makes its products from proprietary nickel-titanium alloys that
have a property called shape memory. These alloys undergo
a transformation from one crystal phase to another over a particular
temperature range. Above this range, the material exists
as austenite. Austenite has a rigid crystal structure.
The shape of a component while in the austenite phase is termed
the memory shape. The low temperature phase, martensite,
is soft and can be deformed about 6% from its original shape
without causing any permanent deformation. Once deformed,
martensitic material will remain in this deformed shape indefinitely.
When heated later, the material transforms to the high temperature
phase and returns to its memory shape.
are typically supplied in the deformed martensitic state.
When heated later by the end user, the devices return to their
memory shape. If the part is constrained from fully
recovering it’s memory shape, it will build up a repeatable
clamping force. This force will be maintained as long
as the material remains in the austenitic state. If cooled
back to the martensitic state, the force exerted by the part
will relax to zero. So, a shape memory fastener must have
a reversion temperature to martensite that is below the minimum
operating temperature of the installed fastener.
is met with our Alloy C which is austenitic at temperatures
down to -65°C and not fully martensitic until about -200°C.
However, since the transformation to austenite begins at about
-120°C, Alloy C fasteners must be shipped and stored in
liquid nitrogen, recovering on warming to room temperature.
This behavior is acceptable, even advantageous, for some applications.
However, for most applications, it is desirable to have fasteners
that can be shipped and handled at normal ambient temperatures,
installed by heating above room temperature, and maintain clamping
force down to minimum terrestrial temperatures.
There is a hysteresis
in the thermal response of shape memory alloys. The transformation
to austenite on heating occurs at a higher temperature than
the transformation to martensite on cooling. Through alloying
and processing, this hysteresis can be significantly enlarged.
Using this approach, our Alloy H achieves the objective stated
above. Alloy H fasteners remain martensitic to 50°C,
recover to austenite and build full clamping stress by 165°C,
maintain clamping stress on cooling to -65°C, and do not
become fully martensitic until cooled below -120°C.
Once a part is
heated to recover its memory shape, it will not spontaneously
return to its original supplied shape when cooled to martensite.
Most products do have some two-way effect, moving back toward
their deformed condition by 1% to 2.5% when cooled. The
magnitude of this 2-way effect is dependent on the alloy and
its processing. The 2-way effect is the basis for multi-cycle
fasteners which release their grip when cooled and re-grip when
technical information, see the paper "Shape
Memory Alloy Fasteners".