Shape Memory Alloys - An Overview

Background

Shape memory alloys are characterised by their ability to return to their original shape after heating to their transformation temperature after having been deformed. This is known as the shape memory effect and is caused by a change in the crystalline structure during the transition from the martensitic phase to the austenitic phase. It gives these materials attractive actuation capabilities.

Shape memory alloys have a high power to weight ratio (up to ten times that of conventional actuation systems) and in the martensitic phase they can withstand large amounts of recoverable strain (up to 8%). When heated to above their transition temperature they can exert high recovery stresses of up to 700MPa which can be used to perform work. On the downside, they are relatively inefficient (less than 10%), having a slow response speed (predominantly dictated by the need for cooling) and are relatively complex to control due to inherent non-linearities and hysteresis in the shape memory effect.

The most common shape memory alloy is Nitinol and alloy of nickel and titanium.

Key Properties

        The ability to return to their original shape after being deformed and heated to their transition temperature

        High power to weight ratio

Applications

Medical Applications

Nitinol devices can be used to catch blood clots in veins. They are chilled so that they collapse prior to insertion. As the wire warms to body temperature, it returns to its functional shape and anchors itself in place. Similar devices can be used to open up restricted tubules. Shape memory patches for heart repairs have also been developed. The medical industry is seen as a large potential market for shape memory alloys.

Other Applications

Shape memory alloys have enjoyed commercial success in the following applications:

        Orthodontics

        Spectacle frames

        Electrical connections

        Pipe and tube jointing systems

        Temperature control

        Brassiers

        Medical Applications

Source: Materials World

For more information on Materials World please visit The Institute of Materials

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