The Stress-Induced R-Phase Transition In Nitinol and Its Impact On Applications

The thermo-induced R-phase transition in Nitinol material can be readily seen in most DSC curves and electric resistance curves. However it doesn't always show up in stress-strain curves and there are confusions in interpreting the results. In this study the stress-induced R-phase transition was investigated using polycrystalline Nitinol tubing and its impact on fatigue was investigated by testing ev3 EverFlex stents. The tubing samples were heat treated differently to achieve different transformation temperatures and were uniaxial tensile tested at different temperatures. The ev3 EverFlex stents were heat treated with the same ways as the tubing samples and were fatigue tested at 37oC. The stress-strain curves clearly showed different deformation mechanisms at different temperatures, such as R-phase re-orientation, stress-induced R-phase and martensitic transformations and the elastic deformation of R-phase and B2 phase, etc. The lack of distinct stress plateau associated with R-phase transition makes it difficult and confused in determining the Young's Modulus of Nitinol material. It was demonstrated that the Young's moduli of R-phase and B2 phase were different and not temperature dependent. The results in this study also indicated that the stress-induced R-phase transition only existed under limited conditions which were determined by the temperature dependence of R-phase transition and its interaction with the martensitic transformation. Fatigue testing results demonstrated that the low hysteresis R-phase transition significantly improves the fatigue resistance of Nitinol material.