline solution is stopped and the body heat induces the martensite-austenite transformation with recovery of the device’s original shape. The filter is now able to block the possible clots of the blood stream.
§ Neurosurgical application :- Coils are devices used for the treatment of cerebral aneurysms, which are localized dilations of the intracranial arteries. Stents are also used for the treatment of intracranial atherosclerotic disease. The stent treatment aims to recover the original blood flow where there is a narrowed lumen. Micro guidewires are used for proper positioning of the stents.
§ ROBOTIC MUSCLE :- Shape memory alloys mimic human muscles and tendons very well, the motion with which they contract and expand are very smooth creating a life-like movement. SMA wires are stretched for holding the finger of the arm straight, to resume the posture the wires are heated by passing electric current.
§ SURGICAL TOOLS :- Bone plates are fabricated using shape memory alloys. The plates are cooled to their transformation temperature then held with the broken bones. As the plates are heated with the body temperature leading to plate contraction thereby providing support to the broken joints while they heal back together.
§ AERONAUTICAL APPLICATIONS :- Aircraft manoeuvrability depends on movement of flaps at rear or trailing edge of the wings. Alternatives for hydraulic systems are being explored by the aerospace industry which include piezoelectric fibres, electro strictive and shape memory alloys. The stretch and fold movements of the wings takes place with the help of wires with the electric currents supply in the wires.
§ SPACE APPLICATIONS :- SMA’S were used as a functional material for the first time it was used as wire actuators of the flaps in a satellite called Nimbus. THE excellent damping ability of these alloys has always.
§ CIVIL EINGINEERING :- They have been used in bridge structures. SMAs possess a property of vibration damping hence they have a wide application in tuning the natural frequency of various structures.
§ Retrofitting Buildings :- The ability of SMAs to undergo large deformations with very little residual strain as well as their ability of reversible shape change with hysteresis is used.
ADVANCEMENTS IN THE SHAPE MEMORY ALLOYS
Recently researchers have taken an experimental approach to enhance the attributes of SMAs, by improving the material compositions to achieve
o Wider pirating temperature
o Better material stability
o Improving material response
The future trends in SMA’s can be expected at three different levels:
o Development of new or improved SMSs
o Combination of the functional properties of SMAs with the structural properties of other materials.
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