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UBC Theses and Dissertations

Dissolution of titanium-nitrogen inclusions in liquid titanium during electron beam melting Xu, Jixiang


Ti-N inclusions, classified as Type I defects in titanium alloys, are nitrogen-enriched areas that locally embrittle and harden the material. The presence of Ti-N inclusions in titanium alloys significantly degrades the fatigue performance, and hence cannot be tolerated in rotor-grade applications. Both reducing the potential for the introduction of these inclusions and removing them in melt-refining processes are therefore critical. The research herein is aimed at understanding: (1) the diffusional transport of nitrogen in Ti and the associated solid-state phase evolution – sub-task 1, and (2) their subsequent dissolution of Ti containing ~ 2 wt. % nitrogen in liquid titanium – sub-task 2. In the first sub-task, nitrogen was introduced to solid commercially pure (CP) titanium rods at 1650 °C in an electric induction furnace. An effective way to avoid the formation of a nitride layer (TiN and Ti₂N) was developed. Microstructure and microhardness were examined on the cross-section of the nitrided samples. Multiple phase layers were observed, and each layer was identified using X-ray diffraction. The effects of temperature and nitriding time on the kinetics of nitrogen diffusion were investigated. Results showed that nitrogen diffusion was accelerated with increasing temperature and nitriding time. Correlations between microhardness and nitrogen concentration were developed for the core and outer layers, respectively. A numerical model has also been developed to simulate nitrogen diffusion. The predicted nitrogen concentration profiles and the displacement of the phase interfaces showed good agreement with experimental observations. In the second sub-task, the nitrided rods were immersed into a molten CP Titanium pool produced by an electron beam button furnace. The evolution of the rod profile over various time periods was observed. Generally, the volume fraction of dissolved Ti-N solid increases with increasing immersion time. A numerical model has also been developed to aid in understanding the transport phenomena involved in the dissolution process. Overall, the predicted dissolved volume fraction across different immersion times agrees well with experimental measurements. Finally, an effective mass transfer coefficient in the range of 4.2×10⁻⁵ to 4.9×10⁻⁵ m/s was derived based on model results, which can be used for evaluating the dissolution kinetics in industrial applications.

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