Shape memory alloy

Shape memory alloy

Shape memory alloy is an alloy that remembers its pre-deformed shape. This alloy gets back to its original shape when heated to a certain temperature.

The two main types of memory alloy are copper-aluminum-nickel and nickel-titanium alloys. Some memory alloys are also created by mixing zinc, copper, gold and iron.

Memory alloys have a special property that their crystal transformation is fully reversible when heated. This special property makes them memory wire. At the time of manufacturing the memory wire is trained in a shape. Due to training, when the wire is heated to the required temperature, the crystal structure of the wire reverses back to that at the time of training, thus gets back to its pre-deformed shape.

This property of the metal makes it useful in making actuators, medical field, making masks, frames of spectacles, jewelry, dentistry, etc. because they are lighter in weight, and easily Handel able. Apart from having so many properties, they have a very limited use at present, since it is highly expensive.

There are two types of memory alloys according to the memory effects.

·        One way memory

·        Two way memory

One way memory-

When the metal remembers only one shape i.e. the shape it forms when heated then it is called one way memory wire. This type of wire can be deformed to any shape when at low temperature, and it retains its shape when heated above transition temperature.

Two way memory-

Two way memory alloys are trained to remember two shapes, one at high temperature and the other at low temperature. There are two transition temperatures for this type of memory alloy. When an alloy remembers two temperatures then it is called two way memory alloy.

Pseudo elastic property-

One of the properties of memory wire is pseudo elasticity. Frames of reading gasses made by memory alloy can undergo deformations in high temperature state but comes back to the original shape when stress is removed.

Memory alloys are manufactured by casting using vacuum arc welding or induction heating. These processes are helpful in keeping the alloy mixed well. After that it is hot rolled and then drawn into wires. After that, the wire so formed is trained. Training is required to impart the shape which the wire would remember. This is done by heating the wire to a temperature below recrystallization temperature so that the dislocations in the crystal structure become stable. Then they are shaped in the hot state and then quenched in water or air. After this the wire can be deformed into any shape and then heated to retain the shape in which it has been trained.

They have lower yield strength than steel. Various compositions of memory alloys have yield strength higher than that of plastic and aluminum.

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