The rates of interdiffusion of Re and Ru, at 1000 and 1100C, in binary diffusion couples with single-phase face-centred cubic microstructures, were characterised and compared to their respective rates in the presence of one another in ternary Ni–Re–Ru couples. The diffusivity of Re in Ni at 1000C (6.12 x 10-18m2/s) and at 1100C (9.31 x 10−17m2/s) was found to be an order of magnitude lower than that of Ru at both temperatures (5.01 x 10−17 and 4.71 x 10−16m2/s at 1000 and 1100C, respectively). The interdiffusion coefficient of Re was found to be unaffected by the presence of Ru in the ternary Ni–Re–Ru alloy (6.08 x 10−18 and 9.16 x 10−17m2/s at 1000 and 1100C, respectively). That of Ru was modestly reduced, by the presence of Re, to 3.45 x 10−17 and 2.36 x 10−16m2/s at 1000 and 1100C, respectively. Up-hill diffusion of Ru was evident in the diffusion zone of Ni–Re–Ru/Ni–Ru couples under all annealing conditions, in spite of the absence of a Ru concentration gradient. The up-hill diffusion of Ru was opposite to, and of the same order of magnitude as Re, the principal diffusing element. This suggested that Re lowered the chemical potential of Ru; thus promoting up-hill Ru diffusion to equilibrate the chemical potential gradient. Lowering of the chemical potential of Ru, by Re, was consistent with thermodynamically stable Re–Ru bonding, which could contribute to enhanced microstructural stability and high-temperature creep performance in Ru-bearing Ni-based single-crystal superalloys.

Uphill Diffusion in Ternary Ni–Re–Ru Alloys at 1000 and 1100C. R.A.Hobbs, M.S.A.Karunaratne, S.Tin, R.C.Reed. C.M.F.Rae: Materials Science and Engineering A, 2007, 460-461, 587-94

Table 10

Diffusivity of 54Mn in Ni2MnGa