Characterization of Non-Metallic Inclusions in Nitinol Tubes

Non-metallic inclusions have been frequently shown to be linked to crack initiation in superelastic Nitinol fatigue specimens.  As the fatigue endurance of Nitinol draws increasing attention for improved device design guidelines in the medical device industry, the effect of inclusions on Nitinol fatigue endurance deserves a more in-depth comprehension. While prior studies have demonstrated that both carbide (TiC) and oxide (Ti₄Ni₂O_x) inclusions can develop in superelastic Nitinol alloys, questions remain on whether or how the chemistry and morphology of these non-metallic inclusions are affected by the melting and subsequent tube manufacturing process.  In the present study, samples of Ti-55.8wt.%Ni alloy were taken from forged and hot rolled bars converted from vacuum induction (VIM) and vacuum arc (VAR) ingots.  Additional samples were taken from tubes at various stages of the tube drawing process.  The chemistry and morphology of non-metallic inclusions in these specimens were analyzed using techniques of optical metallography, scanning electron microscopy, energy dispersive X-ray spectroscopy (EDS) and electron backscatter diffraction (EBSD).  An emphasis focuses on the discussion of the experimental details developed to improve the identification of the different particles present in the material. In another aspect, the correlation between inclusion chemistry and the melting process as well as the influences of tube drawing process and inclusion chemistry on the final morphology of these particles will be analyzed and discussed.