The document provides an overview of shape memory alloys (SMAs), highlighting their ability to undergo phase transformations which allow them to 'remember' their original shape upon heating or applying stress. It covers historical milestones, various types of SMAs, their mechanisms of action, applications in fields like medicine and aerospace, and the advantages and challenges associated with their use. Additionally, it illustrates specific applications such as stents in medical devices and micro-actuators in engineering.

1. Special Technologies
An Introduction to
Shape Memory Alloys (SMAs)
Mehrshad Mehrpouya
mehrshad.mehrpouya@uniroma1.it
Sapienza University of Rome
Department of Mechanical and Aerospace Engineering

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Types of Materials
- Metals
- Ceramics
- Polymers
- Composites
- Semiconductors
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• Piezoelectric materials
• Shape memory alloys
• Magnetic shape memory alloys
• Magnetorheological
• PH sensitive polymers
• Halochromic materials
• Thermochromic materials
• Chromogenic systems
• Electrochromic
• Smart Grease
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What are Shape Memory Alloys?
Shape Memory Alloys (SMAs) are metallic alloys that undergo a solid-to-solid phase
transformation which can exhibit large recoverable strains.
Example: Nitinol

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Timeline of Memory Metals
• 1932 - A. Ölander discovers the pseudoelastic properties of Au-Cd alloy.
• 1949 - Memory effect of Au-Cd reported by Kurdjumov & Kandros.
• 1967 – At Naval Ordance Laboratory, Beuhler discovers shape memory
effect in nickel titanium alloy, Nitinol, which proved to be a major
breakthrough in the field of shape memory alloys.
• 1970-1980 – First reports of nickel-titanium implants being used in
medical applications.
• Mid-1990s – Memory metals start to become widespread in medicine and
soon move to other applications.

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Number of publications per year

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Geographic distribution

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Shape Memory Alloys
Alloy Transformation Composition
Transformation Temp. Rang
(°C)
Hysteresis (°C)
Ag-Cd 44/49 at % Cd -190 to -50 ~15
Au-Cd 46.5/50 at % Cd 30 to 100 ~15
Cu-Al-Ni
14/14.5 wt %Al, 3/4.5 wt %Ni
-140 to 100 ~35
Cu-Sn ~15 at % Sn -120 to 30 −
Cu-Zn 38.5/41.5 wt % Zn -180 to -10 ~10
Cu-Zn-X (X=Si,Sn,Al) few wt % X -180 to 200 ~10
In-Ti 18/23 at % Ti 60 to 100 ~4
Ni-Al 36/38 at % Al -180 to 100 ~10
Ni-Ti ~49/51 at % Ni -50 to 110 ~30
Fe-Pt ~25 at % Pt ~-130 ~4
Mn-Cu 5/35 wn % Cu -250 to 180 ~25
Fe-Mn-Si 32 wt % Mn -200 to 150 ~100

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Ferromagnetic shape-memory alloys
• Shows shape-memory effect in response to a magnetic field
• Deformation due to magnetic field is known as
magnetoelastic deformation.
• Ni-Ti is non-magnetic
• Examples of ferromagnetic SMAs: Ni2MnGa, Fe-Pd, Fe3Pt

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Material behavior
in Micro-size

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Mf Ms As Af
AusteniteMartensite TEMPERATURE
Mf Ms As Af
AusteniteMartensite TEMPERATURE
(twinned)
(twinned)
Characteristic temperatures:
Mf=Martensitic Finish
Ms=Martensitic Start
As=Austenitic Start
Af=Austenitic Finish
Thermally Induced Phase Transformation in SMAs

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How are SMAs working?

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Triggers for
Martensitic
Transformation
(MT)
Stress
Temperature

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How does it work?
Step 1: Austenite Phase
• High Temperature
• The atoms arrange
themselves in their
“permanent” shape

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Step 2: Martensite Phase
• Low temperature
• Cubic structure
becomes folded or
twined

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Step 3:
• Bend the Wire
• It remains in its
Martenesite Phase

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Step 4: Austenite Phase
• Heat the wire above the
transition temperature
of 50 degrees
• It moves back to its
original position!

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Austenite & Martensite Phases

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SMAs
Pseudoelasticity
or Superelasticity
(SE)
Shape Memory
Effect (SME)

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Shape Memory Alloys
• Pseudoelasticity or Superelasticity (SE)
– When the metal is changed to the martensite phase simply by strain.
The metal becomes pliable and can withstand strains of up to 8%.
• Shape Memory Effect (SME)
– These materials have an ability to “remember” its austenite phase. As
the metal is cooled to the martensite phase, it can be easily deformed.
When the temperature is raised to the austenite phase, it reforms to
the original shape of the material.
A mix of roughly 50% nickel and 50% titanium is the most common SMA.
Also CuZnAl and CuAlNi are widely used.

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Superelastic behavior (SE)
SMAs deformed above a critical
temperature show a large
reversible elastic deformation
(recoverable strains up to 10%.
much exceeding the elasticity) as a
result of stress-induced
martensitic transformation.

23. Superelastic behavior
T > Af
Hysteresis loop means
energy dissipation,
hence vibration damping
Stress

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Applications of
superelastic behavior
• Orthodontal braces
• Frames for eyeglasses
• Underwires for brassieres
• Antennas for cellular phones

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Shape Memory Effect (SME)
• Martensitic phase transformation that occurs as a result of
stress or temperature change

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The Shape Memory Effect
s
e
T
Cooling
Detwinning
Heating/Recovery

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Applications of shape memory effect
• Self-expandable cardiovascular stent
• Blood clot filters
• Engines
• Actuators for smart systems
• Couplings
• Flaps that change direction of airflow depending upon temperature
(for air conditioners)

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SHAPE MEMORY ALLOY (NITINOL)
Inactive
Active - Bending Active - Twist

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BEFORE TRANSFORMATION DURING TRANSFORMATION AFTER TRANSFORMATION
SHAPE MEMORY POLYMERS

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AFTER TRANSFORMATIONBEFORE TRANSFORMATION DURING TRANSFORMATION
SHAPE MEMORY POLYMERS

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SHAPE MEMORY POLYMERS

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Types of shape-memory behavior
One-way shape memory:
transformation to the desired
shape occurs only upon heating,
i.e., memory is with the
austenite phase.

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Types of shape-memory behavior
Two-way shape memory: the deformed shape is
remembered during cooling, in addition to the
original shape being remembered during heating,
i.e., memory is with both austenite and martensite
phases (requires training to attain memory during
cooling; formation of favorably oriented twins
during cooling between Ms and Mf)

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Physical properties of Nitinol (versus SS)

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The applications
of SMAs

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• Micro-actuators
• Mobile phone antennas
• Orthodontic archwires
• Penile implant
• Pipe couplings
• Robot actuators
• Rock splitting
• Root canal drills
• Satellite antenna deployment
• Scoliosis correction
• Solar actuators
• Spectacle frames
• Steam valves
• Stents
• Switch vibration damper
• Thermostats
• Underwired bras
• Vibration dampers
• ZIF connectors
• Aids for disabled
• Aircraft flap/slat adjusters
• Anti-scald devices
• Arterial clips
• Automotive thermostats
• Braille print punch
• Catheter guide wires
• Cold start vehicle actuators
• Contraceptive devices
• Electrical circuit breakers
• Fibre-optic coupling
• Filter struts
• Fire dampers
• Fire sprinklers
• Gas discharge
• Graft stents
• Intraocular lens mount
• Kettle switches
• Keyhole instruments
• Key-hole surgery instruments
Currentexamplesofapplicationsof
shapememoryalloys

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Existing and potential SMA applications in the biomedical domain

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SMAs in Bio-medical Devices

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Bone AnchorsRobotic
arms
Medical Stents

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SMA orthodontic wires

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Nitinol is used in medicine for
stents: A collapsed stent can be
inserted into a vein and heated
(returning to its original expanded
shape) helping to improve blood
flow. Also, as a replacement for
sutures where nitinol wire can be
weaved through two structures
then allowed to transform into it's
pre-formed shape which should
hold the structures in place.
SMAs Stents

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Bonestaple

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Photographs of (a) brassiere and (b) various designs of
superelastic NiTi underwires.

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Existing and potential SMA applications in the automotive domain

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Existing and potential SMA applications in the aerospace domain

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Picture of wing with SMA wires
The wires in the picture are used to replace the actuator. Electric
pulses sent through the wires allow for precise movement of the
wings, as would be needed in an aircraft. This reduces the need
for maintenance, weighs less, and is less costly.

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SMAs Microactuators

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SMAs Micro-actuators
SMAs can generate the mechanical work
in shape of large actuation force and
displacement during the phase
transformation. This deformation is
produced due to cooling and heating
cycle as two significant elements in SMA
actuators. The main parts of the SMA
actuators include; SMA part, mechanical
system, electronic control system, the
fixture body and the effective element
with recovering capability based on the
employed stress.

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Advantages and dis-advantages of shape memory materials

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SMAs Micro-actuators
The most common applications of NiTi thin films are
concentrated on microactuators due to their particular
capabilities in MEMS, such as micropumps, microvalves,
microgrippers, micropositioners, microsprings,
microspacers, and microwrappers, etc.

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The schematic of NiTi microvalve
in (a) close position, (b) open
position

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(Left) Scanning electron micrograph of a NiTi micro-gripper (Right) (1) The
sketch of operational mode (2) Thermo-mechanically cycle (3) The working
principle due to heating/cooling

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Magnetic microgripper (a) the
schematic of the body, (b) Gripping
a micro-object by the micro-tip

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A NiTi microgripper with a hook
structure and two C-shape probes
(a) cooled state (b) heated state

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The micro wrapper based on NiTi thin film

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Schematics of the operation of MicroSwitches

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Problems With SMAs
• Fatigue from cycling
– Causes deformations and grain boundaries
– Begin to slip along planes/boundaries
• Overstress
– A load above 8% strain could cause the SMA to completely lose its
original austenite shape
• Difficulty in machining process
• Difficulty with computer programming
• More expensive to manufacture than steel and aluminum

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