Shape memory alloys (SMA)
are metallic materials which are able to return to their original shape after an apparently plastic deformation. Through this phenomenon – ideal conditions provided – between 6 and 8% of reversible elongation can be achieved. 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 will find technical details and backgrounds in our SMA Academy for deepening your knowledge.
What are shape memory alloys?
Shape memory alloys (SMA) are special metals capable of regaining their original shape after large deformation; hence “memorizing” their shape. 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).
Properties of SMA
Shape memory alloys based on NiTi feature an extremely large and reversible deformability (100x larger than steel), exhibit excellent structural and functional properties, possess high corrosion resistance and offer good biocompatibility. A further notable feature lies in the high damping capacity (in this case as pseudo-plastic alloys). This type of SMA display a mechanical behaviour comparable to materials in the human body and are therefore frequently used as implant materials.
Applications of SMA
Due to their extraordinary properties, technical applications of SMA can already be found in many areas today. While the thermally actuated (pseudo plastic) shape memory is used for actuators, inter alia in aerospace, pseudo-elastic SMA are mainly used in medical technology. In medical tech, the shape memory alloys’ property of high flexibility proves advantageous, along with their good corrosion resistance and biocompatibility. In medical technology, in addition to their widespread use as stents SMA, they are applied as guidewires in minimally invasive surgery and as orthodontic wires for tightening and tensioning dental braces. Other applications are spectacle frames, vibration damping elements and solid joints.
Shape memory actuators
Actuators in shape memory allow replacing established systems and developing new products with improved properties. These products can be lighter, more intelligent, autonomous and even more cost-effective by the use of SMA. Compared to the existing solutions, SMA technology often offers the possibility of using a considerably smaller space while providing equal forces or stroke. This means that along with with fewer required components (less complicated mechanisms and electrical control), the system’s weight can also significantly be reduced. Additionally, SMA actuators’ energy consumption is usually lower since the electrical power, as in numerous conventional systems, does not have to be provided permanently.
In general, actuator systems based on shape memory offer numerous benefits:
- large stroke and forces
- low weight
- high performance to weight ratio
- high volume specific working capacity, high energy density → compact, powerful actuators
- intrinsic sensor (with thermal activation)
- high energy absorption and damping capacity