How Does Nitinol’s Shape Memory Work?

Nitinol is a metal alloy of nickel and titanium, where the two elements are present in roughly equal atomic percentages. It has some amazing properties that make it useful for many applications, such as medical devices, robotics, and aerospace engineering.

One of these properties is the shape memory effect, which means that nitinol can remember and return to its original shape after being deformed by an external force.

Another property is super elasticity, which means that nitinol can withstand large strains without permanent deformation.

In this article, we will explain how nitinol’s shape memory works, and what are the factors that affect it.

What is the shape memory effect?

The shape memory effect is the ability of nitinol to undergo deformation at one temperature, stay in its deformed shape when the external force is removed, then recover its original, unreformed shape upon heating above its “transformation temperature“. 

This phenomenon is related to the crystal structure of nitinol, which can change between two different forms: austenite and martensite.

Austenite and martensite

Austenite and martensite are two different ways that the atoms of nitinol can arrange themselves in a regular pattern, called a lattice.

Austenite is the high-temperature form of nitinol, where the atoms form a cubic lattice. Martensite is the low-temperature form of nitinol, where the atoms form a tetragonal lattice, which is slightly distorted from the cubic shape.

The transformation between austenite and martensite is reversible, and depends on the temperature and stress applied to the material.

When nitinol is heated above a certain temperature, called the austenite finish temperature (Af), it transforms completely from martensite to austenite.

When nitinol is cooled below a certain temperature, called the martensite start temperature (Ms), it transforms completely from austenite to martensite. 

Between these two temperatures, nitinol can exist in a mixture of austenite and martensite.

How does the shape memory effect work?

The shape memory effect works by exploiting the difference in shape and size between austenite and martensite.

When nitinol is in its austenite form, it has a well-defined shape and size, which is called its parent shape. This is the shape that nitinol remembers and returns to when heated.

When nitinol is in its martensite form, it can be easily deformed by applying a small amount of stress, which changes the orientation of the tetragonal lattices.

This is called twinned martensite. The deformed shape of nitinol is called its trained shape. This is the shape that nitinol stays in when the stress is removed, as long as it is below the transformation temperature.

To demonstrate the shape memory effect, we can follow these steps:

  1. Heat nitinol above its Af temperature, so that it becomes fully austenitic. This is its parent shape.
  2. Cool nitinol below its Ms temperature, so that it becomes fully martensitic. It still retains its parent shape, but it is more flexible and can be bent or twisted easily.
  3. Deform nitinol into a different shape, such as a coil or a hook. This is its trained shape. The martensite lattices are reoriented, but they are still connected to each other.
  4. Remove the stress that caused the deformation. Nitinol stays in its trained shape, as long as it is below the transformation temperature. The martensite lattices are locked in place by the surrounding atoms.
  5. Heat nitinol above its Af temperature again. The martensite lattices revert to their original orientation, and nitinol transforms back to austenite. Nitinol recovers its parent shape, and the deformation is erased.

What are the factors that affect the shape memory effect?

The shape memory effect of nitinol depends on several factors, such as the composition of the alloy, the processing and treatment of the material, and the environmental conditions. Some of these factors are:

  • Composition: The ratio of nickel and titanium in nitinol affects the transformation temperatures and the mechanical properties of the alloy. For example, increasing the nickel content lowers the transformation temperatures, but also reduces the strength and ductility of nitinol.
  • Processing: The way that nitinol is melted, forged, rolled, drawn, or annealed affects the microstructure and the crystal orientation of the alloy. These factors influence the shape memory effect and the superelasticity of nitinol. For example, cold working nitinol increases the strength and the stability of the martensite phase, but also increases the stress required to deform it.
  • Treatment: The way that nitinol is heated, cooled, or aged affects the phase transformation and the recovery of the parent shape. For example, heating nitinol above its Af temperature for a long time can improve the shape memory effect, but also increase the risk of oxidation and corrosion.
  • Environment: The temperature, humidity, and chemical exposure of nitinol affect the performance and the durability of the alloy. For example, high temperatures can accelerate the degradation of the shape memory effect, while low temperatures can increase the brittleness of nitinol.

Conclusion

Nitinol is a remarkable metal alloy that exhibits the shape memory effect and superelasticity. These properties are related to the reversible transformation between austenite and martensite, two different crystal structures of nitinol.

The shape memory effect allows nitinol to remember and return to its original shape after being deformed, while super elasticity allows nitinol to withstand large strains without permanent deformation.

The shape memory effect of nitinol depends on several factors, such as the composition, processing, treatment, and environment of the alloy. Nitinol has many applications in various fields, such as medicine, robotics, and aerospace engineering.

FAQs

How is nitinol pronounced?

Nitinol is pronounced as “night-in-all”, with the emphasis on the first syllable.

What are some examples of nitinol applications?

Some examples of nitinol applications are:

  • Medical devices: Nitinol is widely used in medical devices, such as stents, catheters, orthodontic wires, surgical instruments, and implants. Nitinol’s shape memory and super elasticity allow these devices to adapt to the body’s anatomy and physiology, and to perform complex functions with minimal invasiveness.
  • Robotics: Nitinol is used in robotics, such as actuators, sensors, grippers, and artificial muscles. Nitinol’s shape memory and super elasticity enable these components to mimic the movement and flexibility of natural systems, and to operate with high efficiency and precision.
  • Aerospace engineering: Nitinol is used in aerospace engineering, such as wings, flaps, valves, and antennas. Nitinol’s shape memory and super elasticity allow these structures to change shape and function in response to temperature and stress, and to reduce weight and complexity.

How can nitinol be trained to have a different parent shape?

Nitinol can be trained to have a different parent shape by applying a shape-setting process, which involves heating, deforming, and cooling nitinol in a controlled manner. The shape-setting process creates a new austenite phase with a different lattice orientation, which becomes the new parent shape of nitinol. The shape-setting process can be repeated to create multiple parent shapes for nitinol, which can be accessed by changing the temperature.

How can nitinol be recycled or disposed of?

Nitinol can be recycled or disposed of by following the appropriate methods and regulations for metal waste. Nitinol can be recycled by melting and reprocessing it into new products, or by recovering the nickel and titanium elements from it. Nitinol can be disposed of by incinerating it at high temperatures, or by burying it in landfills. However, nitinol should not be disposed of in water or soil, as it may release toxic substances or cause environmental damage.

What are the advantages and disadvantages of nitinol?

Some of the advantages of nitinol are:

  • Shape memory and superelasticity: Nitinol’s shape memory and superelasticity enable it to perform complex and adaptive functions, and to withstand large deformations without permanent damage.
  • Biocompatibility and corrosion resistance: Nitinol’s biocompatibility and corrosion resistance make it suitable for medical and biological applications, and for harsh and humid environments.
  • Strength and durability: Nitinol’s strength and durability make it resistant to fatigue, wear, and fracture, and extend its service life.

Some of the disadvantages of nitinol are:

  • High cost and difficulty of processing: Nitinol’s high cost and difficulty of processing limit its availability and accessibility, and require special equipment and skills to handle it.
  • Sensitivity to temperature and stress: Nitinol’s sensitivity to temperature and stress affect its performance and reliability, and require careful calibration and control of the operating conditions.
  • Potential health and environmental risks: Nitinol’s potential health and environmental risks include the release of toxic substances, the generation of heat and sparks, and the interference with magnetic fields.
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Last modified: February 10, 2024

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