Nitinol is an intelligent material with shape memory effect and superelasticity. It is an alloy composed of two elements, nickel (Ni) and titanium (Ti). It is a typical shape memory alloy with viscoplasticity and superelasticity. Its memory effect is manifested as the shape propped up by external force, and the shape recovery is completed by the inherent restoring force, while its superelastic effect shows that it still has good deformation ability within the stress range exceeding its elastic limit, and its rebound degree can reach more than 100%.
What does the superelasticity of nitinol have to do with?
First of all, the superelasticity of Nitinol alloy is related to its inherent structure. It is composed of two elements, nickel and titanium, and its crystal structure is cubic. There are two phases at high temperature: austenite and martensite. When it is cooled, the austenite phase will transform into the martensite phase, forming a memory effect. When it exceeds its Austenitizing Transformation Temperature, the martensite phase will transform into the austenite phase again, and the material will return to its original state. This transformation is caused by the change of spin angular momentum of electrons in the metal. Therefore, this memory effect and superelasticity are one of the unique characteristics of Ni-Ti.
Secondly, Nitinol memory alloy is a crystal structure with a crystallographic hierarchy. It is composed of microscopic lattice subcrystal units. Its microstructure is the basis of its material mechanics and constitutive properties, and is affected by factors such as phase transition temperature, crystal structure, and lattice distortion, which makes it have good superelasticity. When we apply force to the alloy, its interlaced microstructure that captures dislocation distortion provides the material with inherent high elasticity. Therefore, its microstructure has a very important influence on its superelasticity.
Finally, the superelasticity of nitinol material is also affected by the crystal structure. Because the alloy crystal has a strange two-phase structure, this structure enables it to still show excellent deformation ability when it exceeds the Kleeble elastic limit of the material, and its rebound degree can reach more than 100%, and the crystal structure will change differently with different processing processes. Therefore, its superelasticity is closely related to its crystal structure.
Therefore, the superelasticity of nitinol alloy is closely related to its inherent structure, and it has excellent mechanical properties and memory effect. Its lattice structure, crystal structure, crystal defects and other factors have an important influence on the superelasticity of the material. Through structural regulation and surface engineering, its superelastic performance can also be further improved. Therefore, studying the relationship between the inherent structure and superelasticity of nickel-titanium alloy is the key to improving the superelasticity of nitinol.
