The shape memory effect of nickel-titanium foils

The shape memory effect of nickel-titanium foils is essentially the macroscopic manifestation of reversible phase transitions (martensitic transformation and reverse transformation) in their internal crystal structure at specific temperatures.

The specific implementation process can be divided into three core steps:
1. Set the "memory shape": At high temperatures (typically above the phase transition temperature Af point), the nickel-titanium plate is processed into the target shape (such as straight or with a specific curvature), at which point its crystal structure is stable austenite.

2. Low-temperature deformation: When the temperature is reduced below the phase transition temperature Ms point, the crystal structure of the nickel-titanium plate transforms into martensite. At this time, the material becomes soft and can be easily reshaped under external force (such as bending or twisting), and the deformed shape can be maintained at low temperatures.

3. Temperature recovery and shape recovery: When the deformed nickel-titanium plate is heated again above the Af point, the crystal structure reverts from martensite to austenite, and the material automatically returns to the "memory shape" set in step 1, while also regaining its high strength.
In simple terms, temperature changes trigger the crystal structure switch of nickel-titanium alloy from martensite to austenite. This reversible microstructure change ultimately enables the macroscopic nickel-titanium plate to exhibit the characteristic of "remembering its original shape and automatically restoring it".

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