B. Gruez, R. Chantre, P. Comte-Gaz, Minitubes, Grenoble CEDEX 2, France
Literature has well described the tradeoff between superelastic properties and ductility. The desired superelastic properties typically come with an elongation in the range of 10-15%, a limitation to device design. Increasing the ductility while maintaining strong superelasticity potentially opens new fields for device innovation.
A process was designed to obtain high elongation (30-55% range) without compromising superelasticity on stent tubing from the usual binary Ni 50.8% Ti 49.2% alloy.
We present in this paper a unique set of thermo-mechanical properties not yet reported in the literature; “high ductility” processed tubing is compared with “regular process” tubing from the same batch of raw material.
- In the plastic portion of the tensile curve, a high elongation up to 50% is observed, coupled with a drop in the UTS.
- In the superelastic cycle to 8% strain, the “high ductility” process exhibits a slightly longer plateau with similar levels for stress and unrecoverable strain.
- DSC and BFR testing show colder transformation temperatures for the “high ductility” process despite similar plateau stress values, in apparent contradiction with the well known Clausius- Clapeyron equation.
We then show the effects of typical shape setting operations on the high plasticity tubing to predict its behavior on actual devices, before concluding on the application potential for this new process.
Summary: Device design has been dealing with the tradeoff between strong superelastic properties and sufficient ductility. A new process is expanding elongation from the traditional 10 to 15% range up to 50%, without compromising superelasticity of conventional binary nitinol stent tubing.
This paper presents unreported thermomechanical properties and contrasts them with traditional nitinol processing:
- elongation up to 50% with a decrease in UTS,
- identical plateau stress and permanent set, with a slightly longer plateau,
- Colder transformation temperature despite equivalent plateau stress.
Shape setting effect on the high ductility tubing is presented, to predict material behavior on devices after tube processing. We conclude on the application potential for this new nitinol processing.
