Uphill diffusion in ternary Ni–Re–Ru alloys at 1000 and 1100 °C

Abstract The rates of interdiffusion of Re and Ru at 1000 and 1100 °C in binary diffusion couples with single phase face centred cubic (fcc) microstructures have been 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 1000 °C (6.12 × 10 18  m 2 /s) and at 1100 °C (9.31 × 10 −17  m 2 /s) was found to be an order of magnitude slower than that of Ru at both temperatures (5.01 × 10 −17 and 4.71 × 10 −16  m 2 /s at 1000 and 1100 °C, 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 × 10 −18 and 9.16 × 10 −17  m 2 /s at 1000 and 1100 °C, respectively). That of Ru was modestly reduced by the presence of Re to 3.45 × 10 −17 and 2.36 × 10 −16  m 2 /s at 1000 and 1100 °C, respectively. Uphill diffusion of Ru was evident in the diffusion zone of the Ni–Re–Ru/Ni–Ru couples under all annealing conditions despite the absence of a Ru concentration gradient. The uphill diffusion of Ru was opposite to and of the same order of magnitude of Re, the principal diffusing element. This suggests Re lowers the chemical potential of Ru thus promoting uphill Ru diffusion to equilibrate the chemical potential gradient. Lowering of the chemical potential of Ru by Re is consistent with thermodynamically stable Re–Ru bonding which may contribute to the enhanced microstructural stability and high temperature creep performance documented in Ru-bearing Ni-base single crystal superalloys.