ELECTRICAL AND ELECTRONICS ENGINEERING..!!: Shape Memory Alloys:

Shape memory alloys (SMA) are special alloys which, after being deformed at some relatively low temperature and will return to their original shape when it is subjected to the appropriate thermal procedure.
Shape memory alloys can be plastically deformed at some relatively low temperature and will return to their original shape prior to the deformation at some higher. This amazing behaviour is called shape memory effect.

Types of shape memory effect:

One way shape memory.

Two way shape memory.

Certain materials exhibit shape memory effect only upon heating and they are referred to as a one way shape memory. Materials which also exhibit shape memory effect upon recooling are referred to as having two-way shape memory.

Structure of SMA:

The shape memory alloy is easily deformed into any shape and easily remembers the shape before it was deformed respectively at low and high temperatures. It may display two distinct crystal structures or phases.

Martensite

Austenite.

At low temperatures, the crystal structure of SMA is called Martensite. At higher temperatures, the crystal structure of SMA is called Austenite.

    The shape memory alloy is easily deformed into any shape and easily remembers the shape before it was deformed respectively at low temperatures.

Characteristics of SMA:

    The properties of SMA depend upon the amount of each crystal phase present. SMA, can easily be deformed when it is in Martensite phase. Similarly, it can recover its shape when return to Martensite transformation, SMA yields a thermo elastic Martensite and develops from a high-temperatures austenite phase with long range order. Such transformation occurs over a range of temperature and not a single temperature. The range of temperature varies with each alloy system.

    Even though the transformation during heating and on cooling actually extends over much larger temperature range, most of the transformation occurs over a relatively narrow temperature only. The transformation also exhibits hysteresis in that the transformations on heating and on cooling do not overlap.