Formgedächtnislegierungen

Formgedächtnis, commonly translated as shape memory in materials science, describes the ability of certain materials to return to a predefined shape after deformation when exposed to a specific stimulus, typically heat. The effect is most prominently seen in shape memory alloys (SMAs), with nickel-titanium (NiTi, sold as Nitinol) being the best studied example. Other SMAs and shape memory polymers also exhibit related behavior.

The core mechanism involves a thermoelastic martensitic transformation between two solid-state phases: a high-temperature, symmetric austenite phase and a low-temperature, less symmetric martensite phase. When deformed in the martensitic state, the material stores elastic energy. Heating induces a transformation back to austenite, releasing the stored energy and restoring the original shape. Some systems can be trained to exhibit a two-way shape memory, enabling repeated shape changes with each thermal cycle, while most materials show a one-way effect, returning to their remembered shape only after heating.

Materials besides SMAs include shape memory polymers and magnetic or other externally triggered variants. Each system uses different stimuli and offers different performance characteristics, such as recovery stress, actuation temperature, and fatigue resistance.

Applications are broad and growing. Medical devices use NiTi components for stents and guidewires; aerospace and automotive sectors employ shape memory actuators and couplings; robotics and consumer electronics explore compact, silent actuation. Advantages include large recoverable deformations and smooth, quiet operation; limitations involve narrow temperature operating windows, fatigue life, processing challenges, and, in some NiTi systems, biocompatibility considerations due to nickel content. Continued materials research aims to expand operating ranges, durability, and cost-effectiveness.