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Technical Seminar Report Date: 24th March, 2012
Submitted By- TARINI PRASAD MISHRA Page 1 Hall Ticket No- 08261A1856, 4th Year, MME
INTRODUCTION TO SHAPE MEMORY ALLOYS
1. Background
Metals are characterized by physical qualities as tensile strength, malleability and
conductivity. In the case of shape memory alloys, we can add the anthropomorphic qualities of
memory and trainability. Shape memory alloys exhibit what is called the shape memory effect. If
such alloys are plastically deformed at one temperature, they will completely recover their
original shape on being raised to a higher temperature. In recovering their shape the alloys can
produce a displacement or a force as a function of temperature. In many alloys combination of
both is possible. We can make metals change shape, change position, pull, compress, expand,
bend or turn, with heat as the only activator. Key features of products that possess this shape
memory property include: high force during shape change; large movement with small
temperature change; a high permanent strength; simple application, because no special tools are
required; many possible shapes and configurations; and easy to use - just heat [1]. Because of
these properties shape memory alloys are helping to solve a wide variety of problems. In one
well–developed application shape memory alloys provide simple and virtually leakproof
couplings for pneumatic or hydraulic lines [2]. The alloys have also been exploited in
mechanical and electromechanical control systems to provide, for example, a precise mechanical
response to small and repeated temperature changes [3]. Shape memory alloys are also used in a
wide range of medical and dental applications (healing broken bones, misaligned teeth . . )[2, 4].
Shape Memory Alloys (SMA's) are novel materials which have the ability to return to a
predetermined shape when heated. When an SMA is cold, or below its transformation
temperature, it has a very low yield strength and can be deformed quite easily into any new
shape--which it will retain. However, when the material is heated above its transformation
temperature it undergoes a change in crystal structure which causes it to return to its original
shape. If the SMA encounters any resistance during this transformation, it can generate
extremely large forces. This phenomenon provides a unique mechanism for remote actuation.
2. History
First observations of shape memory behaviour were in 1932 by ¨ Olander in his study of “rubber
like effect” in samples of gold–cadmium [5] and in 1938 by Greninger and Mooradian in their
Technical Seminar Report Date: 24th March, 2012
Submitted By- TARINI PRASAD MISHRA Page 2 Hall Ticket No- 08261A1856, 4th Year, MME
study of brass alloys (copper–zinc) [1, 5]. Many years later (1951) Chang and Read first reported
the term “shape recovery” [5]. They were also working on gold–cadmium alloys. In 1962
William J. Buehler and his co–workers at the Naval Ordnance Laboratory discovered shape
memory effect in an alloy of nickel and titanium. He named it NiTiNOL (for nickel–titanium
Naval Ordnance Laboratory) [2, 3]. Buehler’s original task was finding a metal with a high
melting point and high impact resistant properties for the nose cone of the Navy’s missile
SUBROC. From among sixty compounds, Buehler selected twelve candidates to measure their
impact resistance by hitting them with hammer. He noted that a nickel–titanium alloy seemed to
exhibit the greatest resistance to impact in addition to satisfactory properties of elasticity,
malleability and fatigue. One day he took some NiTiNOL bars from melting furnace and laid
them out on a table to cool. He intentionally dropped one on the floor out of curiosity. The bar
produced a bell–like quality sound. Than he ran to the fountain with cold water and chilled the
warm bar. The bar was once again dropped on the floor. On his amazement it exhibited the
leaden–like acoustic response. Buehler knew that acoustic damping signalled a change in atomic
structure that can be turned off and on by simple heating and cooling near room temperature, but
he did not yet know that this rearrangement in the atomic structure would lead to shape memory
effect [3, 6].
It was in 1960 when Raymond Wiley joined Buehler’s research group. He worked on failure
analysis of various metals. He demonstrated to his management the fatigue resistance of a
NiTiNOL wire by flexing it. The directors who were present at this meeting passed the strip
around the table, repeatedly flexing and unflexing it and were impressed with how well it held
up. One of them, David Muzzey, decided to see how it would behave under heat. He was a pipe
smoker, so he held the compressed NiTiNOL strip in the flame of his lighter. To the great
amazement of all, it has stretched out completely. When Buehler heard about that, he realized
that it had to be related to the acoustic behaviour he had noted earlier [3, 6]. After this moment,
NiTi alloys increased interest of developing applications based on a shape memory alloys.
3. Definition of a Shape Memory Alloy
Shape memory alloys are a unique class of metal alloys that can recover apparent permanent
strains when they are heated above a certain temperature. The shape memory alloys have two
stable phases - the high–temperature phase, called austenite (named after English metallurgist
Technical Seminar Report Date: 24th March, 2012
Submitted By- TARINI PRASAD MISHRA Page 3 Hall Ticket No- 08261A1856, 4th Year, MME
William Chandler Austen [3]) and the low–temperature phase, called martensite (named after
German metallographer Adolf Martens [3]).
Figure 1: Different phases of a shape memory alloy [8].
The key characteristic of all shape memory alloys is the occurrence of a martensitic phase
transformation which is a phase change between two solid phases and involves rearrangement of
atoms within the crystal lattice. The martensitic transformation is associated with an inelastic
deformation of the crystal lattice with no diffusive process involved. The phase transformation
results from a cooperative and collective motion of atoms on distances smaller than the lattice
parameters. Martensite plates can grow at speeds which approach that of sound in the metal (up
to 1100m/s). Together with fact, that martensitic transformation can occur at low temperatures
where atomic mobility may be very small, results in the absence of diffusion in the martensitic
transformation within the time scale of transformation. The absence of diffusion makes the
martensitic phase transformation almost instantaneous (a first-order transition). When a shape
memory alloy undergoes a martensitic phase transformation, it transforms from its high–
symmetry (usually cubic) austenitic phase to a low symmetry martensitic phase (highly twinned
monoclinic structure). NiTiNOL’s high temperature phase has B2 crystal structure and its low
temperature phase has B19’ crystal structure. If one ignores the difference between Ni and Ti
atoms, B2 crystal structure is simply body–centred cubic and B19 has the same symmetry as
hexagonal–close packed, except that the two species of atoms break hexagonal symmetry
making the structure to tetragonal. B19’is a small distortion from B19 [7]. As in Figure 2.
Technical Seminar Report Date: 24th March, 2012
Submitted By- TARINI PRASAD MISHRA Page 4 Hall Ticket No- 08261A1856, 4th Year, MME
Figure 2: The cubic B2 cell (shaded box) of NiTi (left). The distortion to the stress–stabilized
B19’ structure (right). Structural parameters for B2 structure are a = 2.949 °A, b = 4.171 °A, c =
4.171 °A and = 90_; structural parameters for B19’ structure are a = 2.861 °A, b = 4.600 °A, c =
3.970 °A and = 97.8_ [9]. A feature of all martensitic transformations is that there are number of
equivalent shear directions through which the martensite can form within a region of parent–
phase. This results in the formation of martensite variants within the microstructure of a
transformed alloy. In Figure 1 at twinned martensite phase, we can see two crystallographically
equivalent martensite variants created by different atomic shears from the parent phase. Two
opposite shears maintain the macroscopic shape of the crystal block. Such a microstructure,
where the shear of one variant is accommodated or “cancelled” by that of the other, is know as a
self–accommodated structure. Three–dimensional self–accommodation requires a large number
of variants (typically up to 48 in many alloys) [5]. Upon cooling without of applied load the
material transforms from austenite into twinned martensite. With heating twinned martensite, a
reverse martensitic transformation takes place and as a result the material transforms to austenite.
This process is shown in Figure 3. There are four critical temperatures defined. Martensitic start
temperature (Ms) which is the temperature at which the material starts transforming from
austenite to martensite. Second is martensitic finish temperature (Mf ), at which the
transformation is complete and material becomes fully in the martensitic phase. Similar
temperatures are defined for reversible transformation. Austenite start temperature (As) is the
temperature at which the reverse transformation starts and austenite finish temperature (Af ) at
which the reverse transformation is finished and the material is in the austenitic phase.
Technical Seminar Report Date: 24th March, 2012
Submitted By- TARINI PRASAD MISHRA Page 5 Hall Ticket No- 08261A1856, 4th Year, MME
Figure 3: Temperature–induced phase transformation of a shape memory alloy without mechanical Loading [8].
4. Engineering Effects of SMAs
Having introduced the key properties of a shape memory alloy, it is now possible to review two
important behaviors exhibited by such materials. These are the shape memory effect (SME) and
the pseudoelastic effect. The usefulness of SMAs is most commonly found in the application of
one of these two engineering effects, with SME used for actuation and pseudoelasticity
employed for applications such as vibration isolation and dampening. These two behaviors will
now be discussed in more detail. The stability of the material response when considering both
effects will also be reviewed.
4.1 The Shape Memory Effect
Recovery of the seemingly permanent deformation observed during detwinning is associated
with the phenomenon known as the stress-free shape memory effect. The nature of the SME can
be better understood by following the process depicted in the stress-temperature phase diagram,
schematically shown in Figure 4. This loading path is experimentally exemplified in s−e−T space
in Figure 5, which shows an actual loading path for a NiTi wire actuator captured during
experimentation at Texas A&MUniversity. At the start of the loading path (indicated by A in
Figures 4 and 5) the SMA is in its parent austenitic phase. In the absence of applied stress, the
Technical Seminar Report Date: 24th March, 2012
Submitted By- TARINI PRASAD MISHRA Page 6 Hall Ticket No- 08261A1856, 4th Year, MME
SMA will transform upon cooling into martensite in the twinned or self accommodated
configuration (indicated by B in Figures 4 and 5). As stress is applied causing the martensitic
phase to be reoriented into a fully detwinned state, deformation takes place and large
macroscopic strains are observed (indicated by C in Figures 4 and 5). The magnitude of this
strain is on the order of 8% for some NiTi alloys [12]. Upon unloading, the elastic portion of the
total strain is recovered while the inelastic strain due to the detwinning process remains due to
the stability of detwinned martensite. This point is indicated by D in Figures 4 and 5. Upon
heating the SMA at zero stress, the reverse transformation to the austenitic parent phase begins
when the temperature reaches As (point E), and is completed at temperature Af (point F Figures 2
and 3). The inelastic strain due to reorientation is recovered, and thus the original shape (before
deformation B–C) is regained. Note that, in this case, the formation of any nonrecoverable
plastic strain has been neglected. Therefore, point A is equivalent to point F in terms of the state
of the material. It is this reversion to an original or “remembered” shape that inspired the names
“shape memory alloy.”
Fig:4 Phase diagram schematic highlighting stress-free SME, Fig:5 Experimental stress–strain–temperature curve of a NiTi SMA
illustrating the shape memory effect isobaric SME, and isothermal pseudoelastic loading paths.
Now consider another loading path denoted by −−− − in Figure 4 and also shown
experimentally in Figure 4 [13]. Such a path is similar to the one previously described, though in
this particular case a constant stress is maintained throughout the thermal cycle. This is
exemplified by hanging a weight on an SMA component such as a wire or spring. If the SMA
material begins in austenite (a) and is cooled through transformation into martensite (−), it will
exhibit large strains associated with the phase transformation. Such strains are the result of both
Technical Seminar Report Date: 24th March, 2012
Submitted By- TARINI PRASAD MISHRA Page 7 Hall Ticket No- 08261A1856, 4th Year, MME
the alteration of the crystal structure from austenite to detwinned martensite as well as the
change in the elastic modulus during phase change. However, this elastic contribution is minor.
Heating the material through the reverse transformation region (−) leads to reversion to
austenite and subsequent recovery of the large macroscopic strains, with the exception of any
non-recoverable plastic strains. Such plastic strains can be observed at the end of heating in
Figure 4. Because the recovered strain is used to provide displacement under a some force, it is
also sometimes referred to as the actuation strain (act
). Note that this shape recovery will cease
to occur if the applied stress exceeds some maximum level. This characteristic maximum
actuation stress is often referred to as the blocking stress and can be easily experimentally
determined.
4.2 The Pseudoelastic Effect
A second commonly utilized phenomenon observed in SMAs is the pseudoelastic effect.
This behavior is associated with stress-induced detwinned martensite (SIM) and subsequent
reversal to austenite upon unloading. The transformation from austenite to detwinned martensite
during pseudoelastic loading is analogous to the reorientation of twinned martensite into
detwinned martensite during detwinning from the point of view that, in both cases, recover- able
inelastic strains are created. However, in the case of the pseudoelastic effect, the starting phase is
austenite, and there is an actual phase transformation that takes place under the influence of
stress. An isothermal pseudoelastic loading path in the stress-temperature space is schematically
shown in Figure 5. Note that any load path which includes formation of SIMand begins and ends
in the austenitic region results in the pseudoelastic effect. Initially, the material is in the
austenitic phase (point 1 in Figures 6 and 7). The simultaneous transformation and detwinning of
the martensite starts at point 2 and results in fully transformed and detwinnedmartensite (point
3). Continued loading will lead to elastic deformation of the detwinned martensite. Upon
unloading, the reverse transformation starts when point 4 is reached. By the end of the unloading
plateau (point 5), the material is again in the austenitic phase and upon unloading to zero stress
all elastic strain (el ) and transformation strain (t ) is recovered. Only plastic strain (
p), if
generated, remains. A typical experimental result generated at Texas A&M and showing the
pseudoelastic response of a NiTi SMA is presented in Figure 5. Here the temperature was
maintained at a constant 80_C. For stresses below sMs the SMA responds elastically. When the
polycrystalline SMA critical stress (sMs) is reached, (AMdt
) transformation initiates and SIM
Technical Seminar Report Date: 24th March, 2012
Submitted By- TARINI PRASAD MISHRA Page 8 Hall Ticket No- 08261A1856, 4th Year, MME
begins to form. During the transformation into SIM, large inelastic strains are generated (upper
plateau of stress-strain curve in Figure 7). This transformation completes when the applied stress
reaches a critical value, sMf . The material is now in a detwinned martensitic state. For further
loading above sMf thematerial responds nearly elastically. Upon unloading, which is initially
elastic, the reverse transformation initiates at a critical stress, sAs, and completes at a stress sA f
because the mechanical load is applied at a temperature above Af . Note that, due to the positive
slopes of the four transformation lines in the phase diagram, increasing the test temperature
results in an increase in the value of each critical transformation stress. A hysteretic loop is
obtained in the loading/unloading stressstrain diagram. If the applied stress exceeds the critical
value sMf , then the width of the hysteresis loop, less any accumulated non-recoverable plastic
strain, is representative of themaximum amount of recoverable strain which can be produced due
to stress-induced phase transformation from austenite to martensite (et ). Another important
material characteristic observed in Figure 7 is the residual plastic strain (epl ) of _ 0.6% seen
remaining at the end of the loading cycle.
Fig 6: Experimental results illustrating the SME under a constant 200MPa stress (NiTi, [13]).
Technical Seminar Report Date: 24th March, 2012
Submitted By- TARINI PRASAD MISHRA Page 9 Hall Ticket No- 08261A1856, 4th Year, MME
5. Applications for Shape Memory alloys
Bones: Broken bones can be mended with shape memory alloys. The alloy plate has a
memory transfer temperature that is close to body temperature, and is attached to both ends of
the broken bone. From body heat, the plate wants to contract and retain its original shape,
therefore exerting a compression force on the broken bone at the place of fracture. After the bone
has healed, the plate continues exerting the compressive force, and aids in strengthening during
rehabilitation. Memory metals also apply to hip replacements, considering the high level of
super-elasticity. The photo above shows a hip replacement.
Reinforcement for Arteries and Veins: For clogged blood vessels, an alloy tube is crushed and
inserted into the clogged veins. The memory metal has a memory transfer temperature close to
body heat, so the memory metal expands to open the clogged arteries.
Dental wires: used for braces and dental arch wires, memory alloys maintain their shape since
they are at a constant temperature, and because of the super elasticity of the memory metal, the
wires retain their original shape after stress has been applied and removed.
Anti-scalding protection: Temperature selection and control system for baths and showers.
Memory metals can be designed to restrict water flow by reacting at different temperatures,
which is important to prevent scalding. Memory metals will also let the water flow resume when
it has cooled down to a certain temperature.
Fire security and Protection systems: Lines that carry highly flammable and toxic fluids and
gases must have a great amount of control to prevent catastrophic events. Systems can be
programmed with memory metals to immediately shut down in the presence of increased heat.
This can greatly decrease devastating problems in industries that involve petrochemicals,
semiconductors, pharmaceuticals, and large oil and gas boilers.
Golf Clubs: a new line of golf putters and wedges has been developed using. Shape memory
alloys are inserted into the golf clubs. These inserts are super elastic, which keep the ball on the
clubface longer. As the ball comes into contact with the clubface, the insert experiences a change
Technical Seminar Report Date: 24th March, 2012
Submitted By- TARINI PRASAD MISHRA Page 10
Hall Ticket No- 08261A1856, 4th Year, MME
in metallurgical structure. The elasticity increases the spin on the ball, and gives the ball more
"bite" as it hits the green.
Helicopter blades: Performance for helicopter blades depend on vibrations; with memory
metals in micro processing control tabs for the trailing ends of the blades, pilots can fly with
increased precision.
Eyeglass Frames: In certain commercials, eyeglass companies demonstrate eyeglass frames that
can be bent back and forth, and retain their shape. These frames are made from memory metals
as well, and demonstrate super-elasticity.
Piping: The first consumer commercial application for the material was as a shape-memory
coupling for piping, e.g. oil line pipes for industrial applications, water pipes and similar types of
piping for consumer/commercial applications.
Automotive: The first high volume product (> 5Mio actuators / annual) is an automotive valve
application build to run low pressure pneumatics /bladders in a car seat to adjust the contour
(lumbar support / bolsters). The accounting of basical all benefits of SMA over solenoids
(noise/EMC/weight/package (form factor)/power consumption) was the crucial factor to replace
the standard technology by SMA. Where in other applications usually just 2 or 3 SMA benefits
are considered and hinder from replacing the 'old' technologies.
Robotics: There have also been limited studies on using these materials in robotics, for example
the hobbyist robot Stiquito (and "Roboterfrau Lara"), as they make it possible to create very light
robots. Weak points of the technology are energy inefficiency, slow response times, and
large hysteresis.
6. Advantages of Shape Memory Alloys
As reviewed above, shapememory alloys are capable of providing unique and useful behaviors.
The shape memory effect, especially when utilized under applied stress, provides actuation. The
pseudoelastic effect provides two very useful advantages to the aerospace designer: a non-
Technical Seminar Report Date: 24th March, 2012
Submitted By- TARINI PRASAD MISHRA Page 11
Hall Ticket No- 08261A1856, 4th Year, MME
linearity which allows vibration isolation and large recoverable deformations as well as an ac-
companying hysteresis which can dissipate energy and therefore dampen vibration. Because of
these, shape memory alloys can provide a highly innovative method of addressing a given design
problem and are often the only viable option. When considering actuation, a single SMA
component represents a significantly more simplified solution than a standard electromechanical
or hydraulic actuator. Compared to other classes of active materials, SMAs are able to provide
substantial actuation stress over large strains. The subsequent high energy density leads to
compact designs. Finally, SMAs are capable of actuating in a fully three-dimensionalmanner,
allowing the fabrication of actuation components which extend, bend, twist, or provide a
combination of these and other deformations. Each of the actuation application examples listed
above exploit one or more of these positive attributes of SMA behavior. Some require simplicity
and resulting reliability (Mars Pathfinder [17], LFSA [14]). Others require compact actuation
(active skin [13], micro space actuation [17]), and still others impose geometric challenges
(active chevrons [24], rotor blade actuation [26]). Because of their unique properties, SMAs are
able to provide solutions to each of these sets of problems. The SMA design process is not
without some challenges, however, and several material attributes must be carefully considered.
One common design challenge is the difficulty in rapidly transferring heat into and especially out
of an SMA component. This is a result of the fact that, as a metallic material, SMAs have a
relatively high heat capacity and density. When considering repeated actuation of SMA
elements, for example, this heat transfer difficulty leads to a limited frequency of system
response. Although the material mechanisms involved in the diffusionless phase transformation
can occur almost instantaneously, the timedependent process of sufficiently changing
temperature to drive that transformation can limit actuation speed. Moreover, while the supply of
thermal energy can be quickly accomplished (e.g. by direct Joule heating via application of
electricity), the speed of energy removal is limited by the mechanisms of heat conduction and
convection. Several methods have been employed in the hopes of expediting heat transfer,
including forced convection via flowing cooled water [19] and forced conduction through the use
of thermoelectric cooling modules [12]. A second challenge is the low actuation efficiency.
Efficiencies can reach levels in the range of 10%–15% [18] though in some studies they have
fallen short of the idealistic Carnot predictions of _ 10% [24,25]. This is often not important for
commercial and military aircraft applications because engine power and waste heat are often in
Technical Seminar Report Date: 24th March, 2012
Submitted By- TARINI PRASAD MISHRA Page 12
Hall Ticket No- 08261A1856, 4th Year, MME
excess. However, this property can present a significant challenge to those proposing SMA use
on spacecraft, where power is more limited. Finally, there are challenges stemming from the
response of an SMA material when subjected to multiple transformation cycles. If a low number
of cycles is required, the issue is material stability. For consistent multi-cyclic actuation, SMA
elements which have developed a sufficiently stable thermomechanical response via repeated
training cycles should be used. SMA components which are not completely trained yet are
repeatedly transformed will lead to system responses which evolve with every cycle. However, if
a device is intended for one-time operation, such as a micro-actuator for satellite use, then
training is not necessary. Designers must also consider the possible degradation of material
response due to the generation of TRIP, especially when considering many actuation cycles. The
topic of SMA fatigue has been discussed in the literature [4,72,73]. For the first 10–100 cycles,
thematerialwill stabilize, as previously discussed. As with all other metals, however, repeated
deformation of sufficientmagnitudewill eventually lead to failure. Experimental studies on NiTi
or NiTiCu SMA wires undergoing up to 2% transformation strain have shown that such SMA
components can survive for _10,000 cycles [4]. This implies a limitation on the number of cycles
an SMA application can provide.
7. Conclusion
Considering the variety of research and development currently being performed in the area of
shape memory alloys, it is clear that new applications will continue to be developed and that this
field will continue to grow. The needs of various defense agencies will continue to present
greater challenges to engineers and designers. The complexity of space operations is ever
increasing and more is demanded of aircraft, both commercial and military, thus more innovative
technological solutions will be required. The fields of SMA research and application are
providing the tools to meet these challenges. The ability to custom order SMA material with
particular properties manufactured to prescribed specifications has improved. The design and
analysis environments are becoming more powerful by becoming more comprehensive. At the
same time, systems integration capabilities have grown. These developments will result in an
increased prevalence of integrated multifunctional SMA systems for aerospace applications.
Such systems will be highly beneficial to the design of new UAVs and micro- and nano-
Technical Seminar Report Date: 24th March, 2012
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satellites. Other industries will also benefit from the advances made in the SMA field. The
automotive and and oil exploration sectors, each of which has already shown increasing interest,
will continue to employ the properties of SMAs in solving design problems where constraints are
imposed by extreme environments and operating conditions. It is widely expected that medical
applications will further increase in number. This overall growth in the utilization of
shapememory alloys and other activematerials will provide designers with more options, and
those in the aerospace industry should continue to take advantage of the unique engineering
solutions provided by shape memory alloys.
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