Microstructural Evolution from Bar to Fine Wire, an Interlaboratory Study

Nitinol medical devices are rarely manufactured from these ASTM F2063-certified wrought products, but rather from semi-finished materials that have undergone additional cross-section reduction. With the fatigue performance being strongly influenced by the microstructure of the finished product form, not necessarily the upstream wrought product, this research establishes a correlation between the certified upstream dimensions and the downstream finished wire microstructures. The evolution of oxides versus carbides is elucidated by evaluating oxide-dominant ingots from Fort Wayne Metals versus carbide-dominant ingots from Resonetics. Additionally, because the ASTM specification does not limit the upstream manufacturing methods, the role of hot- versus cold-work was assessed by subdividing each ingot into two arms – 1) hot-rolled to 1” bar followed by cold-drawing to 0.005”, and 2) hot-rolled to 0.25” coil followed by cold-drawing to 0.005”.

Metallurgical samples were taken after each throughout the drawing process to track the microstructural evolution (nominally 1”, 0.5”, 0.25”, 0.125”, 0.060”, 0.030”, 0.015”, 0.008”, 0.005”) A minimum of twenty five micrographs for each wire diameter at 500X, for a minimum sampled area of 0.5 mm², were examined using backscatter scanning electron microscopy combined with custom image analysis software. The microstructure was quantified per the following measurements taken in both the transverse and longitudinal directions: prior austenite grain size (µm), inclusion area fraction (%), density of inclusions (count/µm²), maximum inclusion size (µm), aspect ratio of diameter-to-inclusion size, and extreme value statistics lambda and standard deviation per ASTM E2283.