Why does nitinol have shape memory?

Nitinol is a shape memory alloy,it is a special alloy, at a certain temperature can automatically restore its plastic deformation to its original shape. Its expansion rate of more than 20%, fatigue life of up to 1×10 to 7 power,damping characteristics are 10 times higher than ordinary springs, corrosion resistance is better than the best medical stainless steel, can meet a variety of engineering requirements, is the best choice for medical applications.

Nitinol grades

• TiNi-01 Phase change temperature: 20℃-40℃
• TiNi-02 Phase change temperature: 45℃-90℃
• TiNi-SS Phase change temperature: 5℃-15℃
• TiNi-YY Phase change temperature:33℃±3℃
• TiNiCu phase change temperature: As-Ms≤5℃
• TiNiNb Phase change temperature:As-Ms

Nitinol is a binary alloy consisting of nickel and titanium. Due to changes in temperature and mechanical pressure, there are two different crystal structure phases, namely Austenite phase and martensite phase.

The phase transition sequence of Nitinol cooling is the parent phase(Austenitic phase)-R phase-martensite phase. R phase is a rhombic body, when the temperature is high or under external force state, austenite is cubic shape and hard，the shape is relatively stable.

The martensitic phase is the state when the temperature is relatively low (less than Mf: the temperature at the end of the martensitic) or when it is activated by an external force. It has a hexagonal shape, is malleable,repeatable, unstable, prone to deformation.

Shape memory is when the parent phase of a shape is cooled from above the Af temperature to below the Mf temperature to form martensite, martensite is deformed at a temperature below the Mf temperature, and the material will be heated below the Af temperature from its shape in the parent phase.

In fact, the shape memory effect is the thermally induced phase change process of Nitinol.