Explore the Secrets of Superelasticity and Shape Memory of Nickel Titanium

Nickel titanium has become a rising star in the field of material research and application with its unique superelasticity and shape memory properties. Nickel titanium, a binary alloy composed of nickel and titanium, exhibits extraordinary performance beyond traditional materials due to its special atomic combination and microstructure. Shape memory is one of the most striking properties of nickel titanium. When the parent phase nickel titanium at a certain temperature is cooled, a phase transformation occurs to form a martensite phase. At this time, an external force is applied to the alloy in the martensite phase to deform it. When it is heated, the alloy will automatically return to its original shape in the parent phase as if it is endowed with memory, accompanied by the reverse phase transformation process. This heat-induced phase transformation process enables the alloy to accurately switch shapes under different temperature conditions to achieve specific functions.

Unimaginable elastic performance
Superelasticity is another amazing property of nickel titanium. Nickel titanium exhibits superelasticity in a narrow temperature range slightly above its transformation temperature. When an alloy is subjected to an external force and produces a strain far greater than its elastic limit strain, the strain can automatically recover at the moment of unloading, and the material can return to its undeformed shape without heating. In this process, the elastic limit of nickel titanium far exceeds that of ordinary materials and no longer follows the traditional Hooke's law. This superelasticity can be divided into linear superelasticity and nonlinear superelasticity. The stress-strain curve of linear superelasticity has a close linear relationship between stress and strain; while nonlinear superelasticity, also known as phase transition pseudoelasticity, is the result of stress-induced martensitic phase transformation and its reverse phase transformation during loading and unloading in a certain range above the Af temperature. The phase transition pseudoelasticity of nickel titanium can reach about 8%. It can apply a constant and gentle correction force to teeth at room temperature, and use its superelastic properties to adapt to the complex movement of teeth during the correction process, realize slow and precise movement of teeth, and improve patient comfort and correction effect. ​

The secret of atomic arrangement​
The reason why nickel titanium has such magical shape memory and superelastic properties lies in the mutual transformation of austenite and martensite phases in its microstructure. At higher temperatures, the alloy is in the austenite phase, where the atoms are arranged in an orderly and compact manner, forming a stable and elastic austenite crystal structure. When the temperature drops, the alloy gradually transforms into the martensite phase, and the atoms are rearranged into a more flexible structure. In the martensite state, the alloy is easy to deform. When subjected to external forces, the twin structure will evolve into a detwinned structure to adapt to the external stress. Once the external conditions change, such as the temperature rises, the alloy will transform from the martensite phase back to the austenite phase, and the atoms will return to the orderly arrangement state again, so that the alloy will return to its original shape. ​

Bright prospects for wide application​
The superelasticity and shape memory properties of nickel titanium make it show broad application prospects in many fields. In addition to the medical field mentioned above, in the aerospace field, it can be used to manufacture deformable wings, allowing the aircraft to adjust the wing shape in real time according to the flight speed and airflow conditions, improving flight efficiency and maneuverability; in the field of intelligent robots, drive components made of nickel titanium can achieve precise and flexible motion control, injecting new vitality into the intelligent development of robots; in daily life, nickel titanium can also be seen, such as super-elastic eyeglass frames, which can quickly return to their original shape after being squeezed and deformed by external forces, bringing convenience and comfort to people's lives. ​