Most technical people know that if you tangle a sample Nitinol wire and then hold it under warm water, it almost jumps back into its original shape. But did you know that the demo wire was alloyed for water temperatures of about 50°C and the temperature is adjustable?

“For medical-device applications, Nitinol is commonly used in wire and small tubes — generally items less than 11/42-in. diameter. It's rare to see demands on large plates of Nitinol,” says Ming Wu, Vice President Technology at Memry Corp., Bethel, Conn., (memry.com).

The material's so-called shape-memory effect comes from its reversible solid-state phase transformation from austenite to martensite on cooling (or by deformation) and the reverse, going from martensite to austenite on heating (or upon release of its deformation). Martensite and austenite phases are particular atomic arrangements, often call solid solutions.

Designers should be aware of several significant transformation temperatures. For instance, a material starts changing from austenite to martensite when it's cooled to its martensite start temperature, M_s, and finishes when cooled to its martensite finish temperature, M_f. Likewise, going from martensite to austenite begins on heating at the austenite-start temperature, A_s, and finishes when at the austenite-finish temperature, A_f. In cold worked and heat-treated materials, an R-phase transformation may come before the austenite-martensite transformation. Transformation temperatures in R-phase materials phase are defined in ASTM F2005.

Superelasticity is Nitinol's second notable characteristic. “It describes a nonlinear yet recoverable deformation above the A_f temperature and stems from stress-induced martensitic transformation on loading and the reversion of the transformation on unloading,” says Wu. Nitinol can recover (change back to an original shape) a strain as high as 8% with little residual deformation. Although heat-treated Nitinol alloys have nonlinear superelasticity, cold-worked alloys recover from strains only as high as 3.5% with little plastic deformation. Superelasticity simplifies medical-device design because a recovery can be triggered by removing a mechanical constraint. Hence, the part needs no heat source. Most Nitinol medical devices are alloyed to trigger the superelastic property at body temperature. When the R-phase transformation is present at temperatures close to body temperature, the Nitinol part feels softer at small deformations because it lowers the apparent modulus before the onset of superelastic stress plateau.

Other material characteristics include thermal and mechanical hysteresis. For binary Nitinol alloys (about 50-50 by atomic weight of nickel and titanium), thermal hysteresis typically has a 30 to 40 Celsius-degree span while mechanical hysteresis spans 30 to 50 Ksi. Hysteresis, like shape memory, can be manipulated by alloying Nitinol. For instance, adding copper (creating ternary material) reduces the thermal-hysteresis width to as little as a 15-degree span. Niobium in a ternary, on the other hand, increases it to as high as 120 degrees.

Stents are a natural medical application for Nitinol because the compressed-wire tube meshes can be triggered to expand by superelasticity or by body heat. “The material also works well in dental drills because of its flexibility and low modulus of elasticity. And its shape-memory characteristics can also be used as an actuator,” says Wu.

Commercial Nitinol alloys are prepared by either a vacuum-induction melting (VIM) followed by vacuum-arc melting (VAR), or by a multiple VAR process. Materials prepared by VIM-VAR tend to have more uniform distributions of transformation temperatures along the ingot but with higher carbon content picked up from the graphite crucible used for processing. Multiple VAR ingots have less carbon but are more varied in their distribution of transformation temperatures.

To set the material temperature, the Nitinol part must be held in the required shape and heat-treated, usually to around 500°C. The length of heat treatment varies with equipment used and thermal mass of the shaping fixture. In a molten salt bath, for example, heat treatment times are generally between two and five minutes.

Lastly, if a Nitinol part must be attached to another, it's best crimped and swaged. Cyanoacrylate and epoxy adhesives also work well. The shape memory effect of the alloy can also effectively connect two parts. Soldering presents problems but can be done with proper flux. Welding by TIG, laser, e-beam, and plasma can also fix Nitinol to itself.

Nitinol by the numbers

A few typical physical characteristics include

Density	6.45 to 6.5 g/cm3
Electrical resistively	76 × 10-6 Ohm-cm in martensite 82 × 10-6 Ohm-cm in austenite (Thermal variations in resistivity come from the composition and thermo-mechanical processing.)
Thermal expansion coefficient	In martensite: 6.6 × 10-6/ °C In austenite: 11 × 10-6/°C
Thermal Conductivity	18 W/m°K
Feet of wire/lb	For round wires: L= 0.4515/D2 where L = length, ft; and D = wire dia., in. For rectangular wire: L = 0.3546/(tw) where t = thickness, in., and w = width, in.