Thermodynamics of oxygen and nitrogen in liquid nickel equilibrated with CaO-TiO x and CaO-TiO x -Al 2 O 3 melts
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I.
INTRODUCTION
IN view of the increasing demand for high-quality refractory alloys, a knowledge of the thermodynamic behavior of dilute solutes in liquid nickel-based alloys is essential for improvement in product cleanliness and compositional control. The thermodynamic behavior of oxygen and nitrogen in liquid nickel has been reviewed and summarized in the literature, tl,z,3J The effect of titanium on the activity coefficients of oxygen and nitrogen in liquid nickel also has been obtained experimentally, t4'Sm However, thermodynamic data on the effects of alloying elements on activity coefficients of nonmetallic solutes in liquid nickel alloys are limited in comparison with the data for liquid iron alloys. In this investigation, the content of oxygen and nitrogen in liquid nickel equilibrated with CaO-TiO~ or CaOA1203-TiOx melts was measured at 1873 K as a function of Ti (or A1) content. Based on the results for nitrogen distribution ratio, LN {= (mass pct N ) / [ m a s s pct N]}, associated with oxygen content, the values for nitride capacity, C~N) {= (mass pct N).o3/4/~1/2~ -0_,/--N, J, were calculated for CaO-A1203-TiO~ melts and compared with those obtained in a gas-slag equilibration experiment.t 71 The activities of A1203 and activity coefficients of TiO2 and TiO~.5 also were evaluated. II.
EXPERIMENTAL
A. Procedure The experimental technique is similar to those in previous w o r k s . [8'9'101 The master slags were prepared from mixtures of r e a g e n t - g r a d e C a C O 3 , TiO2, and/or A1203 in a platinum crucible under dry air atmosphere. A vertical-resistance furnace with heating bars of LaCrO 3 was used. The temperature of an even hot zone with 3 cm within +--2 K controlled by a proportional, integral, and differential S U N G - W O O K CHO, G r a d u a t e S t u d e n t , D e p a r t m e n t of Metallurgical Engineering, and HIDEAKI SUITO, Professor, Institute for Advanced Materials Processing, are with Tohoku University, Sendai, Japan. Manuscript submitted January 19, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS B
(PID) program was measured by a Pt-6 mass pet Rh/Pt-30 mass pct Rh thermocouple. The following equilibrium and supersaturation experiments were carried out at 1873 K, using CaO-TiQ. slags saturated with a CaO crucible and CaO-A1203-TiO.~ slags saturated with an A1203 crucible. In equilibrium experiments, 20 g of pure Ni (8 mass ppm O, 15 mass ppm Fe, and < 3 0 mass ppm C), starting with approximately 200 - 400 mass ppm oxygen and 8 - 11 g of slag, were melted under a deoxidized Ar atmosphere, with an appropriate amount of Ni-(3 10 mass pet) A1 and Ni-(3 -- 10 mass pet) Ti alloys added by dropping afterward. Melts were stirred over a period of 2 hours by an A1203 rod in 3t - 35 mass pet CAO/56 - 59 mass pet A1203/9 -- 10 mass pct TiOx slags (CAT-l) for 2 minutes at 30-minute intervals. To eliminate oxide inclusions by flotation, melts were unstirred for at least 1 hour. In the case of 13 - 15 mass pet CAO/24 - 31 mass pet A1203/55 - 63 mass pet TiO~ slags (CAT-2), the experimental
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