Microstructural Changes and Mechanical Properties of NiTi Microstents Subjected to Fatigue Loading in the LCF to HCF Regime

Wednesday, May 19, 2010 - 10:35 AM

Microstructural Changes and Mechanical Properties of NiTi Microstents Subjected to Fatigue Loading in the LCF to HCF Regime

M. Frotscher, T. Simon, K. Neuking, Ruhr-University Bochum, Bochum, Germany; C. Somsen, G. Eggeler, Ruhr University Bochum, Bochum, Germany

Medical stents are small tubular implants traditionally made out of classic structural materials, such as 316L stainless steel or CoCr. NiTi shape memory alloys (SMA) are gaining importance due to their superior mechanical properties and self-expansion. Present guidelines require a 10-year service life, but premature failures have been reported in literature. Previous work has related processing-induced surface defects, such as intermetallic phases, laser burrs or electropolishing artifacts to these fracture events. Our results indicate that microstructural changes may be involved in this phenomenon. We compare the microstructure of NiTi microstents after a well defined number of fatigue cycles by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Due to small structure sizes, focused ion beam (FIB) was used for sample preparation. The influence on surface roughness was studied by confocal laser scanning microscopy (CLSM). We used nanoindentation to examine differences in nanohardness and shape recovery before and after fatigue.

Summary: NiTi stents have gained significant importance in medicine. However, fatigue is not a well understood topic in shape memory alloys, although this is important with regard to stent applications. We performed high cycle fatigue (HCF) experiments to observe changes in the microstructure of ultrafine-grained NiTi stents and report on our interesting results.