Structure-property relationships in conventional and nanocrystalline NiTi intermetallic alloy wire

The effect of grain size on mechanical property development is examined in fine diameter, nominally 50.8 at% Ni, 49.2 at% Ti wire over a broad domain from tens of nanometers up through conventional grain sizes in the micrometer range. The effect of grain size is elucidated using experimental techniques including cyclic uniaxial tension testing, rotary beam fatigue (RBT) testing, differential scanning calorimetry and bend and free recovery testing for transformation temperature analysis.  Microstructural determination was accomplished using both scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The resistance of the NiTi wires to fatigue damage in RBT testing was found to increase by greater than 30% measured by strain levels in nanocrystalline versus conventional microcrystalline material. In addition, the ultrafine grain structure was found to reduce plastic slip during stress-induced martensitic transformation, thereby reducing permanent deformation under cyclic tensile loading. The significance of these findings is described as they pertain to the use of these wires in the context of medical device applications.