Nitinol specification guidelines

Nitinol specifications

Nitinol is a family of shape memory alloys, which are not completely described using typical engineering properties. In concert with ASTM Standards, it is important that the nitinol specification contain sufficient information to adequately meet the needs of the product for which it is being produced without over specifying properties that are not appropriate.

ASTM standards

ASTM International, in conjunction with the nitinol community, has issued several standards that aid in specifying requirements for nitinol.

• View ASTM Standards

Product form specifications

Current commercially available nitinol forms include: sheet, tube, wire and ribbon, and components fabricated from each product form.

Nitinol sheet

Nitinol sheet is one of the newest, and most exciting product forms. It is available in rolled thicknesses down to 0.0007", widths up to 3.75" (based on thickness), and lengths up to 18” (based on thickness and width).

Typical applications include laser cutting and/or photochemical etching to manufacture complex components.

Nitinol tubing

Johnson Matthey is a leader in large diameter, thin wall, and microlumen tube. Outer diameters as large as 0.325" with wall thicknesses as thin as 0.004” are now available. Microlumen Flextube® with outside diameters as small as 0.0095" can be produced with inside diameters of 0.006”. For most applications, the wall thickness should be chosen to be greater than ten percent of the outer diameter to avoid buckling.

Typical applications include laser cut stents, endoscopic guide tubes, and distal protection devices. Commercial availability is limited to O.D. to I.D. ratios from 1.1 to 1.8.

Components

Many unique nitinol components are fabricated into a wide variety of applications today. These include ground shape set guidewire cores, stents, blood filters, orthodontic arches, surgical instrumentation, dental clips, trocar-pointed rods, photochemically etched stents, and shaped helical forms.

Transformation temperature(s)

Nitinol is a simple binary mixture of nickel and titanium at about 50 atomic percent each (about 55 percent by weight of nickel). However, subtle adjustments in the ratio of the two elements make a large difference in the properties, particularly the transformation temperatures, i.e., the temperatures at which the crystal structure changes from austenite to martensite or vice versa. The sensitivity of the transformation temperature to composition is so great that chemistry is not used to specify the alloy. Instead, transformation temperature is the most accurate means to specify the alloy. The temperature most frequently specified for the finished product is the Active Austenite Finish Temperature, Active A(f). This is determined using ASTM F 2082, commonly called the bend free recovery test. Typical tolerances for Active A(f) are +/- 5 C. For superelastic materials the Active A(f) must be below the product use temperature.

For shape memory materials, the Active A(f) determines the completion of the shape recovery transformation upon heating. The transformation temperatures change due to mechanical processing and annealing, therefore the Active A(f) will be different than the transformation temperature of the original ingot. In most applications, specifying the transformation temperature of the final product, Active A(f), is sufficient; however, the transformation temperature of the original ingot may be specified. The ingot transformation temperature is determined by DSC per ASTM F 2004. Typical tolerances for the Ingot A(p) are +/-5 C. Below is a list of the typical materials offered by Johnson Matthey.

* Common alloy name for reference only. Please specify Active A(f).

Superelastic nitinol

This product form takes advantage of the stress-induced martensitic transformation to achieve incredible amounts of flexibility, strain recovery, and kink resistance. Nitinol behaves superelastically if the Active A(f) temperature is below its use temperature. Applications that are intended to be superelastic at room temperature are generally produced with an Active A(f) temperatures below room temperature in the range of 0 C to 20 C. A superelastic material will remain superelastic up to a temperature from the Active A(f) to a temperature about 50 C above Active A(f). Therefore a material with an Active A(f) of about 15 C will exhibit good superelasticity up to about 65 C. Commonly used super elastic materials are N, S, and C. Alloy C contains a small amount of Chromium which increases the strength of the upper and lower plateau stresses. Please contact Johnson Matthey for more information regarding the use of Alloy C.

Shape memory nitinol

This product form exhibits the ability to recover a shape upon heating above Active A(f). Therefore, the most critical property to specify is the Active A(f). This represents the finish of the transformation from martensite to austenite upon heating, and therefore the temperature at which the shape recovery is also complete. The start of the transformation upon heating is the Austenite Start Temperature, A(s), and is about 15 C to 20 C lower than the Active A(f).

Upon cooling, there are comparable start and finish transformation temperatures for the reverse transformation from austenite to martensite. These are known as M(s) and M(f), respectively. The M(f) temperature is about 15 C to 20 C lower than M(s). There is a hysteresis in the transformation, meaning that the transformation to martensite upon cooling is below the temperature at which the martensite reverts to austenite upon heating. For binary shape memory materials, the difference between M(p) and A(p) is 25 C to 50 C. There is a peak in the transformations from austenite to martensite, and martensite to austenite, and this information is captured as A(p) and M(p) temperatures during DSC testing per ASTM F2004.

Thermomechanical condition

Nitinol is processed by hot working followed by a cold working with complete annealing cycles between cold working steps. The final two processing operations are cold working a precise amount followed by a low temperature heat treatment. For cold drawn wire and rolled ribbon, the final cold work is typically 30 to 50 percent. For FLEXTUBE® and sheet, the final cold work is generally less, but is usually greater than 20 percent. Sheet is typically cold worded greater than 20 percent after the last full anneal. The specific cold work level is determined by the mechanical property requirements and processing limitations. After cold working, the nitinol undergoes a heat treatment to bring out the superelastic or shape memory properties and to achieve the proper balance of final mechanical properties. Active A(f) values can only be determined after this final heat treatment.

The specific terms which may be used to specify nitinol are:

• Cold worked (as drawn or as rolled) - nitinol that has not yet undergone the final strain anneal. This condition is usually specified in cases where the end user intends to perform a shape set strain anneal. It is important to note that in this condition nitinol does not exhibit superelastic or shape memory properties. In that this material has not been heat treated, it does not possess an Active A(f).
• Super elastic strain annealed (SESA) - The term is used to describe materials that have been heat treated to be fully superelastic at room temperature and are straight. Strain annealed materials can be spooled on typical wire spools without taking a permanent set. For guidewires, the material may be specified further to be whip free.
• Flat strain annealed - like straight annealed, but for sheet products that have been held flat during final heat treatment.
• Shape set strain annealed - material that has been formed into a shape, constrained and heat-treated to permanently set a shape.

Surface condition

Nitinol forms a natural oxide during processing which is also used as a lubricant carrier. The oxide is primarily TiO₂. For as-drawn wire and ribbon, the oxide is carefully controlled and is generally a light amber brown oxide or shiny black. For drawn FLEXTUBE, the oxide generally ranges from dark blue to gray. If desired, chemical pickling, mechanical polishing, or centerless grinding can remove the oxide.

Mechanical properties

Depending on the final application, it may be necessary to specify some mechanical properties. These may include ultimate tensile strength (uts) and elongation to failure. For superelastic alloys loading plateau, unloading plateau, and residual plastic strain (permanent set) may also be specified. Typical values for superelastic nitinol are given in the table below:

* All properties are measured at room temperature. The loading and unloading plateaus for straight annealed materials will be approximately 10 KPSI to 20 KPSI higher at 37 C.

** Properties for FLEXITUBE and sheet may be different.