The Influence of the Heat Treatment Process on Important Application Properties of Nitinol Medical Devices

One of the most crucial process steps in manufacturing Nitinol medical devices with respect to the final mechanical properties is the heat treatment during shape setting of the part. While the influence of the heat treatment in a typical process parameter range on the transformation temperatures is well known but not yet fully understood, the influence on the fatigue behavior is not widely investigated. Thus, the purpose of the first part of this study is to examine the influence of different heat treatment temperatures on the fatigue behavior. The heat treatment times are varied to adjust to similar austenite finish temperatures. To eliminate the influence of impurities in the material on the fatigue results as far as possible, fatigue coupons are made of a Generation III Nitinol tube (VAR/EBR processed).

In a second part of this study, a generic stent structure is manufactured with same temperature sets as in the first part using tubes during heat treatment but with different initial mechanical properties. The influence of the heat treatment temperatures and the different mechanical properties of the tubes on the stent radial force and permanent set after several crimp cycles is then investigated and compared.