Nitinol—ageneric trade name for a Ni-Ti alloy, which stands for Nickel (Ni), Titanium (Ti) and Naval Ordnance Laboratory (NOL) where the alloy was discovered in the early 1960s.

Standard Terminology for Nickel-Titanium Shape Memory Alloys((see ASTM F2005 for further details)

3.1 Transformation temperatures are typically determined by Differential ScanningCalorimetry (DSC) which measures the heat flow between the NiTi specimen andthe environment in reference to that of an inert reference as a function of temperature (ASTM F2004).

3.2 Active transformation temperatures can be determined by Bend and Free Recovery (BFR) tests which trace the shape recovery as a function of temperature (ASTM F2082).

3.3 One important drawback tothe DSC method is that tests on partially cold worked materials, such as those
used to optimize superelasticity, can yield poor, inconclusive results. Thissame drawback also may apply to samples which have undergone a heat treatmentin the range of 400 to 600 deg.C following cold working. The Constant Load or Active Af tests are recommended for material in these conditions. Fullyannealed DSC results are often used as the basis for NiTi raw materialselection since they effectively characterize the baseline properties of the material prior to cold working and heat treatment.

Alsotermed “pseudoplasticity”, superelasticity describes a nonlinear recoverabledeformation behavior of NiTi alloys at temperatures above the A_ftemperature, which arises from the stress-induced martensitic transformation on

loading and the spontaneous reversion of the transformation upon unloading. Anatomic model in Figure 1 depicts the mechanism. A transformation-induced strainup to 6% is recoverable. When deformation exceeds 6% strain, the materials canfurther extend the deformation via linear elasticity of the stress-inducedmartensite. A total strain as high as 8% is therefore recoverable. Figure 2 exemplifies a superelastic stress-strain curve of NiTi alloy.

NiTialloys after an apparent deformation in the martensitic phase have the abilityto recover their original shape upon heating through the phase transformationtemperature range above the A_f temperature. Figure 3 depicts anatomic model illustrating the mechanism of shape memory effect while thesequence of temperature change, deformation and shape recovery associated withthe phenomenon is described in Figure 4.

BinaryNi-Ti Alloy

As is 15-20℃lower than Af,

Mf is 15-20℃lower than Ms,

Mp is 25-50℃lower than Ap,

Af is 40-70℃higher than Mf,

Md is 25-50℃higher than Af .

Md>Af>Ap>As>Ms>Mp>Mf

Beijing GEE SMA Technology Co., Ltd.’ Ternary NiTiNb Alloys Transformation Temperature

Optimal TemperatureRange (FullyAnnealed)

Ms=-90°C, Mf=-110℃,As:-55°C, Af=-35℃

Operating TemperatureRange

As'=60°C, Af’=80°C(-60°C, 16% deformation)

As'=70°C, Af’=82°C(-60°C, 18% deformation)

As'=72°C,Af’=89°C (-60°C, 20% deformation)

Mostof the studies on NiTi fatigue are strain-controlled. From this perspective,the fatigue resistance for NiTi is orders of magnitude higher than that of anylinearly elastic material. A typical fatigue limit at 10⁷ cycles isabout 0.5% in outer fiber strain in rotary bending fatigue. Increasing meanstrain up to 4% appeared to extend fatigue endurance. Extending mean strainbeyond 4%, the fatigue characteristics of NiTi follow a strain-based Goodmanrelationship. Fatigue life generally decreases with increasing test temperatureas shown in Figure 10, apparently due to the increase in plateau stresses.Surface finish evidently affects fatigue endurance while the melting technique has negligible effects.

Materialcomposition, amount of cold-work and heat treatment.

Beijing GEE SMA TechnologyCo., Ltd.’ SE508 (nominal composition of Ti-55.85weight %Ni) is the most popular alloy for superelastic applications. Typical mechanical properties of alloy SE508 at 22°C are:

Result of Tension Testing ac ASTM F2516-18

Permanent Set (after 6 % strain) - 0.10 %

Tensile Strength - 1579 MPa

Upper Plateau Strength - 716 MPa

Lower Plateau Strength - 303 MPa

Elongation - 11%

Test temperature - 22℃

Active Af - 16℃

Diameter - 0.40mm(0.016”)

Surface - Polished

Test time - 2024/9/22

Contrarilyto intrinsic alloy transformation temperatures which usually depict the
material transformation temperatures in the fully annealed condition at the
ingot level, the active transformation temperatures characterize the material's
transformation at the final product level; for example, the active
transformation of our SE508 wires is between 5°C to 10°C in the as-supplied
condition, 0° to 15°C for our SE508 ELI.

TransformationTemperature Active Af

_SE508

Fully Annealed Condition ----------------------------- -10 to 5° C(800°C-20min-WQ)

Typical Active Af --_----------------------------------------5 to 10°C (super-elastic straight annealed)

+15 to 25°C (shorttime heat treatment)

+25 to35°C(long time heat treatment)

SE508 ELI

Fully Annealed Condition ----------------------------- -5 to 10 ° C(800°C-20min-WQ)

Typical Active Af -------_---------------------------------- 0 to 15° C (super-elastic straight annealed)

+20 to 30°C (shorttime heat treatment)

+25 to35°C(long time heat treatment)

CommercialNiTi alloys are prepared by either a primary vacuum induction melting (VIM)followed by vacuum arc melting (VAR) or by a multiple VAR process. Materialsprepared by the VIM/VAR process tend to have more uniform distribution oftransformation temperatures along the ingot but with higher carbon contentpicked up from the graphite crucible. The multiple VAR ingots are much cleaner
in carbon content but exhibit more variations in the distribution of transformation temperature.

Super-elastic

NitiAlloy melting is the most critical process step.

Wecommonly used 3 different melting methods，VIM(Graphite crucible), VIM(oxide crucible) and VAR.

l The niti alloy melted using VIM has auniform composition, with strict requirements for transformation temperature.

l VIM (Graphite crucible) has better elasticity and VIM (oxidecrucible) has better fatigue resistance.

l The niti alloy melted using VAR is only suitable for superelastic niti alloyswith less stringent temperature requirements, but not for shape memory application
with strict requirements for transformation temperature.

Anas-drawn wire is a cold-worked wire directly coming off the wire drawingmachine; this wire is not straight and usually exhibits some cast and twist. Inapplications, the material is typically not used in this condition; it has tobe heat treated to shape (or shape-set) to become superelastic and take a finaldesired form. An example of as-drawn wire is our SM product. Straightenedsuperelastic wires like our SE are heat-treated straight and exhibit fullysuperelastic properties.

Acold-work material needs to be heat treated before it can exhibit superelasticor shape memory properties. When subjected to an identical heat treatment, coldwork increases the mechanical characteristics of the alloy, plateau stressesand ultimate tensile strength in the superelastic state. It also decreases thetransformation temperatures i.e. a highly cold-worked wire is slightly ‘colder’than a less cold-worked wire.

Theheat treatment parameters chosen to set both the shape and the properties ofthe part are critical, and usually need to be determined experimentally foreach desired part's requirements. In general, temperatures as low as 400 deg.Cand times as short as 1-2 minutes can set the shape, but generally one uses atemperature closer to 500 deg.C and times over 5 minutes. Rapid cooling of someform is preferred via a water quench or rapid air cool (if both the parts andthe fixture are small). Higher heat treatment times and temperatures willincrease the actuation temperature of the part and often gives a sharperthermal response (in the case of shape memory elements). However, there is
usually a concurrent drop either in peak force (for shape memory elements) orin plateau stresses (for superelastic elements). There is also an accompanyingdecrease in the ability of the NiTi element to resist permanent deformation.

Theunique properties of nitinol have provided the enabling technology for manygroundbreaking applications in the medical and dental industries. Theseapplications have included everything from surgical tools to permanentimplants, including implants within the bloodstream NiTi is generally a safeimplant material has approved several devices for long-term implant applications.