What is Nitinol?

Along with, “What is Truth?” this is one of life’s greatest questions.  According to ASTM, nitinol is a nearly equiatomic mixture of nickel and titanium.  In plain English, this means that nitinol is made up of a nearly equal number of nickel and titanium atoms.  By weight, this works out to roughly 55% nickel/45% titanium.

But, what makes nitinol the most exciting material on the planet is the shape memory effect (SME) and superelasticity (SE).  Shape memory means that the material can be deformed and, all that is required is a bit of heat to restore it to the original shape.  SE means that it can withstand very large deformations without permanent deformation.  Putting it into context, stainless steel breaks at 0.5% elongation, but superelastic nitinol can recover over 50% elongation under the right conditions.  Similarly, shape memory nitinol can recover 12% elongation.

What causes SME/SE?

Both superelasticity and shape memory stem from very similar phenomena.  Nitinol has two crystal structures, known as martensite and austenite, which occur at different temperatures.  The crystal structure of martensite resembles a herringbone shape.  This is known as twinning[i].  Interestingly, the twinned bonds are able to rotate about each other, giving martensitic nitinol the appearance of being highly malleable (soft).  Austinite, however, is a rigid cubic structure that gives the appearance of a stiff, elastic substance.  So, if no bonds are broken when the martensite is deformed, when the nitinol is heted and transformed into austenite, the shape is restored and this is how the shape memory effect works.

As was stated above, superelasticity comes from a very similar phenomena as the shape memory effect.  First off, superelasticity occurs in austenitic nitinol when the austenite transforms into martensite when a force is applied to it.  This form of martensite is known as stress-induced martensite (SIM).  The martensite then allows the nitinol to undergo very large deformation.  When the force is removed from the nitinol, it transforms back into austenite, snapping back into its position.

Along with the radically high deformation recovery is the fact that nitinol has the highest actuation force of any technology readily available.  Pneumatics exert approximately 120psi, hydraulics exert roughly 3,000psi, but nitinol exerts an astounding 25,000 psi!  This means that, by using nitinol, you can pack an incredibly powerful actuator into an extremely small space.

What does this mean to me? 

  • by using nitinol, you can pack a large system into a small space
  • by using nitinol, you can create motion that isn’t possible otherwise
  • by using nitinol, you can replace whole systems, increasing reliability

Learn more about nitinol at www.KelloggsResearchLabs.com

[i] Twinning and Diffusionless Transformations in Metals by E. O. Hall, 1954.

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