Activity of carbon in nickel-rich Ni-Mo and Ni-W alloys
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I.
INTRODUCTION
Q U I T E a few experimental studies have been carried out on phase diagrams in nickel base ternary systems containing carbon, such as Ni-Cr-C, l'z'3 Ni-Mo-C, 4 and Ni-W-C. 5 To construct more widely applicable phase diagrams in these systems on the basis of the thermodynamic equilibrium between existing phases, several thermodynamic quantities should be evaluated. To determine reasonably the solubility of alloy carbides in nickel in these systems, especially, the carbon activity in the nickel alloys is one of the most important thermodynamic quantities. Unlike iron base alloys, very few data are available on the carbon activity in nickel alloys. The present study is undertaken to fill the gap between the traditional phase diagram studies and the thermodynamic equilibrium computations. Two important substitutional alloying elements, molybdenum and tungsten, have been selected in this study, since they are often used as solid solution strengtheners in nickel base alloys. The solubility of carbon in nickel-rich Ni-Mo and Ni-W alloys has experimentally been determined at several carbon activities at 1000, 1100, and 1200 ~ These results have been analyzed using a regular solution model proposed by Hillert and Staffansson. 6 II.
EXPERIMENTAL PROCEDURE
A. Specimen Preparation Seventeen nickel-molybdenum and thirteen nickeltungsten binary alloys were prepared in the form of button ingots of 30 g each by arc-melting in high purity argon with the aid of a nonconsumable tungsten electrode. The starting materials for the alloys were pure metals of nickel, molybdenum, and tungsten with purity higher than 99.9 mass pct. Since the yield of the ingots was better than 99.98 pct, comM. KIKUCHI, Professor, and M. KAJIHARA, Research Associate, are with the Department of Metallurgical Engineering, Tokyo Institute of Technology, Tokyo 152, Japan. S. TAKEDA, formerly Graduate Student, Department of Metallurgical Engineering, Tokyo Institute of Technology, is with Komatsu Manufacturing Co., Hiratsuka 255, Japan. R. TANAKA, formerly Professor, Department of Metallurgical Engineering, Tokyo Institute of Technology, is Professor, Department of Mechanical Engineering and Materials Science, Yokohama National University, Yokohama 240, Japan. Manuscript submitted December 3, 1986. METALLURGICALTRANSACTIONSA
positions calculated from weighed quantities were used as chemical compositions. They are listed in the first columns in Tables I and II. All these nickel-rich binary alloys were located within a single-phase field of the primary solid solution of nickel in the temperature range between 1000 and 1200 ~ 7 The button ingots of the nickel-molybdenum and the nickel-tungsten alloys were homogenized in purified hydrogen at 1200 ~ for 50 hours and at 1250 ~ for 60 hours, respectively. No concentration fluctuation was detected after homogenization by means of electron probe microanalysis.
B. Carburization Two carburization methods were employed in the present study. Most of the carburization experiments were made in a flow of c
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