Ultimate Deoxidation Method of Titanium Utilizing Y/YOCl/YCl 3 Equilibrium
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INTRODUCTION
TITANIUM (Ti) has a strong affinity for oxygen (O), and the solubility of O in Ti is high.[1–3] Because O concentration in Ti drastically changes the properties of Ti, various methods have been used industrially to suppress O contamination in smelting, machining, and working processes. As Ti products have become widespread particularly in the aerospace industry, deoxidation methods of Ti scrap and Ti powder have become increasingly important in recent years.[4] There was no effective method for directly removing O in Ti until the 1990s because of the strong reactivity of Ti. Since then, through the use of molten salt, the method using metal/metal oxide equilibrium (Figures 1(a) and (d)), the calcium-halide flux method (Figure 1(b)), and the electrochemical deoxidation
AKIHIRO IIZUKA is with the Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan and also with the Department of Materials Engineering, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan. TAKANARI OUCHI, and TORU H. OKABE are with the Institute of Industrial Science, The University of Tokyo. Contact e-mail: [email protected] Manuscript submitted August 13, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS B
method (Figure 1(c)) have been actively developed. However, large-scale industrialized direct removal of O from Ti remains challenging. Therefore, we focus on the development of new methods that can reduce the O concentration in Ti more simply and reliably, rather than just the modification of existing methods. In general, calcium (Ca) and rare earth (RE) metals, which have a strong affinity for O, are used in thermochemical deoxidation methods for Ti in metal/ metal oxide equilibrium. In particular, methods to deoxidize Ti in Ca/CaO equilibrium have been widely studied (Figure 1(a)).[5–22] For example, when the activity of each chemical species is unity (aCa = 1, aCaO = 1) at 1300 K (1027 C), O in Ti can be removed to approximately 500 ppm O in the Ca/CaO equilibrium. Additionally, the RMI Titanium company applied this deoxidation method using Ca vapor, referred to as the ‘‘deoxidation in solid state’’ (DOSS) method, on an industrial scale.[18–22] Until recently, many researchers have actively developed deoxidation techniques for Ti bulk or Ti powder using Ca metal.[8–17] In 1992, Okabe et al. demonstrated that O concentration in Ti can be dramatically reduced to 20 ppm O by decreasing the activity of CaO, aCaO, in CaCl2 flux, which has a large solubility of CaO (calcium-halide flux method, Figure 1(b)).[23–26] Recently, the calcium-halide flux method has been studied by Han et al. and Xia et al.[27,28] However, there is no cost-effective process for regenerating Ca metal from CaO produced in the calcium-halide flux method.
Ca (aCa = 1) Ti (s)
O CaO (aCaO = 1) (a)
Ca (aCa = 1) Ti (s)
O CaO (aCaO
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