Shape memory alloy as a technology – what’s behind it?
functions in modern technological applications thanks to their special effects. For example, they can advantageously substitute existing technologies or enable functions that cannot be implemented with conventional solutions under given boundary
conditions. Their ability to return to a defined shape after an apparently plastic deformation characterizes the shape memory alloys. A distinction is made based on the physical quantities that cause a shape memory effect:
• Mechanical: superelasticity / pseudoelasticity
• Thermal: two-way effect
• Thermomechanical: one-way effect
All shape memory effects are based on a solid state phase transformation, more precisely described, the martensitic phase transformation. A martensitic phase transformation proceeds diffusionless in a shear of the crystal lattice. Based on the nature of the phase transformation, the low-temperature phase in shape memory alloys, just as known in steels, is called
martensite. The high-temperature phase is called austenite. The different effects that result from the solid state phase transformation enable it to be used for technical systems in an intelligent way that is adapted to the respective need. At Ingpuls, nickel-titanium-based shape memory alloys, also known as Nitinol, are primarily produced and used.
NiTi-SMA are especially characterized by their high possible effect paths/forces, corrosion resistance and biocompatibility compared to other SMA.
Pseudoelasticity / Superelasticity
Pseudoelasticity, or superelasticity, describes the ability of a metallic material to be subjected to elastic/reversible strains that are orders of magnitude above the elastic strains that a conventional steel can assume. In the case of pseudoelasticity, the material is in its high-temperature phase, austenite, at the application temperature. The component has thus already assumed its defined shape. If it is subjected to mechanical stresses, the material undergoes stress-induced transformation to martensite at a critical point. In the process, martensite variants are formed that are oriented favorably to the direction of stress. The formed “twinned martensite” provides the effect path, which macroscopically manifests itself in a large strain.
When the material is unloaded, this converts back to austenite. Since austenite only permits lattice modification, the material reshapes itself into the original shape in the course of the unloading and associated transformation. The behavior is comparable to that of rubber. Pseudoelasticity is used primarily in medical technology. For example, stents made of pseudoelastic SMA can be guided through microcatheters due to their large elastic stretching capacity. When the stent reaches the vessel to be supported, the catheter is withdrawn and the stent can fully expand and support the vessel wall. Here, even after many years, it survives the cyclic stress of each heartbeat.
In the one-way effect, a material is apparently plastically deformed in its low-temperature phase and then returns to its original shape in the course of heating. The low-temperature phase martensite is characterized in SMA by the presence of certain crystallographic regions within the martensite grains, so-called twins. If a stress is applied to a component as a martensitic phase, twin variants favorable to the stress direction grow at the expense of other variants above a critical stress. In this case, the material can be deformed at a low stress level. In the high-temperature austenite phase, there is only one way in which atoms can arrange themselves in the lattice.Thus, when heated above a critical temperature, the material reshapes into its original shape and remains in this shape after cooling to the low-temperature phase without the influence of a mechanical stress. Therefore, to take advantage of this effect in cyclic applications, an external mechanical stress is needed to deform the component into a different geometry when cold. This is known as the extrinsic two-way effect.
The extrinsic two-way effect is used, for example, in thermostatic valves. Here, an SMA compression spring works against a spring made of a conventional steel. When cold, the steel spring applies a force high enough to compress the SMA compression spring. When warm, the SMA spring “remembers” its longer shape and thus applies a force that can compress the steel spring. Thus, with proper design, this effect can be applied for many 100,000 cycles. The temperatures that produce the effect are adjustable by alloy composition, microstructure and the design layout.
Are you interested in our SMA webinar?
Build in-depth knowledge specifically for your area of application.