Experimental study and thermodynamic assessment of the Ni-Mo-Ta ternary system
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I. INTRODUCTION
THE Ni-Mo-Ta ternary system is of great interest in relation to the development of some Ni-based superalloys. This system has been studied by several investigators. The first investigation of the Ni-Mo-Ta system was carried out by Virkar and Raman.[1] An isothermal section at 1173 K was determined; nevertheless, because the single-phase boundaries were not determined and the number of alloys was too few to determine all three phase boundaries accurately, their results were considered to be rather tentative. By means of electron probe microanalysis (EPMA) and Xray diffraction, Chakravorty and West[2] determined two partial isothermal sections at 1523 and 1273 K. Some data for 1173 K were also obtained. In addition, they suggested a partial liquidus projection diagram based on the analysis of as-cast alloys. The most notable discrepancy between those two groups is that Virkar and Raman found that Ni3Ta and Ni3Mo can form continuous series of solid solution at 1173 K, while Chakravorty and West stated that they cannot. Gupta[3] critically evaluated the previous works of the NiMo-Ta system. Furthermore, he proposed another probable liquidus projection. Kaufman[4] published a tentative calculation of the NiMo-Ta ternary system based on the limited experimental information, in which no ternary solution and compound parameters were employed. Accordingly, it is the objective of this work to determine the phase equilibrium data of the Ni-Mo-Ta system in more detail and to develop a set of consistent thermodynamic descriptions of this system using the CALPHAD method. II. EXPERIMENT The diffusion couple technique, as an effective and powerful approach to determine phase diagrams, plays a dominant
YUWEN CUI, Postdoctoral Candidate, and ZHANPENG JIN, Professor, are with the Department of Materials Science and Engineering, Central South University of Technology, Hunan 410083, People’s Republic of China. XIAOGANG LU, Postdoctoral Candidate, on leave from the Department of Materials Science and Engineering, Central South University of Technology, is with the Department of Materials Science and Engineering, Royal Institute of Technology, S-10044 Stockholm, Sweden. Manuscript submitted August 24, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
role in the study of complex metallic systems. This method was thus adopted in our work. Starting materials were electrolytic nickel (99.97 pct), pure molybdenum bar (99.97 pct), and pure tantalum bar (99.95 pct), provided by GE Company (Schenectady, NY). At first, the Mo-Ta binary couple was prepared by diffusion welding in a GLEEBLE-1500-type Thermal Simulator. The welding was conducted in vacuum (better than 3 3 1023 Pa) at 1473 to 1573 K under a certain force for 20 to 30 minutes. The binary couples and Ni pieces were then polished and welded together, also in the GLEEBLE-1500-type Thermal Simulator, to form well-contacted Ni-Mo-Ta diffusion triples. Figure 1 illustrates the construction of the Ni-Mo-Ta diffusion triple. The triples were sealed in evacuated silica capsu
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