Titanium nickel alloy, also known as Nitinol, is a binary alloy composed of nickel and titanium. The atomic percentages of these two elements are approximately equal.
Austenitic phase and martensite phase
Due to changes in temperature and mechanical pressure, Nitinol has two different crystal structure phases, namely an austenite phase and a martensite phase. In Nitinol, austenite is called the mother phase, which is the crystalline phase of the alloy at high temperatures. When the temperature decreases, austenite will gradually transform into martensite (child phase).
Two characteristics of Nitinol
When the mother phase of a certain shape cools from above the Af temperature to below the Mf temperature and completely forms martensite, shape memory occurs, thereby deforming the martensite below the Mf temperature. After heating below the Af temperature, the material will automatically recover its parent phase shape by reverse phase transition. In fact, the shape memory effect is a thermally induced phase transformation process of nitinol. It refers to the ability of Nitinol to deform at a certain temperature and then restore its original, undeformed shape when the temperature is higher than its “transition temperature”.
The so-called superelasticity refers to the phenomenon that the strain generated by the sample under the action of external force is much greater than the elastic limit strain, and the strain will automatically recover during the unloading process. In the parent phase, due to the effect of external stress, the strain triggers the martensite transformation, so the alloy exhibits different mechanical properties from ordinary materials. Its elastic limit is much larger than ordinary materials. And it no longer follows Hooke& #39;s law. Compared with the shape memory effect, superelasticity does not involve heat.
Application of Nitinol
1. Glasses frame
3. Medical equipment
4. Other applications
A well-known computer manufacturer used Nitinol equipment to eject PCMCIA (International Personal Computer Memory Card Association) cards. In addition, Nitinol is also used to connect the end of the aircraft hydraulic tubing. In less severe applications, nitinol can bend the magic shop spoon when it is placed in hot water.