Thermodynamic Analysis of Oxygen Behavior During Solidification in Titanium–Rare-Earth Alloys in the Presence of Rare-Earth Fluorides

This study presents a thermodynamic analysis of the behavior of oxygen (O) during the early stage of solidification in titanium–rare-earth (Ti–RE) alloys in the presence of rare-earth fluorides (REF 3 ) and rare-earth oxyfluorides ( REₓOyFz RE x O y F z ). The partitioning of rare-earth elements (RE) and O between the melt and β -Ti that precipitates at 1943 K (1670 ∘ ∘ C) during cooling of Ti-rich Ti–RE alloys was estimated using thermodynamic data. The present analysis provides thermodynamic trends rather than exact quantitative predictions, owing to uncertainties in high-temperature thermodynamic data near 1943 K (1670 ∘ ∘ C). For example, when molten Ti–Y alloy with Y concentration of 1 mass pct (10,000 mass ppm) is cooled, β -Ti containing 0.2 mass pct Y (2000 mass ppm Y) is obtained. In addition, when solidification proceeds in equilibrium with liquid YF 3 and solid Y 5 O 4 F 7 , the resulting β -Ti contains approximately 0.053 mass pct O (530 mass ppm O). These results indicate that combining RE–REF 3 flux deoxidation with solidification refining offers a potential route to producing high-purity solid Ti with both low RE and low O concentrations directly from molten Ti. Consequently, this technique provides a promising approach for advancing upgrade recycling (upcycling) technologies for Ti scrap.