06-02-2013, 02:51 PM
Developing a Good Memory Nitinol Shape Memory Alloy
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INTRODUCTION
Very special class of materials can
“remember” their shape. After
deformation you can just heat them
and they return to their original form. A number
of different shape memory materials have
been developed, but one in particular has
become a standby in many applications.
Nitinol. The name comes from the metals
it’s made of and the lab where this remarkable
family of alloys was discovered. NiTi-
NOL; Nickel, Titanium and Naval Ordnance
Laboratory.
Discovery
In 1958, William J. Buehler, a researcher at the Naval Ordnance
Laboratory in White Oak, Md., started looking for a
metallic alloy for missile nose cones that could withstand
the high temperatures of re-entry into the atmosphere,
Buehler wrote in “NITINOL Re-Examination,” an article
about the alloy’s history. He selected 12 likely alloys. One
day he discovered that one of the alloys, a combination of
nickel and titanium, rang like a bell when struck if it was
warmed above room temperature, but at room temperature
it sounded dull, like lead. This attracted some attention
in the lab.
Later, Buehler prepared for a lab management meeting
by bending a thin strip of the material into an accordion
shape, intending to fl ex it repeatedly to demonstrate the
material’s fatigue resistance, he wrote.
Shape memory
To demonstrate shape memory behavior you can deform
a Nitinol item at a low temperature, really bending it out
of shape. Then heat it and watch it return to its original
shape, as if by magic. Nitinol in its martensitic (low temperature)
state is easily bent or twisted into a new shape.
Then, as it is heated it passes through a narrow range of
temperatures where it changes to an austenitic structure
and reverts to its previous shape.
The temperature above which the material remembers
its high-temperature form is called the transformation
temperature.
“The useful range of the material is typically between 0
degrees and 100 degrees Celsius [32 F and 212 F], a very
narrow range when you think of metals,” said Joseph Kain,
product manager, Johnson Matthey, Inc., West Chester,
Pa., a supplier of Nitinol and other metals and parts to the
medical device industry.
Applications
Though many Nitinol applications are invisible to the general
public, some are quite familiar. Eyeglass frames made
from Nitinol can be bent severely out of shape, but then
return perfectly to normal. Also, some years ago, when cell
phones had pull-out antennas, many of the antennas were
made from Nitinol, allowing them to fl ex without breaking or
permanently bending.
Another superelastic application, where you or your
children may have experienced Nitinol, is in arch wires used
for orthodontistry. The orthodontist takes a Nitinol wire and
bends it, attaching it to the teeth. Because the wire is superelastic,
it tries to return to its straight condition, and continually
exerts a force on the teeth. This allows less frequent
visits to the orthodontist to have braces tightened.
A number of lesser-known applications use Nitinol’s
shape memory capability. A well-known computer manufacturer
used a Nitinol device to eject PCMCIA cards. Also,
Nitinol is used in couplings that join the ends of hydraulic
tubing in aircraft. In a less serious application, Nitinol
enables spoons from the magic shop to bend when placed
in hot water.