Characterization of Non-Metallic Inclusions in Superelastic NiTi Tubes

Non-metallic inclusions have been shown frequently to lead to crack initiation in superelastic Nitinol fatigue specimens. While prior studies suggested that both carbide (TiC) and oxide (Ti4Ni2Ox) 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 tubes fabricated from materials of various VIM and VAR melt processes. Additional samples were taken from various stages of the tube drawing process for studying the stringer formation. Our results suggest that both carbide and oxide inclusions are present in VIM/VAR materials and the oxide break-down during tube drawing appears to be the primary mechanism for stringer formation. Carbides in VIM materials generally remain as isolated particles during tube fabrication while the primary inclusion of oxide in the VAR material explains its higher stringer density. In addition, the carbide inclusion has been confirmed to contain a noticeable amount of oxygen; hence, we suggest the “Ti(C, O)” nomenclature. Further study on the role of oxygen on carbide and oxide formation in VIM/VAR materials may be beneficial for improving the future melt quality of NiTi alloys.