Shape memory alloy

Shape memory alloys (SMAs) are alloys that, depending on the temperature, return to their original shape following deformation.

This is caused by transformation from the low-temperature phase (martensite) to the high-temperature phase (austenite). Transformation can be triggered thermally or through mechanical stress.

A distinction is made between:

• The one-way memory effect, which modifies the material's Microstructure in the low-temperature phase (twinned martensite) in such a way during deformation that the martensite is detwinned and stretched. Following stress relief and heating, the Material returns to its high-temperature phase (austenite) and is twinned again during cooling.
• The two-way memory effect enables a material to return to two different shapes, one in the high-temperature range and one in the low-temperature range.
• Pseudoelasticity (superelasticity) is a kind of mechanical material memory. Following deformation, stabilised austenite can only return to its original state (martensite) if the stress is relieved without introducing any heat.
• In the case of magnetic shape memory alloys, the change in shape is brought about magnetically.

Since only a small amount of energy is required to trigger the shape memory effect, such materials are used, amongst other things, as actuators due to their ability to transfer large forces. Many of them exhibit virtually no fatigue, even after substantial movement cycles, but some residual elongation may remain following a large number of cycles.