What are shape memory alloys?
Alloys based on shape memory technology are metallic materials that can remember their original shape after an apparently 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, since 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 suited for applications in
actuator technology, but also for components that must exhibit large reversible deformation (e.g. guide wires or catheters in medical technology). Reversible strains of between 6-8% can be achieved here. The reason for this behavior is a solid-state phase transformation in the metal between two crystal structures, designated martensite (low-temperature phase) and austenite (high-temperature phase). Depending on the alloy composition, ambient temperature and stress condition, different effects can
occur, which can be intelligently used for technical systems in different ways. For example, SMA springs can automatically open or close valves when the temperature of the ambient medium (air, water, oil) exceeds a certain value, the austenite start temperature. Tension wires can realize moderate travel at 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.
How are shape memory alloys characterized?
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: These effects are, 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 phase austenite.
Extrinsic two-way effect
In the extrinsic two-way effect, the initial shape of the material – after an apparent 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 the initial position, the material is initially in the low-temperature phase. Here, a martensitic metal structure prevails. You can now deform the material up to a certain limit in all spatial directions, e.g. bend or stretch it. If you now heat the material, a phase transformation takes place in the material itself above a certain temperature threshold: The martensitic microstructure transforms into austenitic microstructure. The material is therefore now in the high-temperature phase. This structural transformation from martensite to austenite takes place
at the microstructural level and is invisible to the eye. And it is precisely these microstructural processes that take place that cause the material to deform back to its original state. Even if you then allow the material to cool down again, it still retains the shape of its initial state. And you can repeat this process millions of times – depending on the material composition and the degree of deformation – under ideal conditions in such a way that the shape memory material visibly does not fatigue. However, fatigue always occurs in reality. If the component is properly designed and integrated into a system, operation can be reliably guaranteed. The name shape memory alloy derives 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, which we will not go into here for reasons of simplicity and lack of technical relevance (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, SMA can also exhibit another property. This is called pseudoelasticity. The decisive factor is the temperature level prevailing in the material. This results, for example, from the environment or the operating condition.
Pseudoelasticity (also “pseudoelastic behavior” or “superelasticity”)
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 little force. All this with strengths that you know from metals. In the case of pseudoelasticity, you do not want to change the shape to the initial state by increasing the temperature. In the case of pseudoelastic shape memory alloys, the material is designed in a completely different form: It is then modified from the alloy in such a way that austenitic microstructure is already present at room temperature (ambient temperature, operating temperature) – actually the high-temperature phase. In this sense, we are dealing
with shape memory material with a two-way effect, which has very low transformation temperatures. The material at hand has therefore already exceeded its transformation temperature at the prevailing temperatures. That is already the trick! Now you could cool the element below room temperature and produce martensitic microstructure as in 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 the following: The deformation recedes again almost completely and your initial shape is restored. The pseudoelastic effect of shape memory alloy is only possible because the austenitic phase is stabilized at room temperature. And just like the one-way effect, this pseudoelastic deformation can be repeated millions of times, depending on the design.
What are the properties of shape memory alloys?
Shape memory alloys (SMA) are based on the alloying elements nickel (Ni) and titanium (Ti). These are referred to as binary NiTi-SMA. The designation binary comes from the fact that the alloy is composed of these two (binary = two) components. The following material parameters refer to this binary SMA.
Pseudoplastic SMA: Pseudoplastic (thermally activatable) SMAs achieve -strengths of more than 1200 MPa [N/mm²] and -elongations at break of more than 10% in tensile tests.
Pseudoelastic SMA: The material parameters of SMA exceed those of the normal tensile test. In the tensile test at room temperature, pseudoelastic SMA exhibit mechanical hysteresis. Thereby, an upper plateau stress can be recognized. This is above 380 MPa [N/mm²]. For shape memory alloys, the transformation temperatures are crucial. These are the temperatures at which a phase transformation between martensite and austenite begins or ends.
Your introduction to SMA
The better you understand the basic properties of shape memory alloys or Nitinol, the more likely you are to recognize and filter out the best of your ideas and concepts as such. We invite you to explore the topic of SMA and discover your new potentials. You will find the information you need in the following sections (Characterisation, Properties, Manufacturing, Quality Assurance, Application, Webinar) or else in our SMA Course as PDF.