The shape memory alloy: What’s behind it?

Alloys based on shape memory technology are metallic materials that can remember their original shape after an seemingly plastic deformation. The terms memory metal, shape memory metal or even shape memory metal alloy are also widely used. Strictly speaking, however, these are not correct, as shape memory materials are alloys and not pure metals. Alloys based on nickel and titanium (NiTi, Nitinol) are particularly relevant in shape memory technology. Due to their functional properties, shape memory alloys are ideally for applications in actuator technology, but also for components which must be

capable of large reversible deformations (e.g. guide wires or catheters in medical technology) where reversible strains between 6-8% can be achieved. At this point we should take a closer look at the technology behind shape memory alloy, what exactly is behind it? The cause of this behavior is a solid-state phase transformation in the metal between two crystal structures, which are called martensite (low-temperature phase) and austenite (high-temperature phase). Depending on the alloy composition, ambient temperature and stress condition, different effects

can occur, which technical systems can benefit from in various intelligent ways. SMA springs, for example, can automatically open or close valves when the ambient medium (air, water, oil) exceeds a certain temperature, the austenite start temperature. Tension wires can realize moderate travels with very large actuating forces. Under certain circumstances (wire diameter < 1.2-1.4mm), they are suitable for heating with special control electronics directly via the ohmic resistance of the wire.

When you observe the shape memory effect, the ambient or operating temperature plays an essential role. Because depending on how this behaves in comparison to the properties of the alloy of the SM element, you can mainly observe two different effects: On the one hand the extrinsic two-way effect and on the other hand the so-called superelasticity.

From a physical point of view, both effects involve a phase transformation within the material. This is reversible and characterized by diffusionless shearing of the crystal lattice. The transformation takes place between a low temperature phase and a high temperature phase. The low-temperature phase is

called martensite, the high-temperature is known as phase austenite.

In the extrinsic two-way effect, the initial shape of the material – after a seemingly plastic deformation by an external force (e.g. a steel spring or a weight) – is restored by heating. You should distinguish between a low-temperature phase (= initial position) and a high-temperature phase (heated state). In its initial position, the material is in the low-temperature phase where a martensitic metal structure prevails. You can now deform the material up to a certain point in all spatial directions, e.g. bend or stretch it. If you now heat the material above a certain temperature, a phase transformation takes place in the material itself: The martensitic structure transforms into an austenitic structure. The material is now in the high-temperature phase. This structural transformation from martensite into austenite takes place on a microstructural level and is invisible to the eye.

And it is precisely these microstructural processes that cause the material to return to its original state. Even when the material has been allowed to cool down again, it will remain in its original shape. And you can repeat this process a millions times – depending on the material composition and the degree of deformation – under ideal conditions the shape memory material will not be seen to fatigue. However, some form of fatigue will always occur at some point. If the component is properly designed and integrated into a system, however, operation can be reliably guaranteed. The name shape memory alloy comes from the observed shape memory effect.The initial state of the material thus always reflects the shape that has been written into the material’s “memory”.

This is where it wants to deform back to when heated. In addition, a so-called intrinsic two-way effect can also be observed, but for reasons of simplicity and lack of technical relevance we will not go into this. (usable strains and stresses are too low). Depending on your requirements, you can freely determine the “memory” of the material in advance. This is done by a specific heat treatment. Instead of the extrinsic two-way effect, however, shape memory alloys can also have another property called pseudoelasticity. The decisive factor is the temperature level prevailing in the material which derives from the environment or the operating condition.

A pseudoelastic SMA element behaves like a rubber. When you hold a pseudoelastic SMA element in your hands, you can bend it like rubber with very little effort. All this with strengths that you know from metals. In the case of pseudoelasticity, you do not want to achieve a shape change back to the initial state by increasing the temperature.  The material of pseudoelastic shape memory alloys is designed in a completely different way: It is then modified by the alloy in such a way that an austenitic microstructure is already present at room temperature (ambient temperature, operating temperature) – technically the high-temperature phase.

To a certain extent it is a shape memory material with a two-way effect with very low transformation temperatures. The material at hand has therefore already exceeded its transformation temperature at the prevailing temperatures. And that is the trick! Now you could cool the element below room temperature and produce martensitic microstructure as in the extrinsic two-way effect. However, the purpose of pseudoelastic shape memory alloys is different: Instead of cooling, you can mechanically deform the material.  A transformation to a martensitic structure then takes place solely as a

result of your mechanically applied deformation. If you now relieve the element mechanically again, you will observe that the deformation recedes almost completely and the original shape is restored. The pseudoelastic effect of shape memory alloy is only possible because the austenitic phase is stabilized at room temperature. And just as with the one-way effect, this pseudoelastic deformation can be repeated millions of times depending on the design.