Diffusion of Re and Ru in the  0 Phase of Ni Based Alloys Efendi Mabruri 1; * 1, * 2 , Shingo Sakurai 1; * 1 , Yoshinori Murata 1 , Toshiyuki Koyama 2 and Masahiko Morinaga 1 1 Department of Materials, Physics and Energy Engineering, Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan 2 National Institute for Materials Science, Tsukuba 305-0047, Japan The diffusion of Re and Ru in the  0 -Ni 3 Al phase has been investigated at a range of temperatures 1423–1523 K by using the pseudo-binary diffusion couples. It was found that the concentration gradient of Re appeared to promote the uphill diffusion of Al, whereas that of Ru did not promote the Al uphill diffusion. The cross interdiffusion coefficients of ~ D Ni AlRe and ~ D Ni AlRu were independent of the matrix ordering since they were comparable qualitatively between those in the  0 -Ni 3 Al and in the -Ni phases. The tracer diffusion coefficients for Re and Ru in Ni 3 Al were estimated by extrapolation and the obtained activation energies were consistent with the site preferences of the elements reported in the literature. Further, the results of this work suggest that the diffusion of Ru in Ni 3 Al is mostly governed by the diffusion in Ni sublattice and the diffusion of Re is mainly controlled by the formation of anti-site defects in the Ni sublattice with negligible contribution of the anti-site bridge mechanism. [doi:10.2320/matertrans.MRA2007632] (Received December 27, 2007; Accepted March 3, 2008; Published May 1, 2008) Keywords: rhenium, ruthenium, diffusion,  0 phase, nickel three aluminum, superalloys 1. Introduction The development of the next generation single crystal Ni based superalloys pays an intensive attention to the roles of Re and Ru as the potent alloying elements. The combination of these two elements has been found to enhance high temperature capability of the superalloys. 1–5) Providing mobility data for these elements in nickel based alloys is an important issue since it is indeed required in designing the superalloys with respect to high temperature processes such as the precipitation and growth of the  0 , creep, rafting, etc. The diffusion of these elements in the  phase of binary and ternary nickel based alloys have been documented in literature. 6–12) However, to the author’s knowledge there is no report concerning on the diffusion of these elements in the  0 phase. The  0 phase has an order structure of L1 2 type in which Ni atoms are located at the face center site while Al atoms are at cubic corner site. In the stoichiometric Ni 3 Al, a Ni atom is surrounded by the nearest neighbors 8 Ni and 4 Al atoms, while an Al atom is surrounded only by 12 nearest neighbor Ni atoms. Due to the order structure, the diffusion of atoms occupying the Al site is expected to be slower than the atoms occupying the Ni site. Thus, the diffusion of solutes in the  0 depends strongly on their occupation behaviors in the Ni 3 Al lattice. 13,14) The present paper presents the diffusion of Re and Ru in the Ni 3 Al within a range of temperatures 1423–1523 K. The extrapolated tracer diffusion coefficients in Ni 3 Al are estimated and the activation energies are correlated with the existing data of the elements from a point of view of the site preferences. In addition, some of the cross interdiffusion coefficients in the systems of Ni 3 Al-Re and Ni 3 Al-Ru are also evaluated qualitatively from the redistribution profile of Al which is initially a constant. 2. Experimental Procedure The experiments used the pseudo-binary diffusion couples with a constant Al concentration. Due to the limited solubility of Re and Ru in the Ni 3 Al (about 2 mol% for Re at 1313 K 15) and about 4 mol% for Ru at 1373 K 16) ) dilute concentration of these elements was used in the alloys for the diffusion couples. The composition of the diffusion couples in mol% after homogenization and the heat treatment condition used in this study are presented in Table 1. The aluminum concentration of the alloys shown in the table deviates slightly from stoichiometric composition of Ni 3 Al and this may be due to the melting condition. Thus, these alloys can be considered to be the Al-rich Ni 3 Al. The button ingots of the alloys were prepared by arc melting in high purity argon gas atmosphere. The ingots were annealed at 1523 to promote homogenization and grain growth. Sufficient large grain sizes were obtained for the three alloys as shown in Fig. 1. Then, the ingots were cut into plate-shape and ground flat and parallel. The plates were polished with a 0.25 mm finish of diamond slurry and cleaned by acetone in an ultrasonic cleaner. The diffusion couples were assembled with alumina fibers of several micrometers in diameter placed between the polished surfaces of the plates as the inert marker of initial interface. The diffusion couples were then clamped with molybdenum holders. Before the arrangement, the surfaces Table 1 Composition (mol%) of the diffusion couples and condition of diffusion annealing. No. Diffusion Couple Diffusion Annealing 1423 K, 516.06 ks 1 Ni-25.5Al/Ni-25.5Al-1Re 1473 K, 486.24 ks 1523 K, 240.55 ks 1423 K, 516.06 ks 2 Ni-25.5Al/Ni-25.5Al-1Ru 1473 K, 399.60 ks 1523 K, 259.20 ks * 1 Graduate Student, Nagoya University * 2 Corresponding author, E-mail: efendi@silky.numse.nagoya-u.ac.jp Materials Transactions, Vol. 49, No. 6 (2008) pp. 1441 to 1445 #2008 The Japan Institute of Metals