Elementary Deformation and Damage Mechanisms During Fatigue of Pseudoelastic NiTi Microstents

In the present study, we investigate the fatigue behavior of Nickel Titanium (NiTi) microstents at 22 °C (room temperature) and 37 °C up to 30 × 106 load cycles. We briefly describe our test procedure, which applies displacement‐controlled pull–pull fatigue cycling between displacements corresponding to apparent strains of 5 and 7.5%. The response of the microstents to mechanical loading indicates cyclic softening during 30 × 104 cycles. Subsequently, the maximum load remains constant throughout the remainder of the test. We use transmission electron microscopy (TEM) to clarify the microstructural reasons for cyclic softening. A focused ion beam (FIB) technique is used to take out thin foil specimens from critical microstent locations. Our TEM results show that the dislocation density increases during cycling. We also find that microstructural regions with stabilized stress‐induced B19′ martensite can be detected. The fatigue behavior of NiTi stents is investigated up to 30 million load cycles. The response of the stents to mechanical loading indicates cyclic softening during cycling. FIB and TEM are used to clarify the microstructural reasons. The results show that the dislocation density increases and that microstructural regions with stabilized stress‐induced martensite can be detected.