What is Nitinol?
What is the driving force behind the shape memory effect and
superelasticity?
What are transformation temperatures?
How does one measure transformation temperatures?
What is R-phase?
What is superelasticity?
What is linear superelasticity?
What is shape memory effect?
How does stress affect the transformation temperatures?
What is the typical hysteresis for NiTi alloys?
What are the typical physical characteristics of NiTi alloys?
How many feet are there per pound material?
What are the typical fatigue properties of NiTi alloy?
What influences transformation temperatures and mechanical
characteristics?
What are the typical mechanical properties of superelastic NiTi
alloys?
What does ACTIVE transformation temperatures mean?
How are NiTi alloys melted?
How are NiTi ingots converted into mill products?
What is the difference between an as-drawn wire and a
straightened superelastic wire?
What is the influence of the amount of cold work in the material?
How do I set a shape in a NiTi component?
What material condition to start with?
How to join NiTi to dissimilar materials?
Can NiTi be soldered?
Can it be welded?
Can it be machined?
Can it be laser or EDM machined?
What are the finishes available?
Can it be electro-polished?
Can NiTi be sterilized by EtO or radiation sterilization techniques?
How corrosion resistant is NiTi?
How do dissimilar materials affect the corrosion resistance and biocompatibility of NiTi?
< Back
GE
FABR
JO
MAC
FIN
CORR
BIOCOM
COATING
Is NiTi biocompatible and can it be used as an implant material?
Can NiTi be plated?
Can NiTi be Teflon™ or PTFE coated to enhance its lubricity?
What about other polymeric coatings?
GENERAL
What is Nitinol?
A generic trade name for NiTi alloys, which stands for Nickel (Ni), Titanium (Ti) and Naval Ordnance Laboratory
(NOL) where the alloy was discovered in the early 1960s.
What is the driving force behind the shape memory effect and superelasticity?
A reversible solid-state phase transformation from austenite to martensite on cooling (or by deformation) and
the reverse transformation from martensite to austenite on heating (or upon release of deformation).
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What are transformation temperatures?
Martensite start temperature (Ms): the temperature at which the transformation from austenite to martensite
begins on cooling.
Martensite finish temperature (Mf): the temperature at which the transformation from austenite to martensite
finishes on cooling.
Austenite start temperature (As): the temperature at which the transformation from martensite to austenite
begins on heating.
Austenite finish temperature (Af): the temperature at which the transformation from martensite to austenite
finishes on heating.
The definitions of these temperatures are illustrated in Figure 1. NiTi material specification is generally defined
by one of these transformation temperatures (most commonly A s or Af) in the fully annealed condition (see
ASTM F2063) while transformation temperature range (TTR) is a generic term used to describe the range of
these temperatures.
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Figure 1. Definition of transformation temperatures, A s, Af, Ms and Mf, based on the amount of transformation.
How does one measure transformation temperatures?
Transformation temperatures are typically determined by Differential Scanning Calorimetry (DSC) which
measures the heat flow between the NiTi specimen and the environment in reference to that of an inert
reference as a function of temperature (ASTM F2004). Figure 2 illustrates a typical DSC curve and the
measurement of transformation temperatures of a fully annealed NiTi alloy. Active transformation temperatures
(see question on active transformation temperature) can be determined by Bend and Free Recovery (BFR)
tests which trace the shape recovery as a function of temperature (ASTM F2082). An illustrated example of
BFR test result and the determination of As and Af temperatures is shown in Figure 3. For actuator or fastener
applications, transformation temperatures may be measured by Constant Load Dilatometry (CLD) to evaluate
the effects of applied stress on the transformation. Figure 4 shows an example of CLD test result and the
determination of transformation temperatures, Ms, Mf, As and Af, on the curve.
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Figure 2. A typical DSC curve of a fully annealed NiTi alloy.
Figure 3. An illustrated example of a BFR test result.
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Figure 4. An illustrated example of a CLD test result.
What is R-phase?
An intermediate phase having a rhombohedral distortion of the cubic austenite lattice that forms from austenite
prior to martensite (see ASTM F2005 for further details).
What is superelasticity?
Also termed “pseudoelasticity”, superelasticity describes a nonlinear recoverable deformation behavior of NiTi
alloys at temperatures above the Af temperature, which arises from the stress-induced martensitic
transformation on loading and the spontaneous reversion of the transformation upon unloading. An atomic
model in Figure 5 depicts the mechanism. A transformation-induced strain up to 6% is recoverable. When
deformation exceeds 6% strain, the materials can further extend the deformation via linear elasticity of the
stress-induced martensite. A total strain as high as 8% is therefore recoverable. Figure 6 exemplifies a
superelastic stress-strain curve of NiTi alloy.
Figure 5. An atomic model depicting the mechanism of superelasticity
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Figure 6. An exemplified superelastic stress-strain curve of NiTi alloy.
What is linear superelasticity?
While heat-treated NiTi alloys exhibit nonlinear superelasticity, cold worked NiTi alloys exhibit extended linear
elasticity where a strain as high as 3.5% is recoverable with minimal plastic deformation.
What is shape memory effect?
NiTi alloys after an apparent deformation in the martensitic phase have the ability to recover their original shape
upon heating through the phase transformation temperature range above the Af temperature. Figure 7 depicts
an atomic model illustrating the mechanism of shape memory effect while the sequence of temperature change,
deformation and shape recovery associated with the phenomenon is described in Figure 8.
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Figure 7. An illustration depicting the atomic mechanism of shape memory effect.
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Figure 8. The sequence of temperature change, deformation and shape recovery associated with shape
memory effect.
How does stress affect the transformation temperatures?
The presence of stress typically raises the transformation in a linear fashion as shown in Figure 9.
Figure 9. Effects of stress on the transformation temperatures.
What is the typical hysteresis for NiTi alloys?
For binary NiTi alloys, the hysteresis is typically 30 to 40° C in thermal hysteresis and 30 to 50 Ksi in
mechanical hysteresis. Hysteresis can be manipulated by alloying addition. For example, the addition of Copper
to NiTi can reduce the thermal hysteresis width as low as 15°C while Niobium in a ternary NiTiNb alloy (Alloy X)
will increase it as high as 120°C.
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What are the typical physical characteristics of NiTi alloys?
Density = 6.45 to 6.5 g/cm3
Electrical Resistivity = 76x10-6 Ohm.cm in martensite, 82x10-6 Ohm.cm in austenite
* Variations in resistivity with temperature are complex functions of composition and thermo-mechanical
processing.
Thermal expansion coefficient = 6.6x10-6/°C in martensite, 11x10-6/°C in austenite
Thermal Conductivity = 18 W/m°K
Magnetic Susceptibility = 2.4x10-6 emu/g in martensite, 3.7x10-6 in austenite
How many feet are there per pound material?
It is a function of the cross-section of the wire:
For round wires: 0.4515/(DxD) ft/lbs (D=wire diameter in inches)
For square or rectangular wire: 0.3546/(TxW) ft/lbs (T=thickness and W=width
in inches)
What are the typical fatigue properties of NiTi alloy?
Most of the studies on NiTi fatigue are strain-controlled. From this perspective, the fatigue resistance for NiTi is
orders of magnitude higher than that of any linearly elastic material. A typical fatigue limit at 10 7 cycles is about
0.5% in outer fiber strain in rotary bending fatigue. Increasing mean strain up to 4% appeared to extend fatigue
endurance. Extending mean strain beyond 4%, the fatigue characteristics of NiTi follow a strain-based
Goodman relationship. Fatigue life generally decreases with increasing test temperature as 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.
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Figure 10. A typical strain-controlled fatigue life of NiTi alloy at various test temperatures. Fatigue life typically
decreases with increasing test temperature.
What influences transformation temperatures and mechanical characteristics?
Material composition, amount of cold-work and heat treatment.
What are the typical mechanical properties of superelastic NiTi alloys?
Alloy BB (nominal composition of Ti-55.8 weight %Ni) is the most popular alloy for superelastic applications.
Typical mechanical properties of alloy BB at 37C are:
Loading plateau stress:
Unloading plateau stress:
Permanent strain after 8% strain:
Ultimate tensile strength:
Tensile elongation:
Young’s modulus (austenite):
Young’s modulus (martensite):
60-80 Ksi
10-30 Ksi
0.2-0.5%
160-180 Ksi
10-20%
12 Msi
5 Msi
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What does ACTIVE transformation temperatures mean?
Contrarily to intrinsic alloy transformation temperatures which usually depict the material transformation
temperatures in the fully annealed condition at the ingot level, the active transformation temperatures
characterizes the material's transformation at the final product level; for example the active transformation of
our GUIDE-BB wires is between -5°C to +10°C in the as-supplied condition, 12° to 15°C for our GUIDE-BC.
FABRICATION
How are NiTi alloys melted?
Commercial NiTi alloys are prepared by either a primary vacuum induction melting (VIM) followed by vacuum
arc melting (VAR) or by a multiple VAR process. Materials prepared by the VIM/VAR process tend to have
more uniform distribution of transformation temperatures along the ingot but with higher carbon content picked
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.
How are NiTi ingots converted into mill products?
NiTi ingots are hot forged/swaged and hot rolled to bars and coils. Wires are subsequently drawn to finish sizes
from large diameter hot rolled coils. Hollowed barstock with 0.5-1.5 inch diameter is subsequently drawn down
to finished tubing sizes.
What is the difference between an as-drawn wire and a straightened superelastic wire?
An as-drawn wire is a cold-worked wire directly coming off the wire drawing machine; this wire is not straight
and usually exhibits some cast and twist. In applications, the material is typically not used in this condition; it
has to be heat treated to shape (or shape-set) to become superelastic and take a final desired form. An
example of as-drawn wire is our CW-xx- product. Straightened superelastic wires like our GUIDE- or SE-Sf- are
heat-treated straight and exhibit fully superelastic properties.
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What is the influence of the amount of cold work in the material?
A cold-work material needs to be heat treated before it can exhibit superelastic or shape memory properties.
When subjected to an identical heat treatment, cold work increases the mechanical characteristics of the alloy,
plateau stresses and ultimate tensile strength in the superelastic state. It also decreases the transformation
temperatures i.e. a highly cold-worked wire is slightly ‘colder’ than a less cold-worked wire.
How do I set a shape in a NiTi component?
The material needs to be fixtured and constrained in the desired shape and heat-treated. Typically for
superelastic material, a heat treatment in the 500°C range is adequate; the length of heat treatment varies with
the equipment used for the heat treatment and the thermal mass of the shaping fixture. In a molten salt bath for
example, the heat treatment time is generally between 2 and 5 minutes.
What material condition to start with?
For intricate parts, it is better to start with an as-drawn or stress-relieved material (ex: CW-xx- wires), beware
that the material is less ductile in its cold-worked state. For components for which a large portion of the part has
to be straight and for example one end hooked, it is better to start with a pre-straightened material, like our
GUIDE- or SE-BB for example.(links to Semi Finished)
JOINING
How to join NiTi to dissimilar materials?
Mechanical techniques are preferred; crimping, swaging can be used.
Adhesives: cyanoacrylates, epoxies, etc. Soldering: see soldering question. In some cases the shape memory
effect of the NiTi alloy can be used effectively to connect two mating parts. For example a superelastic tube
NiTi tube can be chilled, expanded while in martensite and then recovered onto its mating counterpart. A postrecovery interference or contact strain of about 1.5% is recommended for an effective joint. Designers shall
take into account tolerance stack-ups and installation clearance when designing a ‘shape memory’ joint.
Back
Memry’s alloy X (NiTiNb) alloy is particularly useful for shape memory joints that require maintaining
mechanical integrity at cryogenic temperatures.
Can NiTi be soldered?
The problem in soldering NiTi is the passive oxide layer that covers NiTi components. Sn-3.5Ag solder can
work effectively if combined with a very aggressive flux (like aluminum paste flux and others). Some companies
use plating (Ni, Au, etc.) on NiTi to enhance solderability.
Can it be welded?
TIG, laser, e-beam, plasma techniques can be used to weld NiTi to itself. A protective inert atmosphere shall be
used as well during the welding process. By careful practice, weld strength at about 70% of the raw material
tensile strength can be achieved, sufficient to retain the superelastic and shape memory properties. Welding
NiTi to stainless steels is much more difficult because of brittle intermetallics that form in the weld zone. To
avoid the problem, interlayer such as Ta can be used to bridge the two materials.
MACHINING
Can it be machined?
Although it is very difficult and creates a lot of tool wear, NiTi can be machined using conventional machining
equipment and techniques: milling, turning, grinding, etc. Carbide tooling and a coolant flood are strongly
recommended.
Can it be laser or EDM machined?
Yes. NiTi stents are routinely laser machined out of NiTi tubing. After both processes, however, recast layer
and heat-affected zone are typically present and must be removed to enhance fatigue endurance and corrosion
properties.
FINISHING
What are the finishes available?
Straightened Wires come in the as-drawn (slight yellow straw color) finish, black oxide, sandblasted or
mechanically polished. As-drawn wires come with the as-drawn finish or can be mechanically polished. Tubing
comes with OD in the as-drawn finish (oxide) or centerless ground condition. Tubing ID is typically slurry
cleaned and/or micro-blasted. Components may retain the oxide finish after heat treatment or may be delivered
with subsequent surface preparation such as mechanical polishing, chemical polishing, electro-polishing and/or
acid passivation.
Can it be electro-polished?
Yes, although most companies keep their electrolyte chemical composition very confidential.
Can NiTi be sterilized by EtO or radiation sterilization techniques?
Yes
CORROSION AND BIOCOMPATIBILITY
How corrosion resistant is NiTi?
The corrosion resistance of NiTi alloys is highly sensitive to the surface condition. Materials with as-drawn and
heat-treated surfaces are more susceptible to pitting corrosion due to the presence of heavy oxide and
processing contamination. Materials with a passive oxide layer, such as mechanically polished and passivated
part, are highly corrosion resistant and have the ability to repassivate in the event of a small local destruction of
the passive film.
How do dissimilar materials affect the corrosion resistance and biocompatibility of NiTi?
It is highly dependent on the coupling material. Materials such as stainless steels, Ti, and Ta have weak
galvanic effects with NiTi and are safer to use as compared to precious metals such as Au and Pt that have
strong galvanic effects.
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Is NiTi biocompatible and can it be used as an implant material?
NiTi is generally a safe implant material as FDA has approved several devices for long-term implant
applications. A large amount of data is available in various publications. According to an in-vitro study of
passivated NiTi in Hank’s solution, the Ni release rate was the highest of 14.5 x 10 -7 g/cm-2sec-1 in the first day
but decreased quickly to an undetectable level in 10 days. In-vivo studies of NiTi implants in soft tissues
indicated that the overall inflammatory response to NiTi was very similar to that of stainless steels and Ti-6V4Al alloy. Studies on NiTi vascular stents showed a mild inflammatory response, minor atrophy of vessel media,
acceptable fibrocellular tissue growth and endotheliazation, indicating that the biocompatibility of NiTi stents is
equal to or better than that of stainless steel stents. A comparative in-vivo study of NiTi and stainless steel
intramedullary rods on osteotomy healing indicated more healed bone unions and closer bone contact for NiTi
when compared to the stainless steel group. The callus size and the mineral density were similar between the
two groups. Studies on the use of NiTi bone implants in humans generally reported good clinical results. The
existing data suggest that NiTi with proper surface finish is a safe biomaterial for vascular, soft tissue and
orthopedic applications.
COATING AND PLATING
Can NiTi be plated?
Yes, nickel, gold, copper, and silver are routinely being plated on NiTi. It is not an easy task as the adhesion
between the plating and the NiTi substrate needs to be able to withstand high strains without flaking.
Can NiTi be Teflon™ or PTFE coated to enhance its lubricity?
Spayed coatings of PTFE require a curing cycle at high temperature (>300°C) that can affect the superelastic
or shape memory characteristics of a NiTi component.
Sprayed PTFE coating of guidewires shall be performed with the wire maintained in the straight condition and
under slight tension. Heavy thicknesses of coating can be achieved.
Vacuum or plasma deposition techniques are effective but will leave only a very thin layer of coating.
What about other polymeric coatings?
The same reserve and precautions will apply if process and/or curing cycle at high temperature (>300°C) are
required during the coating. Our antenna wire can be polyurethane-coated for example.
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Back
ching June 12, 2007
European Sales Office
Memry Corporation is proud to serve the rapidly expanding European Medical Device Marke
through our European Sales Office. Existing or prospective customers seeking techincal sales
service support for either Memry or PPC product platforms may seek assistance by contacting
Eoin O'Madagain
Telephone: +353 67 22 901
Fax: +353 16 335 100
Mobile: +353 87 611 4757
europe@memry.com
europe@putnamplastics.com
europe@ppcplus.com (Launching June 12, 2007)
Memry 2008 European Tradeshow Schedule
Medtec Ireland
September 19-20, 2008
Radisson SAS Hotel
Galway, Ireland
Medtec Stuttgart
March 11-13, 2008
New State Trade Fair Centre
Stuttgart, Germany
INTRODUCTION TO SHAPE MEMORY
& SUPERELASTICITY
Shape Memory Alloys, such as Nickel Titanium, undergo a phase
transformation in their crystal structure when cooled from the str
high temperature form (Austenite) to the weaker, low temperatur
(Martensite). This inherent phase transformation is the basis for t
unique properties of these alloys -- in particular, Shape Memory
Superelasticity.
Shape Memory
When a shape memory alloy is in its martensitic form, it is easily deformed to a new shape.
However, when the alloy is heated through its transformation temperatures, it reverts to aust
and recovers its previous shape with great force. This process is known as Shape Memory.
The temperature at which the alloy remembers its high temperature form when heated can b
adjusted by slight changes in alloy composition and through heat treatment. In the Nickel
Titanium alloys, for instance, it can be changed from above +100 deg.C to below -100 deg.C
The shape recovery process occurs over a range of just a few degrees and the start or finish o
transformation can be controlled to within a degree or two if necessary.
Schematic of the Shape Memory Effect
Superelasticity
These unique alloys also show a Superelastic behavior if deformed at a temperature which
is slightly above their transformation temperatures. This effect is caused by the stress-induce
formation of some martensite above its normal temperature. Because it has been formed abo
normal temperature, the martensite reverts immediately to undeformed austenite as soon as
stress is removed. This process provides a very springy, "rubberlike" elasticity in these alloy
Typical Loading and Unloading Behavior of Superelastic NiTi
Typical Properties of NiTi Shape Memory Alloys
Martensite is...
Fairly Weak: 10,000 to 20,000 psi deformation stress
Able to absorb up to 8% recoverable strain
Austenite is...
Strong: 35,000 to 100,000 psi yield strength
Both forms of the alloy are...
Ductile: elongation to failure over 25%
Strong: tensile strength up to 200,000 psi
Biocompatible and extremely corrosion resistant
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SPECIFYING NITI MATERIALS
Specifying Nitinol
Nitinol is a family of materials that are not completely described using typical engineering
properties. In concert with ASTM Standards, it is important that the 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 Nitinol.
> ASTM F 2004-05 Standard Test Method for Determination of Transformation Temperature
of Nickel-Titanium Shape Memory Alloys by Thermal Analysis
> ASTM F 2005-05 Standard Terminology for Nickel-Titanium Shape Memory Alloys
> ASTM F 2063-05 Standard Specification for Wrought Nickel-Titanium Shape Memory
Alloys for Medical Devices and Surgical Implants
> ASTM F 2082-02 Standard Test Method for Determination of Transformation Temperature
of Nickel-Titanium Shape Memory Alloys by Bend and Free Recovery
> ASTM F2516 -05 Standard Test Method for Tension Testing of Nickel-Titanium
Superelastic Materials
ASTM 2004 Commentary
This specification provides guidance on the determination of transformation temperature of
fully annealed materials, and is not appropriate for testing products that are heat treated to
impart superelasticity. A sample can be solution annealed and tested, but the transformation
temperature will be that of the ingot, not product. There is also controversy over the exact
temperature, time, and protective gas used during solution annealing. Please contact Johnson
Matthey should you have questions regarding test methods to insure compliance.
ASTM 2063-05 Commentary
From its inception, 2063 was created to govern the manufacture of wrought products, not
finished products. There is an ongoing debate in the industry as to whether it properly specifies
chemistry and inclusion size and distribution. Johnson Matthey supports ASTM 2063-05 as a
raw material specification. It is important to discuss this topic prior to adding to an existing or
new specification.
Product Forms
Current commercially available forms include: sheet, tube, wire and ribbon, and components
fabricated from each product form.
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). Applications include laser cutting and/or photochemical
etching to manufacture complex components.
Flextube®
Johnson Matthey is a leader in large diameter, thin wall, and microlumen Flextube®. 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”. The overall length of the Flextube® is a function of outside
diameter. For most applications, the wall thickness should be chosen to be greater than 10
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.
Wire and Ribbon
Wire and ribbon are the most mature product forms of Nitinol, being commercialized in the
mid 1980’s. Wires from 0.250” down to 0.010" are commercially available from Johnson
Matthey. Ribbon is wire with a rectangular cross-section and radiused edges. Common wire
flattening methods are generally limited to width to thickness ratios of less than 10 to 1 aspect
ratio, although this limit can be exceeded for some special applications. Strands and cables are
also available in 1X3, 1X7, 1X19, and 7X7 configurations. Typical applications include
guidewires, support members in a catheter wall, and wire wound stent fabrication.
Components
Many unique 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.
Table 1: Typical Properties of Nitinol
Common Name*
Ingot A(p)
Active A(f)*
Description
N
-20 C to -5 C
0 C to 20 C
High Nickel Super Elastic Nitinol
S
-5 C to 15 C
10 C to 20 C
Super Elastic Nitinol
C
-20 C to -5 C
0 C to 10 C
Chromium Doped Super Elastic Nitinol
B
15 C to 45 C
20 C to 40 C
Body Temperature Nitinol
M
45 C to 95 C
45 C to 95 C
Mid Temperature Range Nitinol
H
> 95 C
95 C to 115 C
High Temperature Range Nitinol
* 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 A(f).
Straight Strain Annealed
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:
Table 2: Typical Mechanical Properties of Standard Superelastic NiTi Wires**
Typical as
Drawn Properties*
Materials
UTS
(KPSI)
Cr Doped
A(f) 10 C
270 +/- 15
270 +/- 15
%
Elongation
at Failure
6% Min
6% Min
Typical as
Straight Annealed Properties*
UTS
(KPSI)
220 +/- 15
210 +/- 15
%
Elongation
at Failure
Loading
Plateau
(KPSI)
12% Min
12% Min
70 Min
65 Min
Unloading
Plateau
(KPSI)
~50
~30 (15 to 45)
Residual
Strain
Percent
< 0.25
< 0.25
A(f) 20 C
250 +/- 15
6% Min
200 +/- 15
12% Min
65 Min
~20 (5 to 35)
< 0.25
* 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 Flextube® and sheet may be different.
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TRANSFORMATION
TEMPERATURE HYSTERESIS IN NITI
ALLOYS
Upon heating or cooling, NiTi alloys do not completely undergo their phase transformation at one
particular temperature. Instead, the transformation begins at one temperature (known as the start
temperature) and is completed at another temperature (known as the finish temperature). Further,
there is a difference in the transformation temperatures upon heating from martensite to austenite and
cooling from austenite to martensite, resulting in a delay or "lag" in the transformation. This difference,
known as the transformation temperature hysteresis, is generally defined as the difference between
the temperatures at which the material is 50% transformed to austenite upon heating and 50%
transformed to martensite upon cooling. This value can be approximated by the difference between Ap
and Mp on a DSC curve. Typical values for binary NiTi alloys are about 25 to 50 deg.C.
In addition to the hysteresis, the overall span of the transformation may be important. If the device
being designed requires complete transformation upon both heating and cooling, then the difference
between Af and Mf (the finish temperatures of the transformations to austenite and martensite,
respectively) must be considered. Typical values for the overall transformation temperature span are
about 40 to 70 deg.C.
Both the hysteresis and the overall transformation temperature span are slightly different for different
NiTi alloys. Further, alloying can greatly affect the transformation hysteresis. Copper additions have
been shown to reduce the hysteresis to about 10 to 15 deg.C and Niobium (Columbium) additions
can expand the hysteresis to over 100 deg.C. Cold working and heat treatment have less dramatic, but
still measurable effects on the transformation hysteresis. The table below shows the differences in
hysteresis and overall temperature span for some different binary alloys.
Example Transformation Temperature Values
Example
Numbers
Mf
Mp
Ms
As
Ap
Af
Hysteresis
(Ap-Mp)
Overall
Temp. Span
(Af-Mf)
1
-53
-40
-33
-24
-14
-5
26
48
2
-45
-30
-24
-15
-3
+7
27
53
3
-3
+3
+6
+23
+30
+35
27
38
4
24
31
36
54
66
71
35
45
5
59
68
79
100
114
121
46
62
These numbers should provide some assistance when designing a device which utilizes the shape
memory effect. For example, if one were designing a device to activate at boiling water temperature
(100 deg.C) that also must be fully retransformed to martensite at room temperature (20 to 25 deg.C),
there is a narrow set of binary alloys which meet the criteria. From the above table, one can estimate
that one should consider alloys with As of approximately 60 to 80 deg.C to satisfy both criteria.
Similarly, an alloy designed to be completely transformed by body temperature upon heating (Af <
37 deg.C) would require cooling to about -10 deg.C to fully retransform to martensite.
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SELECTED PROPERTIES OF NITI
Transformation Properties
Transformation Temperature.............................-200 to 110 deg. C
Latent Heat of Transformation..................................5.78 cal/g
Transformation Strain (for polycrystalline material)
for a single cycle.............................................max 8%
for 100 cycles.....................................................6%
for 100,000 cycles.................................................4%
Hysteresis**..............................................30 to 50 deg. C
Physical Properties
Melting Point...................................1300 deg. C (2370 deg. F)
Density.....................................6.45 g/cu.cm (0.233 lb/cu.in)
Thermal Conductivity
austenite..............0.18 W/cm * deg. C (10.4 BTU/ft * hr * deg. F)
martensite.............0.086 W/cm * deg. C (5.0 BTU/ft * hr * deg. F)
Coefficient of Thermal Expansion
austenite.............................11.0E-6/deg. C (6.11E-6/deg. F)
martensite.............................6.6E-6/deg. C (3.67E-6/deg. F)
Specific Heat..................0.20 cal/g * deg. C (0.20 BTU/lb * deg. F)
Corrosion Performance***........................................excellent
Electrical and Magnetic Properties
Resistivity [resistance = resistivity * length / cross-sectional area]
austenite............approx. 100 micro-ohms * cm (39 micro-ohms *
in)
martensite............approx. 80 micro-ohms * cm (32 micro-ohms *
in)
Magnetic Permeability.............................................< 1.002
Magnetic Susceptibility.......................................3.0E6 emu/g
Mechanical Properties
Young's Modulus****
austenite...................................approx. 83 GPa (12E6 psi)
martensite......................approx. 28 to 41 GPa (4E6 to 6E6 psi)
Yield Strength
austenite..............................195 to 690 MPa (28 to 100 ksi)
martensite...............................70 to 140 MPa (10 to 20 ksi)
Ultimate Tensile Strength
fully annealed......................................895 MPa (130 ksi)
work hardened......................................1900 MPa (275 ksi)
Poisson's Ratio......................................................0.33
Elongation at Failure
fully annealed..............................................25 to 50%
work hardened................................................5 to 10%
Hot Workability................................................quite good
Cold Workability....................difficult due to rapid work hardening
Machinability....................difficult, abrasive techniques preferred
** Values listed are for a full martensite to austenite transition.
Hysteresis can be significantly reduced by partial transformation or
ternary alloys.
*** Similar to 300 series stainless steel or titanium
**** Highly nonlinear with temperature
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SETTING SHAPES IN NITI
The use of a NiTi shape memory or superelastic element for a particular application generally requires
the setting of a custom shape in a piece of NiTi. The process required to set the shape is similar
whether beginning with NiTi in the form of wire, ribbon, strip, sheet, tubing, or bar. Shape setting (or
training) is accomplished by constraining the NiTi element on a mandrel or fixture of the desired shape
and applying an appropriate heat treatment. The heat treatment methods used to set shapes in both
shape memory and superelastic forms of NiTi are similar.
The heat treatment parameters chosen to set both the shape and the properties of the part are critical,
and usually need to be determined experimentally for each desired part's requirements. In general,
temperatures as low as 400 deg.C and times as short as 1-2 minutes can set the shape, but generally
one uses a temperature closer to 500 deg.C and times over 5 minutes. Rapid cooling of some form is
preferred via a water quench or rapid air cool (if both the parts and the fixture are small). Higher heat
treatment times and temperatures will increase the actuation temperature of the part and often gives a
sharper thermal response (in the case of shape memory elements). However, there is usually
a concurrent drop either in peak force (for shape memory elements) or in plateau stresses (for
superelastic elements). There is also an accompanying decrease in the ability of the NiTi element to
resist permanent deformation.
A final caution that one should observe is that heat treatment fixtures can be surprisingly sluggish in
reaching the desired temperature in air or vacuum furnaces. The desired NiTi shapes and properties
are imparted largely by the time at the maximum temperature, so be careful that your parts actually
reach the desired temperature and time. More massive elements (e.g., bars and rods) will therefore
require longer times to allow for the temperature of the part to equilibrate.
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MAKING SHAPE MEMORY SPRINGS
With the possible exception of straight wire, the most popular and useful form of NiTi shape memory
alloy for actuators is helical coil springs. Such springs can be used either in extension (tension) or
compression, can provide an impressively large stroke, and may be designed to exert significant
forces.
The method of making shape memory springs is chosen largely by the quantity required. If only one or
a few springs are needed, it is easiest to wind the chosen wire on a cylindrical mandrel having a
diameter that will yield the chosen I.D. Both ends of the wire must be fastened to the mandrel, such as
by capturing them under a screw head, and one should wind the wire on the mandrel tightly and with
the desired pitch (wire spacing). Heat treating on the mandrel (see below) will then set the shape of the
spring.
For larger numbers of springs, a standard spring winding machine is used. The shape memory wire
should be in the as-drawn condition to respond best to the winding, and because of the very high
elasticity of these alloys you will find that you need to set the machine to wind a tighter coil than you
would expect to achieve a desired size. The as-wound springs can be heat treated in two different
manners. In the first, you can maintain the as-wound dimensions by putting the coils into a holding
fixture of the chosen size and then heat treating the coils to set their shape. The second method is to
wind the coils tighter than the desired final size, and then heat treat the coils without confinement.
Because of the shape memory effect, the coils will grow significantly in diameter (on the order of 25%)
during heat treatment. This method requires more trial and error to achieve the desired final size,
and gives poorer size control, but is less expensive than the first method.
The heat treatment chosen to set the shape and properties of the spring is critical, and usually needs to
be determined experimentally for each desired spring's requirements. In general, temperatures as low
as 400 deg.C and times as short as 1-2 minutes can set the shape, but generally one uses a
temperature closer to 500 deg.C and times over 5 minutes. Rapid cooling of some form is preferred via
a water quench or rapid air cool (if both the springs and the fixture are small). Higher heat treatment will
increase the actuation temperature of the spring and often gives a sharper thermal response, but
there is usually a concurrent drop in peak force for springs and in the ability to resist permanent
deformation.
A final caution that one should observe is that heat treatment fixtures can be surprisingly sluggish in
reaching the desired temperature in air or vacuum furnaces. The desired spring properties are imparted
largely by the time at the maximum temperature, so be careful that your springs actually reach the
desired temperature and time.
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APPROXIMATE SURFACE STRAINS
IN WIRE, RIBBON & SHEET
Outer Fiber Strain
Ratio of Bend Diameter to
Wire Diameter/Thickness
Ratio of Bend Radius to
Wire Diameter/Thickness
1%
100.0 to 1
50.0 to 1
2%
48.9 to 1
24.4 to 1
3%
32.2 to 1
16.1 to 1
4%
23.9 to 1
11.9 to 1
5%
18.9 to 1
9.44 to 1
6%
15.5 to 1
7.74 to 1
7%
13.1 to 1
6.56 to 1
8%
11.3 to 1
5.64 to 1
9%
10.0 to 1
5.00 to 1
10%
8.83 to 1
4.41 to 1
20%
3.95 to 1
1.97 to 1
30%
2.33 to 1
1.16 to 1
40%
1.49 to 1
0.74 to 1
50%
0.99 to 1
0.50 to 1
60%
0.65 to 1
0.33 to 1
70%
0.42 to 1
0.21 to 1
80%
0.25 to 1
0.13 to 1
To use these numbers, divide the bend diameter by the wire diameter in question. Find the closest
value in the middle column and read the corresponding value in the first column. For example, for a 1
mm diameter wire being bent around a rod with a diameter of 10 mm, the ratio of bend diameter to
wire diameter is 10 to 1, which represents approximately 9% strain on the surface of the wire. In simple
bending, this strain will be in tension along the outer surface of the wire and in compression on the
inner surface of the wire. The interior of the wire will see smaller strain magnitudes with strain
of approximately 0% near the midline of the wire (neutral axis).
The maximum recoverable strain limit for both superelastic and shape memory NiTi is about 6 to 8%.
However, 3 to 4% is the recommended limit for product design. The ductility of NiTi wire which has
been cold worked and heat treated is typically 10 to 20% in tension. Fully annealed NiTi wire has a
ductility limit of about 60 to 70%.
Note: The numbers in the above table are approximate values based on general beam theory
calculations for all materials. They have not been specifically calculated for NiTi alloys.
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