Phase Stability of Nonmetallic Interstitials in Ni-rich Ti-Ni Alloys

Nonmetallic inclusions and the resulting particle–void assembly (PVA) formed in Ti-Ni alloys are key factors influencing fatigue resistance. A previous study by the authors' group demonstrated that, in Ti-51.0 at%Ni alloys, setting the carbon-to-oxygen (C/O) weight ratio to 1.5 or higher with vacuum induction melting leads to the sole retention of TiC(O) with a NaCl structure during subsequent manufacturing processes such as hot forging and rolling, thereby yielding rather excellent fatigue resistance [1]. Conversely, when the C/O ratio is below 1.5, a phase transition from TiC(O) to Ti₄Ni₂O_X with a face-centered cubic (fcc) structure occurs during subsequent manufacturing processes, leading to a higher frequency of PVA formation.

In the present study, the characteristics of nonmetallic inclusions formed in Ni-rich Ti-Ni alloys with systematically varied carbon and oxygen contents were investigated using metallographic techniques. The transition from TiC(O) to Ti₄Ni₂OX was found to exhibit unique microstructural features, including core–shell structures (core: TiC(O), shell: Ti₄Ni₂O_X), and cores containing a dispersed secondary phase with plate-like morphology.