Nitinol Shape-Memory Alloy
Nitinol is the near-equiatomic nickel-titanium alloy — about 55–56 % nickel by weight — that switches between martensite and austenite within a few degrees of a transformation temperature set by its exact composition. Below that temperature it is a soft shape-memory metal that recovers up to 8 % strain on heating; above it, it is superelastic, springing back from 6–8 % strain where steel takes a permanent set at 0.5 %. It is 6.45 g/cm³, non-magnetic and biocompatible under its titanium-oxide skin.
Popular Nitinol grades
Compare in the explorer| Grade | Yield, MPa | Tensile, MPa | Density, g/cm³ | Machinability | Weldability | Best for |
|---|---|---|---|---|---|---|
| Nitinol | 400 | 950 | 6.45 | Not recommended | Fair | Superelastic or shape-memory Ni-Ti; medical devices |
All 1 Nitinol grades
Choose Nitinol for
Self-expanding stents, heart-valve frames and filters; guidewires, catheters and endoscopic instruments that must flex and recover; orthodontic archwire and root-canal files; eyeglass frames; thermal actuators, pipe couplings and release mechanisms; superelastic springs and dampers.
Look elsewhere when
The part is a spring within the elastic range of steel (17-7 PH, music wire or beryllium copper are far cheaper), it runs above about 100 °C where superelasticity fades, it needs stiffness (the austenite modulus is about 75 GPa), millions of cycles at high strain are expected (superelastic strain should stay below about 2–3 %), or it is a complex machined part — Nitinol is made as wire, tube, strip and sheet and finished by laser cutting, grinding and EDM, not milling.
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Nitinol by application
- Stents and implants
- Superelastic Nitinol tube per ASTM F2063 with Af around 15–25 °C, laser-cut, shape-set expanded, electropolished and passivated so the surface is titanium oxide rather than nickel.
- Guidewires and instruments
- Superelastic wire and tube (Af 5–20 °C) for cores, cannulas and graspers that bend around anatomy and return straight; ground to taper, joined to stainless by crimping or adhesive.
- Actuators and couplings
- Shape-memory Nitinol with Af set 20–30 °C above ambient: wire actuators that pull 4–5 % stroke when heated by current, shrink-fit pipe couplings and release pins that trigger at a chosen temperature.
Working with Nitinol
- Machining and cutting
- One of the hardest metals to machine conventionally: it work-hardens, springs back on the tool and grabs drills. Tube and sheet are laser-cut, wire is centreless-ground, and features are finished by EDM, waterjet or grinding; unavoidable turning runs sharp carbide at low speed and high feed with short tool life.
- Shape setting
- The final shape is set by constraining the part on a fixture at 500–550 °C for 5–20 minutes and water-quenching; the same treatment shifts Af upward by a few degrees, so it is verified by DSC afterwards. Cold work before shape setting gives superelastic wire its high plateau stress.
- Joining and finishing
- Nitinol laser- or resistance-welds to itself; to stainless steel it forms brittle intermetallics, so devices are crimped, swaged, bonded or joined through a nickel interlayer. Medical parts are pickled, electropolished and passivated (ASTM F86) to a smooth titanium-oxide surface that improves fatigue life and lowers nickel release.
Other nickel alloy series
Nitinol Shape-Memory Alloy — frequently asked
What is Nitinol?
Nitinol is the near-equiatomic nickel-titanium alloy — about 55–56 % nickel by weight — that switches between martensite and austenite within a few degrees of a transformation temperature set by its exact composition. Below that temperature it is a soft shape-memory metal that recovers up to 8 % strain on heating; above it, it is superelastic, springing back from 6–8 % strain where steel takes a permanent set at 0.5 %. It is 6.45 g/cm³, non-magnetic and biocompatible under its titanium-oxide skin.
Which grade of Nitinol should I use?
One alloy, two ways to specify it, and the austenite-finish temperature (Af, measured by DSC per ASTM F2004) is the whole specification. Superelastic Nitinol has Af below the working temperature — typically 5–25 °C for a device that must be superelastic in the body at 37 °C — and is what stents, guidewires, archwire and eyeglass frames use, with a loading plateau around 400–550 MPa and 1,000–1,200 MPa tensile strength. Shape-memory Nitinol has Af above the working temperature (40–90 °C) so the part stays deformed until heated; it is the actuator, coupling and thermal-valve grade. Medical material is bought to ASTM F2063 with Af on the certificate.
Superelastic or shape-memory — how do I specify Nitinol?
By the austenite-finish temperature. If Af is below the working temperature the alloy is austenitic in service and superelastic — it bends up to 8 % and springs back, which is what a stent at 37 °C needs; medical grades are usually set at 5–25 °C. If Af is above the working temperature the part sits in soft martensite, holds whatever shape it is bent to, and snaps back to its heat-set shape when warmed past Af — the actuator behaviour. Order to ASTM F2063 with Af stated and tested by DSC.
Can Nitinol be machined by conventional CNC?
Only reluctantly. It work-hardens instantly, its superelasticity makes it spring away from the cutting edge, and the chips weld to carbide, so turning and milling run at very low speeds with short tool life and poor finish. Almost every Nitinol product is made instead from wire, tube or strip by laser cutting, centreless grinding, EDM and photochemical etching, then shape-set and electropolished; conventional machining is kept for simple turned features on large sections.