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The unique class of materials known as shape memory alloys, such as nickel titanium (Nitinol), undergo an inherent phase transformation in their crystal structure as they are cooled from the stronger, high temperature form called austenite to the weaker, low temperature form called martensite.
These alloys exhibit two distinct crystal structures or phases, which determine the metal’s unique properties, including shape memory and superelasticity. Martensite exists at lower temperatures and is easily deformed, while austenite — which is stable at higher temperatures — “remembers” the shape that was originally programmed into it.
Moreover, the alloys’ shape memory effect can be controlled and improved by changes in alloy composition, mechanical working, and heat treatment. However, these adjustments should be implemented carefully to ensure that the material has the desired properties.
Nitinol is also biocompatible, which makes it ideal for use in medical applications such as vascular stents, anchors for attaching tendons to bone, medical guidewires, medical guidespins, root canal files, and bendable surgical tools. The material’s high damping capacity provides a great degree of comfort and reduces the risk of tissue damage during surgery.
Another fascinating and useful property of Nitinol is superelasticity. This property is a result of stress-induced formation of some martensite above its transformation temperature. This martensite then immediately reverts to undeformed austenite as the stresses are removed, providing an extremely springy, “rubberlike” elastic behavior.