1. Atomic - level mechanism

The shape - memory effect is based on a reversible phase transformation between two different crystal structures. At a lower temperature, the alloy is in a martensitic phase, which is relatively soft and can be easily deformed. When the alloy is heated above a certain temperature (the transformation temperature), it reverts to an austenitic phase, and it returns to its original shape. This phase transformation is a diffusion - less transformation that involves the rearrangement of atoms. For example, in nickel - titanium (Ni - Ti) alloys, the atoms shift their positions during the phase change, enabling the material to recover its shape.

2. Properties    

Superelasticity: In addition to the shape - memory effect, SMAs also exhibit superelasticity. This means that the material can undergo large deformations (up to about 8 - 10%) and still return to its original shape when the applied stress is removed. This property is due to the stress - induced martensitic transformation.

Good biocompatibility: Many SMAs, especially Ni - Ti alloys, have excellent biocompatibility. They are corrosion - resistant and do not cause significant immune responses in the body.

1. Medical field

Stents: As mentioned earlier, SMA - based stents are widely used in cardiovascular medicine. They can be inserted into narrowed blood vessels in a compressed state and then expand to their original shape as they reach body temperature. This helps to keep the blood vessels open and improve blood flow.

Orthodontic wires: Ni - Ti wires are used in orthodontics. They can exert a constant, gentle force on teeth over a long period. Their superelastic property allows them to provide the necessary corrective force as the teeth gradually move to their desired positions.

Surgical tools: Some surgical instruments are made using SMAs. For example, endoscopic graspers can be designed to have a shape - memory effect. They can be inserted through a small incision in a deformed state and then regain their original shape to perform the intended function inside the body.

2. Aerospace and automotive industries

Aerospace: In aerospace applications, SMAs can be used for morphing structures. For example, the shape of an aircraft wing could potentially be adjusted using SMA components to optimize aerodynamic performance during different flight conditions such as take - off, cruise, and landing.

Automotive: They can be used in actuators for various functions. For example, in engine components, SMA - based actuators could help control the flow of fuel or air by changing their shape in response to temperature changes.

3. Consumer products

Eyeglass frames: Some eyeglass frames are made from SMAs. They can be bent and deformed without breaking and will return to their original shape.

Smart textiles: There is research into incorporating SMAs into textiles to create clothing that can change its shape or fit according to the wearer's body temperature or other environmental factors.