Interdiffusion and Diffusion Mobility for fcc Ni-Co-Al Alloys
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the Ni-Co-Al ternary system plays an important role in developing both Ni-based and Co-based superalloys. However, current existing investigations on diffusion in fcc Ni-Co-Al alloys are very limited. Up to now, a few assessments on the diffusion mobility for the fcc phase of Ni-Al binary system have been published. The earliest one was conducted by Engstro¨m and A˚gren.[1] However, the self-diffusion coefficients for elemental metals were usually assessed at high temperatures in most of the early studies, such as Jo¨nsson[2] for pure Ni and Engstro¨m and A˚gren for pure Al.[1] For this reason, Cui et al.[3] reviewed the self-diffusivity data for pure Al and Zhang et al.[4] updated the self-diffusion parameters for pure Ni and Al by considering all the experimental data over a wide temperature range.
YANG WANG, NAQIONG ZHU, and HAO WANG are with the School of Materials Science and Engineering, Shanghai University, Shanghai 200436, China. XIAO-GANG LU is with the School of Materials Science and Engineering, Shanghai University, and also with the State Key Laboratory of Advanced Special Steel, Shanghai University, Shanghai 200072, China. Contact email: [email protected]. edu.cn Manuscript submitted April 26, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
Campbell et al.[5] reported the development of a diffusion mobility database for Ni-based superalloys, including the Ni-Al and Ni-Co binary systems. Recently, Cui et al.[6] reassessed the mobility of the fcc phase by considering two more recent experimental data[7,8] for the Ni-Co binary system. In addition, Cui et al.[9] measured the interdiffusion coefficients for fcc Co-Al and assessed the corresponding diffusion mobilities. For the Ni-Co-Al ternary system, there was only one work[10] that studied one Ni-rich diffusion couple (Ni-5Al-28.9Co/Ni-7.7Al-25.8Co, wt pct) at 1373 K (1100 °C) for 24 hours. No investigation on the diffusion behavior of fcc Co-rich Ni-Co-Al alloys has been reported. Therefore, it is imperative to supplement more experiments of the ternary system in order to study its diffusion behavior. Sample alloys in the composition range of Ni-(0 to 40) at. pct Co-(0 to 10) at. pct Al were prepared by arc melting of appropriate mixtures of pure Ni (99.995 pct), Al (99.999 pct), and Co (99.99 pct) in an argon atmosphere. The alloy ingots, each of which was about 20 g, were fabricated by wire-electrode cutting into blocks with a size about 6 9 6 9 6 mm3. These blocks were made into diffusion couples. The annealing experiments for ternary diffusion couples were carried out at 1173 K (900 °C), 1373 K (1100 °C), and 1573 K (1300 °C), respectively. Details are listed in Table I. After annealing, the samples were quickly quenched into water. All diffusion couples were then mounted and polished by standard metallographic techniques. The diffusion concentration profiles were determined by electron probe microanalysis (EPMA) in a JEOL*JXA 8100 equipment. The above-measured concentration profiles were analyzed by the error function expansion (ERFEX) method as shown in the following equati
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