Superelasticity and Shape Memory of Nitinol

MIRAMAR, Fla. - Jan. 11, 2021 - PRLog -- Nitinol has two characteristics: shape memory effect (SME) and superelasticity (SE).

Shape memory
Shape memory happens when mother phase of a certain shape is cooled from above the Af temperature to below the Mf temperature and completely forms the martensite, deforming the martensite below Mf temperature. After being heated to below Af temperature, with reverse phase transformation, the material will automatically restore its shape in mother phase. In fact, the shape memory effect is a thermally induced phase transition process of nitinol. It refers to ability of nitinol to deform at a certain temperature and then restore the original, non-deformed shape when the temperature is higher than its "transition temperature".

Superelasticity
The so-called superelasticity refers to the phenomenon in which the sample produces a strain that is far greater than the elastic limit strain under the effect of external forces and the strain can automatically restore during unloading. In mother phase, due to the effect of external stress, the strain triggers martensitic phase transition so that the alloy exhibits mechanical behaviors that are different from those of ordinary materials. Its elastic limit is much larger than that of ordinary materials. And it no longer follows Hooke's Law. Compared with shape memory effect, superelasticity does not involve heat.
Nitinol (https://www.edge-techind.com/Products/Refractory-Metals/T...) is an aromatic or near-equiatomic intermetallic compound of titanium and nickel.