Volume 4, Issue 7 (July 2007)
Martensite Transformations and Fatigue Behavior of Nitinol
In addition to excellent biocompatibility and corrosion resistance, the unique flow behavior of NiTi pseudo-elastic alloys renders them ideally suited for several medical device applications utilizing minimally invasive technologies. For example, self-expanding stents are indicated chronic therapies for treatment of a variety of vascular diseases. Long-term device integrity is thus critical with the FDA recommending a minimum 10-year life. Historically, the majority of published fatigue data on NiTi alloys have been generated under strain-controlled conditions using wire or single-diamond cell samples designed to replicate the behavior of the repetitive unit within a stent. Resulting data are then presented on modified Goodman or Soderberg diagrams in order to define regions of acceptable device life. Consistent with their unusual monotonic flow characteristics these alloys are found to exhibit unique cyclic behavior in fatigue: published data clearly demonstrate that increasing mean strain often increases high-cycle fatigue life. This anomalous behavior is generally attributed to any of a number of possible microstructural or mechanical effects associated with the reversible stress-assisted austenite-to-martensite phase transformation. Although the majority of published work has focused on these macroscopic properties in order to characterize and adequately define device life, various efforts are currently underway to better understand the more fundamental metallurgical and mechanical aspects controlling fatigue life in this alloy. Such studies include crack initiation and propagation rates, determining the roles of absolute and relative inherent flow behavior of the transforming structures, mechanical instabilities resulting from the heterogeneous nature of the transformation, latent heat effects during cyclic transformation, as well as the effects of testing frequency, crystallographic texture, grain-shape anisotropy, and stress-state on fatigue life. A review of existing literature delineating these behaviors and current efforts is presented in light of known beneficial effects of transformation plasticity and toughening in enhancing fatigue and fracture properties of a variety of metallic and nonmetallic materials.