[ Materials ]
[ Materials ]
Nitinol - the metal that remembers
Nitinol ( UNS N01555 ) is a metal alloy of nickel and titanium , where the two elements are present in roughly equal atomic percentages ( 55 to 56 % weight percent nickel ). Nitinol exhibits the unique properties of Shape Memory ( SM ) and Super-Elasticity ( SE ).
By Geir Moe , Nickel Institute
Nitinol ’ s shape memory property allows it to be bent out of shape but revert to its original structure upon heating above its ‘ transformation temperature .’ These properties are due to a phase transformation ( a change in crystal structure ). Below the transformation temperature the microstructure is known as martensite , while above this temperature it is known as austenite . Nitinol displays SE above its transformation temperature , due to martensite forming in areas that are stressed . When the stress is removed , this martensite returns to the undeformed austenite state . While most metals can tolerate only a small fraction of a percent of strain without permanent deformation , nitinol can take up to an eight percent strain and return to its original shape . Similarly , SM enables nitinol to revert to its original structure after deformation . To fix the original ‘ parent shape ,’ the alloy must be held in position and heated to about 500 ° C ( 930 ° F ). This process is called shape setting . Nitinol will revert to its original shape when deformed
A deformed paper clip returns to shape after immersion in hot water . Photo : Wikimedia Commons , Petermaerki , CC BY-SA 3.0
below its transformation temperature . Simply heating the nitinol causes the martensite to transform back to
the undeformed austenite . Making small changes in the composition can change the transition temperature of the alloy significantly . Transformation temperatures in nitinol can be controlled within a range from about −20 to + 110 ° C ( −4 to 230 ° F ). Healthcare and medical device applications are increasingly making use of nitinol ’ s properties including ear implants , eyeglass frames and stents . A medical stent needs to be severely compressed so that it can be inserted into an artery , but when the compression is removed it springs out to support and hold open the artery .
Nitinol stents can be fabricated at one temperature , folded smaller at another temperature , then inserted into an artery where the body heats the material above its transformation temperature and it returns to its original size . Photo Dreamstime .
About Nickel Institute
The Nickel Institute is the global association of leading primary nickel producers . Its mission is to advocate for the responsible supply and sustainable development of nickel and the nickel industry . For information , please visit nickelinstitute . org
20 Stainless Steel World April 2025 www . stainless-steel-world . net