Shape Memory Alloys

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Shape Memory Alloys
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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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Two Phases

• Austenite
• Hard, firm
• Inelastic
• Resembles titanium
• Simple FCC structure
• Martensite
• Soft
• Elastic
• Complex structure

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Shape Memory AlloyQualities

• 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.
• Pseudoelasticity
• When the metal is changed to the martensite phase
  simply by strain. The metal becomes pliable and
  can withstand strains of up to 8.
• 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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Shape Memory
The twinned martensite phase resemble the
austenite phase from our point of view, but on an
atomic level, the structure is different. There
are phase planes where the martensite can
reconfigure itself with 24 crystallographically
equivalent habit planes. This is called twinning
because of the symmetry across the planes.
Phase Changes in NiTi (2001 SMA/MEMS Research
Group)
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Pseudoelasticity

• Pseudoelasticity (superelasticity) occurs when
  the alloy is above the martensite temperature,
  but there is a load strong enough to force the
  austenite into the martensite phase. The alloy
  will not return to the austenite phase until the
  loading is decreased or there is a large enough
  change in temperature.

The figure shows load versus temperature on an
SMA. 2001 SMA/MEMS Research Group
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The figure below shows NiTis ability to change
its shape along phase planes. Other metals, as
we know, slide along slip planes when there is an
induced stress.
The above figure shows the Martensitic
transformation and hysteresis ( H) upon a change
of temperature. As austenite start, Af
austenite finish, Ms martensite start, Mf
martensite finish and Md Highest temperature to
strain-induced martensite. Gray area area of
optimal superelasticity. (Jorma Ryhänen 2000)
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Flexible Nitinol wires.
University of Alberta
Wires have the ability to flex the robotic
muscles according to electric pulses sent through
the wire.
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Nitinol Wires

• Nitinol is generally doped with other materials
  like Cr, Cu, Al, or Fe.
• Flexinol is a popular brand of SMA wire.
• Flexinol is designed to take more repeated stress
  cycles than pure NiTi mixes.
• Specifically designed to manufacturers needs.
• 0.0010 inch diameter wire can lift 7 grams in 1
  second with a 20 mA current.
• 0.010 inch diameter wire can lift 930 grams in 1
  second with a 1 A current.
• Wires are also made to change states at different
  temperatures generally between -30 C and 120 C
  within 5 C.

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Biological Applications

• Bone Plates
• Memory effect pulls bones together to promote
  healing.
• Surgical Anchor
• As healing progresses, muscles grow around the
  wire. This prevents tissue damage that could be
  caused by staples or screws.
• Clot Filter
• Does not interfere with MRI from
  non-ferromagnetic properties.
• Catheters
• Retainers
• Eyeglasses

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Aircraft Maneuverability

• Nitinol wires can be used in applications such as
  the actuators for planes. Many use bulky
  hydraulic systems which are expensive and need a
  lot of maintenance.

USAF Aircraft Pictures
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Typical actuator in the wing of a plane.
University of Alberta
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Picture of wing with SMA wires. University of
Alberta
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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Other Applications

• Small incision tweezers
• Eyeglass frames
• Anti-scalding devices/Fire sprinklers
• Household appliances
• A deep fryer that lowers the basket into the old
  at a certain temperature
• Underwire bras
• Prevent structural damage to bridges/buildings
• Robots

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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 with computer programming
• More expensive to manufacture than steel and
  aluminum
• Relatively new

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References
http//www.mkt-intl.com/tungsten/images/niti_1.jpg
Shape Memory Alloys. University of
Alberta 2001 SMA/MEMS Research Group How Memory
Metals Shape Product Design. Design News June
1993 Ryhänen, Jorma. Biocompatibility
evaluation of nickel- titanium shape memory metal
alloy. 2000 Lin, Richard. Shape Memory Alloys
and Their Applications. Hornboden, E. Review
Thermo-mechanical Fatigue of Shape Memory
Alloys. Journal of Material Science.
2004 Martensitic Transformation. Encyclopedia of
Materials Science and Technology. 2001