Thermodynamic Considerations of Direct Oxygen Removal from Titanium by Utilizing the Deoxidation Capability of Rare Eart
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INTRODUCTION
METALLIC Ti has a strong binding affinity with oxygen.[1] Furthermore, as shown in Figure 1, oxygen is highly soluble in metallic Ti.[2,3] At 1300 K (1027 C), the solubility limit of oxygen in b-Ti is approximately 1 mass pct and in a-Ti, the solubility of oxygen is as high as 14 mass pct. Due to these inherent properties, it is well known that deoxidation of metallic Ti is extremely difficult. Oxygen dissolved in metallic Ti cannot be removed by employing vacuum melting processes such as electron beam melting and vacuum arc melting. The oxygen content in Ti usually increases during melting, casting, and machining processes. As shown in Figure 2,[4] the oxygen content in typical Ti scrap is usually higher than in virgin metals (300 to 2000 mass ppm) for producing
TORU H. OKABE is with the Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan. Contact e-mail: [email protected] CHENYI ZHENG is with the Institute of Industrial Science, The University of Tokyo 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. YU-KI TANINOUCHI is with the Institute of Industrial Science, The University of Tokyo Manuscript submitted April 12, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS B
ingots, such as the Ti sponge produced by the Kroll process. Ti scraps often contain oxygen concentrations ranging between 2000 and 4000 mass ppm (0.2 to 0.4 mass pct). Considering the high production cost of metallic Ti, it is desirable to remelt scraps with virgin metals to produce primary ingots of Ti or its alloys. However, the usage of scrap as a raw material increases the oxygen level of the resultant Ti ingots. Commercial processes that can effectively and directly deoxidize Ti have not yet been established. Therefore, scraps with relatively high oxygen contents cannot be reused as raw material for the production of Ti ingots. The demand for Ti and its alloys has been increasing in various fields and particularly in the aerospace industry. Thus, the recycling of Ti scraps as a raw material for primary ingots has become more important, and the development of effective deoxidation processes for Ti scraps is essential to meet the future demand. Significant attention has been focused on developing methods for the direct removal of oxygen from Ti, and many techniques have been proposed and examined.[4–41] For example, there are many reports on the deoxidation of solid Ti using Ca as a deoxidant.[7–15,20,24–27,29,33–35,37] Oxygen removal during melting process using Ca,[31,32] Al,[28] and H2[30,36] as deoxidants was also studied. More recently, deoxidation with Mg under a H2 atmosphere was reported and gained a significant amount of attention in the field.[38,40,41]
O content, CO (mass pct) 1
5
10
15
2200
Liquid 2000
γ-TiO
1993 K 1943 K
Temperature, T / K
1800
β-Ti
1600
α-Ti
1400 1200 1155 K
1000 800 600
(Ti2O) (Ti3O) 0
10
20
30
O content, xO (mol pct)
Ti
40
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