Nitinol

Nitinol is roughly equal parts nickel and titanium, and it does two related things no ordinary metal does. Deformed while cool it can be returned to a remembered shape by warming it. And at body temperature it is superelastic: it can be strained perhaps ten per cent and spring back completely, where a good spring steel manages under one per cent.

Both come from a reversible transformation between two crystal structures. Deformation moves the boundaries between the two rather than moving dislocations permanently, and the transformation reverses, so the shape returns. The composition has to be controlled to within a fraction of a per cent, because the transformation temperature moves by around ten degrees for every tenth of a per cent of nickel — which is why the alloy is difficult and expensive to make.

The superelasticity is what most applications actually use, and it is the property that made the self-expanding stent possible: a device compressed into a catheter and released to open an artery by itself.

Melted under vacuum, because oxygen and carbon form inclusions that both weaken the alloy and shift the transformation temperature. Shape setting is a heat treatment — the component is constrained in the desired geometry and held at around 500 °C, and that is the shape it will remember.

Machining is poor for the same reason titanium's is, and worse. Most nitinol components are laser cut from tube or sheet, which is exactly how a stent is made, and then electropolished to remove the recast layer and leave a passive titanium oxide surface.

Self-expanding vascular stents, which is the dominant use by value. Guidewires that follow a vessel without kinking. Orthodontic archwires, which apply a gentle constant force over a wide range of movement instead of a large force that fades. Bone staples and surgical instruments.

Outside medicine: eyeglass frames that survive being sat on, actuators and thermal switches, aerospace couplings that are fitted cold and grip as they warm, and vibration damping.

Found in 1959 by William Buehler at the US Naval Ordnance Laboratory — the name is Nickel Titanium Naval Ordnance Laboratory — while looking for a fatigue-resistant nose cone material. The shape memory effect was noticed later, and its most-repeated demonstration came at a laboratory management meeting where someone applied a pipe lighter to a folded strip and it straightened itself.

Medical use took until the 1990s, and needed both the manufacturing control to hit the transformation temperature reliably and the regulatory evidence that a nickel-rich alloy is safe implanted — which it is, because the surface is titanium oxide and the nickel does not reach the tissue.