Study On Increased "Stiffness" In Binary and Ternary NiTi-Based Alloys

If the term „stiffness“ is used for metals it generally refers to the Young’s Modulus. In relation to Nitinol alloys the stiffness usually refers to the height of the plateaus in the stress-strain-curve of a superelastic sample. Especially in many guidewire applications but also for use in design of very fine stent structures, the combination between high flexibility (i.e. superelasticity) and a high torqueability or radial force whilst maintaining a small cross section is desirable. For this purpose the development of alloys with improved plateau heights as compared to standard binary Nitinol has been successfully initiated.

The current paper will compare the results achieved on NiTiCo, NiTiCr and NiTiTa alloys with standard binary Nitinol alloys in terms of static mechanical properties but also in terms of fatigue properties. For this purpose all produced alloy formulations were drawn to fine wire and subsequently tested in tensile tests, active A_f tests and in rotary beam tests to determine the fatigue performance.

A design of experiment was setup to determine the best set of parameters for thermomechanical treatment of some of the manufactured wires in order to define the optimal process window.