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What are Shape Memory Alloys?

Shape Memory Alloys

Shape Memory Alloys

Shape Memory Alloys, Nickel-Titanium alloys, NiTi or simply Nitinol, are a very special kind of metals able to recover its original shape after very large deformations, usually larger than conventional materials.

What is NiTi? Stress-Strain

Initially, the alloy’s crystal structure arranges in zig-zag position, forming twins along symmetry lines. This is the twinned martensite. When the alloys stresses, this mesh deforms to a detwinned martensite. This phase is stable and the material is ready for the shape memory effect. Once heat is applied, the alloy gets in a higher energy level state or austenite. The mesh changes again to a stronger ortogonal arragement. As soon as the material gets cold again, the crystal structure shifts to the initial zig-zag shape, finishing the cycle.

NiTi Shape memory effect

On this video, you can see a SMA spring in action. The Nickel-Titanium element is holding the weight of a tool. When is cold, the springs stretches and the tool goes down due to the force of gravity. When is hot, the SMA is stronger than the weight of the tool, therefore it goes up. This is just a simple example of how Shape Memory Alloys can be used. The rest of potential applications are up to your imagination. In your particular project, you can design your own mechanism.

The heat source can be electricity (Joule effect), hot water or liquid, gas, etc. To return the mechanism to the initial position, you can use gravity, another spring, material stiffness, etc. You can also use the heat of the sun or the difference between day and night as a way to activate/deactivate your alloy/actuator.

The actuation is fast, reliable and silent. This is especially important in applications where you want to save weight or cannot make any sound.

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