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?
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).
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 As
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.
Figure
1. Definition of transformation temperatures, As,
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.
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
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.
Figure
7. An illustration depicting the atomic mechanism of shape
memory effect.
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.
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 107 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.
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: |
60-80 Ksi |
| Unloading
plateau stress: |
10-30
Ksi |
| Permanent
strain after 8% strain: |
0.2-0.5% |
| Ultimate
tensile strength: |
160-180
Ksi |
| Tensile
elongation: |
10-20% |
| Young’s
modulus (austenite): |
12 Msi |
| Young’s
modulus (martensite): |
5
Msi |
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.
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 post-recovery 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. 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.
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 mg/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-6V-4Al 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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