Experimental and Thermodynamic Investigation of the Ni-Al-Mo System XIAOGANG LU, YUWEN CUI, and ZHANPENG JIN The diffusion triple and electron probe microanalysis (EPMA) techniques have been adopted to study the isothermal section of the Ni-Al-Mo system at 1373 K. Experimental data from present work and literature have been assessed to evaluate the thermodynamic parameters of the ternary system by means of the calculation of phase diagram (CALPHAD) method. A substitutional-solution model is used to describe liquid, fcc, and bcc phases, while a sublattice model is used to describe intermetallic phases. Two sets of thermodynamic descriptions have been obtained, and comparison has been made between them. Also, calculations of equilibrium-phase diagrams compared to the experimental data have been presented. There is good agreement between the calculations and the experimental data. I. INTRODUCTION effective and powerful approach to determining phase dia- grams, plays a dominant role in the study of complex metallic A NEW generation of turbine-blade materials, necessary systems. Accordingly, this method is utilized in this study. for advanced high-performance engines, has been under development for decades. The Ni-Al-Mo system was chosen A. Experimental Procedures as a base, since great attention has been focused on direc- tional solidification of fcc(Ni)/L1 2 (Ni 3 Al) + bcc(Mo) eutec- Starting materials were high-purity aluminum (99.99 wt tic superalloys, which show excellent properties both at room pct), electrolytic nickel (99.97 wt pct), and a molybdenum and at elevated temperatures. Much work has been done to bar (99.97 wt pct). investigate phase stabilities and to establish phase relations, Alloys 1 and 2 were prepared by argon are melting on such as that by Henry, [1] Yoshizawa et al., [2,3] Wakashima a cooled copper hearth, using a nonconsumable tungsten et. al., [4] Nash et al., [5] Miracle et al., [6] and Maslenkov et electrode. Each alloy button was turned over three times al. [7,8] However, they are not always consistent with each and remelted to make it more homogeneous. No significant other and even result in contradictions. To fully understand loss in weight occurred, so the alloys were believed to have the microstructural evolution of Ni-Al-Mo superalloys dur- the desired compositions. The nominal compositions of the ing processing and long-term exposure to various working alloys are listed in Table I. conditions, all related information should be assessed and a Polished bulk Mo and Ni were first bound with thin Mo complete thermodynamic description should also be cords and put into vacuumed (10 -1 Pa) quartz tubes. After obtained for the purpose of further theoretical prediction and heating in a GK-2B–type diffusion furnace at 1373 K for simulation. Early work by Kaufman and Nesor [9] has been 96 hours, a layer of compound appeared, binding the bulk done to calculate some isothermal sections of the Ni-Al-Mo Mo and Ni together to form the so-called diffusion couple. system. However, due to the lack of experimental data at Then, the couples and alloys were polished and assembled that time, that work needs to be improved. Recent work by to form well-contacted metal-alloy diffusion triples, as illus- Huang and Chang, [10] Ansara et al., [11] Saunders, [12] and Cui trated in Figure 1. Triples 1 and 2 were again sealed in and Jin, [13] which focused on sub-binary systems of the Ni- evacuated (10 -1 Pa) quartz tubes, and triple 3 was argon Al-Mo system, have formed the basis of the present work. protected. All the triples were heated in the same type of The objective of the present work is mainly centered on the furnace, at 1373 K, for different times. The heat-treatment assessment of thermodynamic parameters of each phase as conditions are shown in Table II. The temperature was con- well as experimental study. trolled to within 5 K. After annealing, the triples were air- quenched, during which the cooling rate is fast enough to keep the microstructure at room temperature. The triples II. EXPERIMENTAL INVESTIGATION were then polished parallel to the diffusion direction. Finally, the local compositions were analyzed by electron probe Almost all experiments conducted on the Ni-Al-Mo sys- microanalysis (EPMA) performed on a CAMECA SX-50 tem to establish phase relations are based on the method of electron probe. The measurement was operated at 15 kV analyzing the microstructure of alloys (Wakashima et al., [4] using pure Ni, Al, and Mo as standards. The microprobe Nash et al., [5] Miracle et al., [6] and Maslenkov et al. [7,8] ). intensity data were corrected using the ZAF computer soft- The diffusion triple technique, on the other hand, as an ware program. The relative error of the compositional analy- sis is estimated to be less than 1 pct of the individual element. XIAOGANG LU and YUWEN CUI, Postdoctoral Students, and ZHAN- B. Results and Discussion PENG JIN, Professor, are with the Department of Materials Science and Engineering, Central South University of Technology, Hunan 410083, Peo- Figure 2(a) is the back-scattered electron (BSE) image of ple’s Republic of China. Manuscript submitted August 24, 1998. triple 2 in the contacted region, which contains a two-phase METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 30A, JULY 1999—1785