Shape memory alloys (SMAs) undergo a solid-to-solid phase transformation from martensite to austenite phases in response to temperature changes or stress, allowing large recoverable strains. SMAs such as nitinol exhibit one-way or two-way shape memory behavior and are used in applications like couplings, MEMS actuators and sensors, and microvalves. Common SMAs include nitinol, Cu-Zn-Al, and Cu-Al-Ni alloys.

1. SHAPE MEMORY ALLOYS

2. 1.0 INTRODUCTION
• A material that can remember its shape
• Shape Memory Alloys (SMAs) are metallic
alloys that undergo a solid-to-solid phase
transformation which can exhibit large
recoverable strains.
WACHIRA J.
NDUNG'U/MEMS/MECHATRONIC
ENGINEERING/JKUAT

3. Origin of
shape-memory effect
• Martensitic phase transformation that occurs
as a result of:
a) stress or
b) temperature change
WACHIRA J.
NDUNG'U/MEMS/MECHATRONIC
ENGINEERING/JKUAT

4. Thermally Induced Phase
Transformation in SMAs
WACHIRA J.
NDUNG'U/MEMS/MECHATRONIC
ENGINEERING/JKUAT

5. Martensitic transformation
temperatures
• Ms: temperature at which austenite begins to
transform to martensite upon cooling
• Mf: temperature at which transformation of
austenite to martensite is complete upon cooling
• As: temperature at which martensite begins to
transform to austenite upon heating
• Af: temperature at which transformation of
martensite to austenite is complete upon heating
WACHIRA J.
NDUNG'U/MEMS/MECHATRONIC
ENGINEERING/JKUAT

6. 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.
• 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)
WACHIRA J.
NDUNG'U/MEMS/MECHATRONIC
ENGINEERING/JKUAT

7. Applications of SMA
Use of memory alloys
for coupling tubing: A
memory alloy coupling
is expanded (a) so it fits
over the tubing (b).
When the coupling is
reheated, it shrinks
back to its original
diameter (c), squeezing
the tubing for a tight fit
WACHIRA J.
NDUNG'U/MEMS/MECHATRONIC
ENGINEERING/JKUAT

8. SMA in MEMS
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Desirable properties for MEMS applications
high power to weight ( force to volume) ratio,
pseudoelasticity (or superelasticity),
high damping capacity,
good chemical resistance and biocompatibility
WACHIRA J.
NDUNG'U/MEMS/MECHATRONIC
ENGINEERING/JKUAT

9. SMA ACTUATORS
• TiNi actuators
• TiNi films in MEMS are focused on
microactuators, such as
micropumps, microvalves, microgrippers, springs, microspacers, micropositioners,
TiNi cantilever
showing the
actuation during
heating and
cooling
WACHIRA J.
NDUNG'U/MEMS/MECHATRONIC
ENGINEERING/JKUAT

10. SMA SENSORS
• TiNi Sensors
• TiNi thin films are sensitive to environmental changes
such as thermal, stress, magnetic or electrical fields.
• Examples: a gate of metal-on-silicon (M.O.S.) capacity
sensor, switches or microrelays include on-chip circuit
breakers against overheating caused by short circuit or
overload, probe tips for automatic test
equipment, fiber optics switching, automotive fuel
injectors, micro-lens positioner, light valve or on-off
optical switch for spatial light modulators, a far infrared
radiation (FIR) imaging sensor
WACHIRA J.
NDUNG'U/MEMS/MECHATRONIC
ENGINEERING/JKUAT

11. Examples of SMAs
• Cu-Zn-Al
• Cu-Al-Ni
• Ni-Ti (50 at.% Ti, nitinol, which stands for
Nickel Titanium Naval Ordinance Laboratory)
WACHIRA J.
NDUNG'U/MEMS/MECHATRONIC
ENGINEERING/JKUAT

12. SMA Applications
Scanning Electron Microscope picture of the TWSME micro-gripper for
sub-millimeter lens handling
WACHIRA J.
NDUNG'U/MEMS/MECHATRONIC
ENGINEERING/JKUAT

13. SMA- Example of a Microvalve
Micro-valve with SMA actuator (Ti-Ni ribbon) .The valve is
shown in its closed configuration. Dimensions of the assembly are 5 x
8mm for a thickness of 2mm. The film thickness is a few microns.
WACHIRA J.
NDUNG'U/MEMS/MECHATRONIC
ENGINEERING/JKUAT

14. The Olympus Co. micro-endoscope .
Left: The endoscope winding around a match; right: exploded view of the
inside structure.
WACHIRA J.
NDUNG'U/MEMS/MECHATRONIC
ENGINEERING/JKUAT