Using Heat Treatment and Thermal Shape Memory to Tailor Nitinol to Your Application

Shutterstock |  Alexey Kamenskiy

The shape-memory alloy Nitinol is in great demand for use in medical devices. Each device needs the Nitinol wire in order to exhibit specific properties and shapes. Using heat treatment and shape-setting processes, Fort Wayne Metals can offer Nitinol wires with enhanced properties and customizable shapes – with regards to its resistance to mechanical stress or fatigue performance, for example.

Nitinol is a “smart” material made up of almost equal atomic ratios of titanium (Ti) and nickel (Ni). Initially developed in 1961 at the U.S. Naval Ordnance Laboratory (NOL), Nitinol is considered the most well-known example of shape-memory alloys.1 Due to the shape memory effect, it is possible to deform Nitinol at one temperature and return it to its original shape when heated to its particular transformation temperature.2

Besides temperature change, the application of mechanical stress can also temporarily deform Nitinol – a phenomenon known as superelasticity. Superelasticity and the shape memory effect are the result of a reversible phase transition of the material between a monoclinic martensite and a cubic austenite structure.2

Nitinol’s shape memory and superelastic properties allow it to be used in a wide range of applications in the aerospace, medical, consumer technology, telecommunications and automotive industries.3 For instance, in the medical field, Nitinol is used for vascular guidewires, stents and embolic protection filters.4 In particular applications, such as the self-expanding stent, the alloy is delivered into the body in a compact shape where it changes into its original shape, triggered by body temperature.

Shape-Setting Produces Custom Shapes

Fort Wayne Metals matches the shape requirements of particular device designs by offering customized Nitinol wires using shape setting.5 The Nitinol wire is wound around a fixture or mandrel and developed into the desired ‘parent’ shape in the shape-setting process. This is followed by heat treating the wire and fixture to about 500°C and then quenching in a water bath.1

The shape set wire will display the shape memory and superelastic properties indicative of Nitinol when it is removed from the fixture. The material rearranges to its martensite phase when cooled and can then be deformed as desired. When the Nitinol is heated to its transition temperature, it readopts to the earlier set parent shape.

For instance, shape setting is utilized for arch wires in dental braces. Shape set wires in the martensitic condition are deformed and inserted into the patient’s mouth. When warmed by the patient’s body temperature, the wire will then adapt to the right shape.1 Moreover, the parent shape is mostly manufactured to be somewhat smaller so that it exerts a small pressure on the teeth.

Heat Treatment Enhances Nitinol’s Properties

Usually, heat treatment and thermomechanical processes can change the parent shape of a Nitinol wire, and can also improve other properties. For example, low-temperature heat treatment is capable of reducing the residual internal strains in austenite during superelastic deformation.6 A material’s microstructure can also be refined to nanoscale in order to increase their ability to resist high cycle mechanical loading.7

On a microscopic level, heat treatment can modify the micro-mechanical behavior of the Nitinol wire by altering the texture of the martensite and austenite structures.6 This enhances the material’s fatigue performance, which allows it to cycle the wire thousands, if not millions, of times – a property that is important for wire used in life saving medical devices.

Specialized and Customizable Nitinol Wire

The properties of Nitinol can be optimized to match a specific application by using heat treatment and shape-setting techniques. The product range of Fort Wayne Metals features different customizable and specialized Nitinol wires, such as:

  • Silk® Nitinol has undergone treatment resulting in an oxide-free, ultra-smooth wire surface.8 Silk® Nitinol is of specific interest for stent braiding operations in which the smooth surface permits the filers in the braid to glide past one another effortlessly, preventing fractures and snags during processing.
  • DPS® wire or “Dynamic Plateau Strength” wire is manufactured using a special thermomechanical straightening process that produces a perfectly straight wire and approximately a 30% increase in plateau strength than the standard Nitinol wire.9 This highlights the fact that the wire will feel stiffer in applications in which the material is strained to >1%. This is mainly useful in guidewire applications where superior stiffness and straightness enables the Doctor to have better control when guiding the wire through a tortuous path in the body.
  • USN® wire is treated to exhibit increased stiffness at tensile strains smaller than 1%.10 The wire is optimal for applications where increased initial column strength is required, as it shows a great resistance to being bent initially.
  • Niti#1-DFT® is a composite of Nitinol, wrapped around another core material, such as platinum, silver or tantalum. In order to merge the superelasticity and strength of the Nitinol outer material and the conductivity, resiliency, radiopacity or MRI enhancement of the core.11 For instance, tantalum and platinum cores are used to improve the visibility of the wire in X-ray fluoroscopy, and silver cores are utilized for pacemaker leads due to their exceptional high cycle fatigue, high flexibility and low electrical resistance.7

Thermomechanical processes such as shape setting and heat treatment can customize the properties and shape of Nitinol, matching the requirements for different non-medical and medical applications.

References

  1. AZO Materials (2015) The Properties and Applications of Nitinol Wire, available at: http://www.azom.com/article.aspx?ArticleID=11852 (assessed: 25/05/2017).
  2. Fort Wayne Metals (2017) http://www.fwmetals.com/materials/nitinol/ (assessed at: 25/05/2017).
  3. Stebner et al. (2011) Neutron diffraction studies and multivariant simulations of shape memory alloys: Empricial texture development-mechanical response relations of martensitic nickel-titanium, Acta Materialis 59, 2841-2849.
  4. AZO Materials (2015) Manufacturing of Nitinol Wire for the Medical Industry: an Interview with David Plumley, available at: http://www.azom.com/article.aspx?ArticleID=11685 (assessed: 25/05/2017).
  5. Fort Wayne Metals (2017) http://www.fwmetals.com/materials/nitinol/nitinol-shapesetting/ (assessed: 25/05/2017).
  6. S. Cai et al (2014) Effect of heat treatment temperature on nitinol wire, Appl. Phys. Lett. 105, 071904; doi:10.1063/1.4893595.
  7. AZO Materials (2016) Smart Wire for Next Generation Medical Devices, available at: http://www.azom.com/article.aspx?ArticleID=12706 (assessed: 25/05/2017).
  8. Fort Wayne Metals (2017) Silk® Nitinol, http://www.fwmetals.com/materials/nitinol/silk-nitinol/ (assessed: 25/05/2017).
  9. Fort Wayne Metals (2017) DPS® wire, http://www.fwmetals.com/materials/nitinol/dps-nitinol/ (assessed: 25/05/2017).
  10. Fort Wayne Metals (2017) USN® wire, http://www.fwmetals.com/materials/nitinol/usn-nitinol/ (assessed: 25/05/2017).
  11. Fort Wayne Metals (2017) Niti #1-DFT, http://www.fwmetals.com/materials/nitinol/niti-1-dft/ (assessed: 25/05/2017).

This information has been sourced, reviewed and adapted from materials provided by Fort Wayne Metals.

For more information on this source, please visit Fort Wayne Metals.

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