Shape Memory NiTi spring
Let’s take a look at this Shape Memory NiTi spring in action. First, let’s examine the Stress-Strain-Temperature graph. Initially the crystal structure is in a zig-zag arragement (twinned martensite). The material forms twins along symmetry lines. When the alloy deforms in cold state, a temporary deformation is set and the zig-zag setup is stretched to a diagonal arragement. This is the detwinned martensite. The twins transform into a new tilted structure.
As a result, the crsytal structure is ready for the shape memory effect. Applying heat, makes the detwinned martensite transforms again into a regular, straight and cubic form and therefore, the remembered shape is set back again. This is the austenite, which is the strongest arragement of the alloy. During this state, the material is stronger, has a higher elacticity modulus and deformations are recoverable up to 8%.
Finally, when the material gets cold again, it returns to the initial twinned martensite, finishing and completing the cycle.
We can check this on the next video. On screen, there are two glasses of water. Hot on the left and cold on the right. This spring is a Nickel, Titanium and Copper alloy. The percentage of Copper in the alloy is 5% and the rest is Nickel and Titanium. Wire diameter is 1mm and outer diameter is 10mm. It deforms in cold state and when inserted in hot water, it recovers its original shape. Cold water helps to speed up the cooling process of the Ni-Ti alloy.
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