I n t r i n s i c  D e v i c e s  I n c o r p o r a t e d

Shape Memory Technology


Intrinsic 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.

Our products 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. 

This requirement 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 heated.

For detailed technical information, see the paper "Shape Memory Alloy Fasteners".


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