SHAPE MEMORY ALLOYS (SMAs) refer to a group of materials
which have the ability to return to a predetermined shape when heated. The
shape memory effect is caused by a temperature dependent crystal structure. When
an SMA is below its phase transformation temperature, it possesses a low
yield strength crystallography referred to as Martensite (see Stress-Strain
figure). While in this state, the material can be deformed into other shapes
with relatively little force. The new shape is retained provided the material
is kept below its transformation temperature. When heated above this
temperature, the material reverts to its parent structure known as Austenite
causing it to return to its original shape (see Phase Transformation
figure). This phenomenon can be harnessed to provide a unique and
powerful actuator.
The most widely used
shape memory material is an alloy of Nickel and Titanium called
Nitinol. This particular alloy has excellent electrical and
mechanical properties, long fatigue life, and high corrosion
resistance. As an actuator, it is capable of up to 5% strain and
50,000 psi recovery stress, resulting in ~1 Joule/gm of work output.
Nitinol is readily available in the form of wire, rod, and bar stock
with transformation temperature in the range of -100° to +100°
Celsius. More recently applications in
Micro-Electro-Mechanical-Systems (MEMS) has led to the development
of Nitinol in the form of sputter deposited thin film.
For more information about Shape Memory Alloy and its applications,
contact TiNi Aerospace. | | Stress-Strain Characteristics
Phase Transformation
Properties of Nitinol
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Density
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6.45 gm/cm3
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(0.23 lb/in3 )
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Thermal Conductivity
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10 W/moK
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(5.78 Btu/hr ftoF)
|
|
Specific Heat
|
322 j/kgoK
|
(0.08 Btu/lboF)
|
|
Latent Heat
|
24,200 J/kg
|
(10.4 Btu/lb)
|
|
Ultimate Tensile Strength
|
750-960 MPa
|
(110-140 ksi)
|
|
Elongation to Failure
|
15.5%
|
(15.5 %)
|
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Yield Strength (Austenite)
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560 MPa
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(80 ksi)
|
|
Young's Modulus (Austenite)
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75 GPa
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(11 mpsi)
|
|
Yield Strength (Martensite)
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100 MPa
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(15 ksi)
|
|
Young's Modulus (Martensite)
|
28 GPa
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(4 Mpsi)
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