How does nitinol wire work?

Nitinol wire is a type of shape memory alloy (SMA) that exhibits unique properties such as superelasticity and shape memory effect. These properties arise from the unique crystal structure of nitinol, which allows it to undergo a reversible phase transformation under certain conditions.

At high temperatures, nitinol wire exists in the austenite phase, which is a highly ordered crystalline structure. When the wire is cooled below a certain temperature, called the martensite start temperature, it undergoes a phase transformation to the martensite phase, which is a less-ordered crystal structure. In this phase, the wire can be easily deformed, and it will retain its new shape when the deformation is removed. However, when the wire is heated above the martensite finish temperature, it undergoes a second phase transformation back to the austenite phase, and it will return to its original shape.

Let us consider a 45-degree high-temperature nitinol wire as an example. When this wire is heated to its austenite phase temperature (above 500℃), it will straighten out completely. If it is then cooled to room temperature while being held straight, it will retain its straight shape because it is in the martensite phase. However, if the wire is bent or wrapped around a cylinder while in the austenite phase and then cooled to room temperature, it will retain its deformed shape. When the wire is heated to its austenite phase temperature again, it will return to its original straight shape.

Now, let us consider the scenario where the 45-degree nitinol wire is cooled to below its martensite finish temperature (around -10 to 0℃) while being bent or wrapped around a cylinder. In this case, the wire will retain its deformed shape even when it is heated to its austenite phase temperature because it has undergone a permanent deformation due to the martensite phase transformation. Therefore, the wire will not return to its original straight shape even at high temperatures.

Similarly, if the wire is bent or tied in a knot while in the austenite phase and then cooled to room temperature, it will retain its deformed shape even when it is heated back up to its austenite phase temperature. This is because the permanent deformation caused by the knot cannot be undone by the martensite phase transformation.

In conclusion, the working principle of high-temperature nitinol wire is based on its reversible phase transformation between the austenite and martensite phases, which allows it to exhibit superelasticity and shape memory effect. However, if the wire is subjected to a permanent deformation while in the martensite phase, it will not be able to recover its original shape even at high temperatures.