What are Shape Memory Alloys (SMA) / Nitinol?

SMA are metallic materials which are able to return to their original shape after an apparently plastic deformation. Often, they may be referred to as memory metals, shape memory metals or even as shape memory metal alloys. Technically speaking, these terms may be considered incorrect since shape memory alloys constitute an alloy and not a pure metal. In shape memory technology, in particular alloys based on nickel and titanium (NiTi, Nitinol®) are mostly relevant. Due to their functional properties, these shape memory alloys are ideal for actuated applications, as well as for parts and components required exhibit high reversible deformations (e.g. guidewires or catheters in medical technology). Reversible strains between 6-8% can be achieved here. The reason for this behaviour is a phase transformation within the solid state of the metal, occurring between two crystal structures: One being martensite (low temperature phase), the other being austenite (high temperature phase). Different effects can occur depending on the alloy composition, the ambient temperature and the state of stress. These can be used intelligently in different ways for technical systems. Due to the functional properties of shape memory alloys, they constitute an excellent option for many fields: Be it applications in actuator systems, or as components where the property of large reversible deformation is required (e.g. guidewires or catheters in medical technology). For example, SMA springs are capable of automatically opening or closing valves as soon as the temperature of the ambient medium (air, water, oil) exceeds a certain value, namely the austenite start temperature. Tension wires can achieve moderate actuation paths while at the same time providing very large actuating forces. Under certain conditions (wire diameters < 1.2-1.4mm) they are suitable for being heated directly via ohmic resistance of the wire, thereby being controlled by special electronics.

You can find more information in our SMA course as PDF or on the following pages: