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It is well known that NiTi shape memory alloys exhibit superelasticity where a large elastic deformation proceeds under a constant stress due to the formation of stress-induced martensite. However, for most commercial NiTi alloys used in medical devices, the materials are processed in such a way that the R-phase transformation is apparently separated from the martensitic transformation. The B2 to R-phase change exhibits a much smaller shape change and a narrower hysteresis than does the B2 to B19' martensite transformation. Hence, the transformation temperatures as well as the thermomechanical properties associated with the R-phase are much more stable during cyclic deformation when compared to those associated with the B19' martensite. The crystallographic features for both martensitic and R-phase transformations are reviewed, and their effects on the tensile properties and rotating-beam fatigue endurance are studied in the present paper.
NiTi, B19', martensite, R-phase, superelasticity, rotating-beam fatigue
Wu, Ming H.
Memry Corporation, Bethel, CT