Nickel titanium

Nickel titanium, also known as Nitinol (part of its name derived from Nickel Titanium Naval Ordnance Laboratory), is a metal alloy of nickel (Ni) and titanium (Ti) in roughly equal atomic percentages. Nitinol exhibits two closely related and unique properties: shape memory effect and superelasticity (also called pseudoelasticity), which make it extremely useful in various applications, most notably in medical devices and practices.

Composition and Properties

Nickel titanium is composed of approximately 55% nickel and 45% titanium. Small changes in the composition can significantly affect the properties of the alloy, including its transition temperature, which is the temperature at which the alloy changes its phase from austenite to martensite and vice versa. The alloy is biocompatible, highly resistant to corrosion, and capable of undergoing large deformation and returning to its original shape upon heating (shape memory effect) or upon unloading (superelasticity).

Shape Memory Effect

The shape memory effect is the ability of Nitinol to undergo deformation at one temperature, then recover its original, undeformed shape upon heating above its transformation temperature. This property is utilized in various medical devices, such as stents and dental braces, where the material is deformed, inserted into the body, and then returns to its original shape to perform its function.

Superelasticity

Superelasticity occurs at a narrow temperature range just above its transformation temperature, where the material can undergo large strains without permanent deformation. This property is particularly useful in applications requiring the material to absorb significant amounts of strain energy, such as in eyeglass frames and orthodontic archwires.

Applications

Nickel titanium's unique properties have led to its use in a wide range of applications, particularly in the medical field. Some of its applications include:

• Stents: Used in cardiovascular treatments to keep arteries open.
• Orthodontic appliances: Such as braces and archwires that apply gentle, continuous pressure on teeth to move them to the desired position.
• Surgical tools: Instruments that can be inserted in a compact form and then assume their functional shape at body temperature.
• Implantable devices: Including filters and grafts that can be inserted in a compact form and then expanded to their functional shape within the body.

Biocompatibility

One of the reasons Nitinol is so valuable in medical applications is its high degree of biocompatibility. The alloy's resistance to corrosion and its ability to remain inert within the human body make it an ideal choice for long-term implantation. However, concerns have been raised about nickel leaching and potential allergic reactions, which are areas of ongoing research.

Manufacturing and Processing

The manufacturing and processing of Nitinol are complex and require precise control of composition and mechanical working conditions to achieve the desired shape memory and superelastic properties. Techniques such as metal injection molding, laser cutting, and electro-discharge machining (EDM) are commonly used in the production of Nitinol components.

Future Directions

Research into Nickel titanium continues to explore new applications and improvements in its properties. Areas of interest include enhancing its fatigue resistance, developing new methods for processing and shaping the alloy, and expanding its use in non-medical fields such as aerospace and automotive industries.

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  Nickel_titanium

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  Austenite and martensite phases of Nitinol

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  Transformation hysteresis of Nitinol

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  Structure transformation in Nickel-titanium

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  Ms temperature vs Nickel content in Nitinol