Shape memory alloys, such as nickel titanium, undergo a phase transformation in their crystal structure when cooled from the stronger, high temperature form (austenite) to the weaker, low temperature form (martensite). This inherent phase transformation is the basis for the unique properties of these alloys -- in particular, shape memory and superelasticity.

 

Shape memory

When a shape memory alloy is in its martensitic form, it is easily deformed to a new shape. However, when the alloy is heated through its transformation temperatures, it reverts to austenite and recovers its previous shape with great force. This process is known as shape memory.

The temperature at which the alloy remembers its high temperature form when heated can be adjusted by slight changes in alloy composition and through heat treatment. In the nickel titanium alloys, for instance, it can be changed from above +100 deg.C to below -100 deg.C. The shape recovery process occurs over a range of just a few degrees and the start or finish of the transformation can be controlled to within a degree or two if necessary.

 

Superelasticity

These unique alloys also show a superelastic behavior if deformed at a temperature which is slightly above their transformation temperatures. This effect is caused by the stress-induced formation of some martensite above its normal temperature. Because it has been formed above its normal temperature, the martensite reverts immediately to undeformed austenite as soon as the stress is removed. This process provides a very springy, "rubberlike" elasticity in these alloys.

 

Properties of nitinol shape memory alloys

 

Both forms the nitinol alloy are...

  • Ductile: elongation to failure over 25%
  • Strong: tensile strength up to 200,000 psi
  • Biocompatible and extremely corrosion resistant

Martensite is...

  • Fairly Weak: 10,000 to 20,000 psi deformation stress
  • Able to absorb up to 8% recoverable strain

Austenite is...

  • Strong: 35,000 to 100,000 psi yield strength
Loop of nitinol tubing

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