Mechanisms of impurity migration and removal during iodine chemical vapor transport purification of high-purity low-oxygen titanium

Iodine chemical vapor transport (iodine-CVT) is a promising route for the direct production of high-purity, low-oxygen bulk titanium. However, the mechanisms governing impurity migration during titanium transport remain insufficiently understood. In this work, the migration behavior and removal mechanisms of selected metallic impurities and oxygen during titanium growth by iodine-CVT were investigated through combined experimental and thermodynamic analyses under controlled temperature and pressure conditions. Using Fe as a representative impurity, a critical concentration was identified at the growth front, marking the transition from an initial diffusion-dominated transient stage to a later quasi-steady stage. In the latter stage, the Fe content in the deposited layer remains close to the critical concentration under the coupled action of solid-state diffusion and gas-phase transport. The same mechanistic framework also accounts for the migration behavior of Ni and Al. In contrast, oxygen incorporation into the deposit is limited by gas-phase mass transfer, and the oxygen is inferred to originate primarily from oxygen-bearing species in the background atmosphere rather than from transport through the iodine-CVT pathway from the source.