Nitinol alloys exhibit two closely related and very unique properties: shape memory and superelasticity (also called pseudoelasticity).
Nitinol (°°50% nickel, °°50% titanium) alloys exhibit two closely related and very unique properties: shape memory and superelasticity (also called pseudoelasticity). Shape memory refers to the ability of Nitinol to undergo deformation at one temperature, and then recover its original, undeformed shape upon heating above its "transformation temperature." Superelasticity occurs at a narrow temperature range just above its transformation temperature. In this case, no heating is necessary to cause the undeformed shape to recover, and the material exhibits enormous elasticity – some 10-30 times that of ordinary metal.
Nitinol's extraordinary ability to accommodate large strains, coupled with its physiological and chemical compatibility with the human body have made it one of the most commonly used materials in medical device engineering and design. One such application for nitinol alloys is medical stents.
Shape setting refers to the process used to form Nitinol. Whether the Nitinol is superelastic or shape memory, in the cold work or straightened condition, it is often necessary to form the material into a new “memory” shape. This is done by firmly constraining the material into its new shape in a fixture or on a mandrel and then performing a heat treatment. The heating method can be an air or vacuum furnace, salt bath, sand bath, heated die or other heating method. The temperature should be in the range of 500-550°C (930-1020°F) with higher temperatures resulting in lower tensile strengths. Cooling should be rapid to avoid aging effects, a water quench is recommended. The heat-treatment time should be such that the material reaches the desired temperature throughout its cross-section. This depends on the mass of the fixture and material and the heating method. Times may be less than a minute for heating small parts in a salt bath or heated die. Times may be much longer (10-20 minutes) for heating massive fixtures in a furnace with an air or argon atmosphere. In these cases, a thermocouple in contact with the material or fixture is recommended. In all cases, experimentation for the proper time and temperature will be required to determine the combination that gives the desired results.
Aging can be done to raise the Aftemperature of superelastic Nitinol components. Aging is done by heat treating to about 475°C (890°F) for extended periods. Aging and shape setting can be done simultaneously by firmly constraining the material to its new shape in a fixture and heating to around 475°C (890°F) for up to an hour. Longer times result in higher Aftemperatures. Aging can also be done on material that was previously shape set. As with shape setting, aging times must be determined experimentally because they depend on the processing history of the material, the heating method and temperature. It is advisable to perform a water quench after aging to sharply define the heating time.
Heat Treatment of Nitinol Alloys (Part 1)
Dan Herring is president of THE HERRING GROUP Inc., which specializes in consulting services (heat treatment and metallurgy) and technical services (industrial education/training and process/equipment assistance). He is also a research associate professor at the Illinois Institute of Technology/Thermal Processing Technology Center. tel: 630-834-3017; e-mail: email@example.com; web: www.heat-treat-doctor.com
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