Galvanic cell measurements on supersaturated activities of oxygen in Fe- Al - M (M=C, Te, Mn, Cr, Si, Ti, Zr, and Ce) me
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I.
INTRODUCTION
IN order to understand the fundamental aspects of the precipitation mechanism of nonmetallic inclusions in deoxidation, reoxidation, and solidification processes, knowledge of supersaturation is of crucial importance. In previous laboratory scale experiments, the supersaturation with respect to alumina precipitation was extensively investigated and reported in the literature.[1–12] As shown in our previous article,[7] the supersaturated oxygen for a given aluminum content has been observed by means of not only chemical analysis, but also electromotive force (EMF) measurement, when the initial oxygen levels in premelted Fe-0.005 to 0.21 mass pct Al alloys are located below the value for the critical supersaturation but above the equilibrium value. It was also observed that the degree of supersaturation in aluminum deoxidation decreased with the addition of surface active elements such as Te[10] and S[11] and was influenced by the addition of alloying elements such as C, Mn, and Ti in the reoxidation experiments.[12] Although the understanding of supersaturation has been improved on the basis of these previous experimental results, much is still unknown about the reasons for the time dependence of supersaturation, the effect of alloying elements on supersaturation, and other aspects. The present investigation was undertaken with the objective of clarifying the time-dependent supersaturation effect and the influences of alloying elements such as C, Te, Mn, Cr, Si, Ti, Zr, and Ce on the degree of supersaturation in Fe-0.0017 to 0.41 mass pct Al melts at 1873 K in an Al2O3 crucible, using EMF measurements. The results obtained by oxygen analysis are discussed in a separate article.[13]
GUANGQIANG LI, Research Student, and HIDEAKI SUITO, Professor, are with the Institute for Advanced Materials Processing, Tohoku University, Sendai 980, Japan. Manuscript submitted May 6, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS B
II.
EXPERIMENTAL
A. Cell Construction A tube-type mullite probe (f5.0 3 f2.4 ; 3.0 3 40 mm) supplied by NIKKATO Corporation (Sakai) was made as follows: 3Al2O3 z 2SiO2 coprecipitated by the sol-gel method was sintered at 1923 to 1973 K for 2 hours after rubber press. The chemical composition of a mullite probe with stoichiometric composition is 71.8 pct Al2O3, 28.1 pct SiO2, 0.06 pct TiO2, ,0.01 pct Fe2O3, 0.04 pct Na2O, 0.02 pct K2O, and 0.04 pct ZrO2 in mass content. A ZrO2-based plug-type (ZR-plug) probe was made as follows: a 9 mol pct MgO stabilized ZrO2 rod (f4.0 3 10 mm) supplied by NIKKATO Corporation, thick enough to eliminate electrochemical oxygen permeability, was fixed into a high-purity alumina tube (f6 3 f4 3 35 mm) with zirconia cement. A Mo rod (f3 mm) protected by a high-purity Al2O3 tube was dipped into a liquid Fe-Al-M alloy only enough to measure the EMF values. By so doing, the residual Mo content dissolved in a liquid alloy was analyzed to be in the range of 0.005 to 0.15 mass pct. B. Apparatus and Procedure A vertical resistance furnace with LaCrO3 heating bars
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