Deoxidation of titanium aluminide by Ca-Al alloy under controlled aluminum activity
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Removal of oxygen in titanium aluminide (TiA1) by chemically active calcium-aluminum (Ca-A1) alloy was carried out around 1373 K with the purpose of obtaining extra-low-oxygen TiAI. The deoxidation experiments were preceded by an investigation of the phase equilibria of the system Ti-A1-Ca at 1273 and 1373 K. The compositions of the Ca-A1 alloy deoxidant, which equilibrates with TiA1, and the experimental conditions suitable for the deoxidation were of particular interest. In experiments in which Ti-A1 samples were submerged in liquid Ca-A1 alloys at 1373 K, the surfaces of the samples severely deteriorated and became nodular. When TiA1 powders were mixed with CaO and the deoxidant was supplied in vapor form, powders which initially contained 510, 1100, and 4200 ppm O were deoxidized to about 160, 490, and 670 ppm O after deoxidation at 1373 K in 86.4 ks (1 day). Among many conditions tested, the use of TiA1 powders mixed with CaCl2 was most effective for deoxidation at 1373 K. CaC12 was used as a flux to facilitate the deoxidation by decreasing the activity of the deoxidation product CaO. In the case that TiA1 powders mixed with CaC12 and reacted with Ca-A1 vapor at 1373 K for 86.4 ks, the powders initially containing 510, 1100, and 4200 mass ppm O were deoxidized to a level of 62, 140, and 190 mass ppm O, respectively. No significant change in morphology of the particle after deoxidation was observed. The titanium and nitrogen concentrations in the powders remained constant, whereas calcium, which was present only in trace amounts initially, increased up to 160 mass ppm after the deoxidation treatment.
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INTRODUCTION
I N recent years, there has been considerable interest in developing titanium aluminide (TiA1) as a structural material and mechanical properties of TiA1 have been intensively investigated. In particular, plasticity of TiAI has proven to be influenced by impurities such as oxygen.t l] For laboratory use, the grade of TiA1 is between 99.9 and 99.999 pct pure (excluding gaseous elements), where the main impurity is oxygen, at a level of at least several hundred mass ppm oxygen. This oxygen mainly originates from impurities in the titanium starting material which is produced by the Kroll process. Even though low oxygen-containing titanium has been successfully produced using other less widely used methods, such as electrolytic refining or iodide refining, oxygen contamination of TiA1 is inevitable during subsequent processing. Oxygen removal directly from TiA1 to a level below 100 mass ppm is deemed to be very difficult, because TiA1 has a strong affinity for oxygen. For this reason, titanium and aluminum are usually alloyed to make titanium aluminide in an oxygen-free atmosphere, taking great care to avoid oxygen pickup. From a thermodynamic viewpoint, external gettering used in solid state refining is one of the most promising T.H. OKABE, Ph.D. Candidate, T. OISHI, Associate Professor, and K. ONO, Professor, are with the Department of Metallurgy, Kyoto University, Kyoto 606, Japan. Manuscript subm
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