Shenzhen Wuge Industrial Co., Ltd

The principle of nickel titanium shape memory alloy involves the interaction between crystal structure changes, temperature, and stress. The editor will provide a detailed introduction from the following aspects:

1、 Fundamentals of Crystal Structure

(1) Two key phases

Austenitic phase (high temperature phase): At higher temperatures (above the phase transition temperature), the atoms are arranged neatly, and the crystal structure is a body centered cubic (B2) or more complex body centered tetragonal (B19 ') structure, with high symmetry and stiffness.

Martensitic phase (low-temperature phase): When the temperature decreases (below the phase transition temperature), the crystal structure transforms into a monoclinic or triclinic structure, the symmetry decreases, the atomic arrangement becomes looser, and it is easy to deform under external forces.

(2) Phase transition characteristics: The transition between two phases is a diffusion shear free process, where atoms do not undergo long-distance migration and only undergo structural transformation through lattice distortion, which is the core basis of shape memory effect.

2、 The Implementation Process of Shape Memory Effect

1. High temperature shaping (initial shape setting)

In the austenite phase temperature range (such as above the austenite end temperature Af), external force is applied to the alloy to shape it (such as bending into a specific shape), and the alloy maintains its austenite structure, with the shape fixed to the "target shape".

2. Low temperature deformation (temporary shape assignment)

Cooling to the martensitic phase temperature range (such as below the martensitic end temperature Mf), the alloy transforms into a martensitic structure. At this point, when external force is applied, the martensitic crystal can undergo significant deformation through "twinning deformation" (internal shear of the crystal), forming a "temporary shape" (such as being straightened).

Key feature: The twinning deformation of martensite is reversible. After the external force is removed, the crystal structure may partially recover due to internal stress, but the macroscopic shape remains in a temporary state.

3. Heating recovery (shape memory implementation)

When the temperature rises to the austenite transformation temperature range (beyond the austenite starting temperature As), the martensitic phase begins to transform into the austenite phase.

Due to the characteristic of "memory" of the initial shape in the crystal structure of austenite phase, the atomic arrangement returns to the state of solidification, driving the macroscopic shape to gradually return to the target shape at high temperature solidification, until the temperature exceeds the austenite end temperature Af and fully recovers.

3、 Principle of hyperelasticity (pseudo elasticity)

Phenomenon: When subjected to high stress at room temperature (in the austenite phase), the alloy can undergo significant deformation. After removing the stress, it quickly returns to its original state, with a deformation amount of over 8%, far exceeding the elastic limit of ordinary metals.

Principle:

Stress induced martensitic transformation: Under stress, the austenite phase at room temperature will locally transform into martensite phase (known as "stress-induced martensite"), achieving large deformation through the twinning deformation of martensite.

After stress removal, the martensitic phase spontaneously reverses to the austenitic phase due to the temperature being higher than the martensitic transformation range, the deformation disappears, and the shape is restored.

4、 Application and advantages

In the medical field, such as vascular stents (which restore shape and expand blood vessels at body temperature) and orthodontic wires (which apply sustained force under temperature changes).

Aerospace: Pipeline connection fasteners (installed at low temperatures and tightened after heating up).

Daily life: eyeglass frame (heated and restored after deformation), temperature control element (temperature sensitive switch).

Advantages: Large deformation, high recovery accuracy, good fatigue resistance, and strong reversibility of phase transformation process, which can be reused tens of thousands of times.

5、 Influencing factors and optimization

Composition ratio: The atomic percentage of nickel in nickel titanium alloys is usually around 50%~51%, and the precise ratio affects the phase transition temperature and properties.

Heat treatment process: Annealing, quenching and other processes can adjust grain size and phase transition temperature, optimize shape memory effect and superelasticity.

Alloying modification: Adding elements such as copper and niobium can adjust the phase transition temperature range and expand the application scenarios.

The principle of nickel titanium shape memory alloy is essentially temperature/stress-induced martensitic austenite transformation and its reverse process, which achieves macroscopic shape "memory" and recovery through reversible changes in crystal structure. This unique physical property gives it irreplaceable advantages in multiple fields, and precise control of phase transition temperature and crystal structure is the key to optimizing its performance.