Elementary Deformation and Damage Mechanisms During Fatigue of Pseudoelastic NiTi Microstents

Abstract 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.