Effect of Different Heat Treatment Equipment on the Mechanical Properties of Nitinol Wire

Abstract

Nitinol is a nickel-titanium shape memory alloy commonly used in the medical device field for many implanted devices because of its unique properties (shape memory and superelasticity). The purpose of this study is to analyze the mechanical behavior differences between Nitinol wire heat treated in a salt bath and Nitinol wire heat treated in a sand bath. The goal was to determine if one heat treatment method is superior to the other when considering the mechanical properties for the design of a transcatheter aortic valve. Eighteen test groups of Nitinol wire were evaluated by using two different types of heat treatment equipment and varying heat treatment temperature and heat treatment time. Samples from each test group underwent tensile testing with upper plateau strength, lower plateau strength, residual elongation, ultimate tensile strength and elongation recorded. Reducing delivery forces requires a low UPS (UPS less than 89,888 psi). Low chronic outward force was desired for improved fatigue resistance and to reduction of conduction issues caused by high radial force. Therefore, a low LPS (LPS less than 43,220 psi) is desired. A residual elongation less than 0.10% to limit the permanent deformation from loading and unloading. Higher UTS (212,372 psi) and Elongation (18.0%) are also desired for greater design space of stronger and more ductile wire. The tensile data was used to determine what equipment and process parameters yields superior mechanical performance. Results show that there is a limited process space to reach the desired mechanical properties. Heat treatment equipment showed a statistically significant impact in the lower plateau strength, ultimate tensile strength and residual elongation. Sample groups heat treated in the salt bath were more repeatable than the corresponding sand bath groups. There was no combination of tested time and temperature that yielded positive results. The salt bath had a limited design space to obtain an optimal process. A heat treatment for a time of 2 minutes and 20 seconds at a temperature of ~515°C in the salt bath is located approximately in the middle of the acceptable area determined by a contour plot. Conducting a range finding study for salt bath heat treatment is recommended to verify the study and potentially expand the processing parameter options.