Nitinol Bounces Back
Nitinol is a family of intermetallic materials that contain a nearly equal mixture of nickel and titanium. Its claim to fame lies with its unique behavior: nitinol undergoes a unique phase transformation in its crystal structure whenever it transitions between its stronger, high temperature state - Austenite phase - and its weaker, low temperature state - Martensite phase.
Unique Features
This unusual behavior endows nitinol with the properties of "superelasticity" and "shape memory." You can adjust or tune these properties, including the temperatures at which the properties exist, by adding other elements.
Shape memory refers to the process of restoring the original shape of a plastically deformed sample by heating it. Below its transformation temperature, nitinol stays in its softer, martensitic phase. Heating nitinol, however, converts it to its high strength, austenitic condition.
Superelasticity refers to the ability of nitinol to spring back to its original shape when a deforming stress is removed. This elasticity is also known as "pseudoelasticity" or transformational "superelasticity."
Uses
Nitinol-based products are not new. Early products in the late 1960's made use of nitinol's thermal shape memory behavior. The tradition continues with current products such as hydraulic line couplings for military jets and ships, electrical connectors, and actuators for appliances and automotive transmissions.
In the medical device industry, nitinol is in demand for reusable medical instruments. Surgeons can shape an instrument on-site to fit a patient's geometry. After heat sterilization, the device, such as a heart valve sizer tool, can return to its original shape for the next procedure.
Today the emphasis has shifted to using nitinol's superelastic behavior – about 10 times greater than stainless steel's – for products in the telecommunications, sporting goods, and eyeware industries. Medical devices, in particular, benefit because nitinol alloys, when processed correctly, reach their optimum superelastic behavior at body temperature. |
