Ternary diffusion in a RuAl-NiAl couple

A ternary diffusion couple assembled with NiAl and RuAl disks and annealed at 1100 degrees C was examined by scanning electron microscopy and analyzed for concentration profiles by electron microprobe analysis. Complete mutual solid solubility with continuous variations in compositions was observed between the binary B2 aluminides. Ternary interdiffusion coefficients were determined with the aid of a program called MultiDiFlux over two composition ranges, one = Al Ru-rich and the other Ni-rich, within the diffusion zone. The interdiffusion coefficient, <(<(D)over tilde>)over bar>(Al)(RuRu) varies little with variation in composition, but the interdiffusion coefficient, <(<(D)over tilde>)over bar>(Al)(NiNi) decreases by an order of magnitude from the Ni-rich region to the Ru-rich region. <(<(D)over tilde>)over bar>(Al)(NiNi) is larger than <(<(D)over tilde>)over bar>(Al)(RuRu) in the Ni-rich region by an order of magnitude. The cross coefficients, <(<(D)over tilde>)over bar>(Al)(NiRu)and <(<(D)over tilde>)over bar>(Al)(RuNi) are both positive. <(<(D)over tilde>)over bar>(Al)(NiRu) is comparable in magnitude to the main coefficient <(<(D)over tilde>)over bar>(Al)(NiNi) in the Ni-rich region; hence, Ni interdiffusion flux is enhanced down a Ru concentration gradient but decreased against it. Similarly, Ni interdiffusion is reduced down Al gradients. Characteristic depth parameters calculated for Ni and Ru are larger on the NiAl side than on the RuAl side. Approximate calculations of cumulative intrinsic diffusion fluxes past a Kirkendall plane suggest that the atomic mobility of Ni is larger than that of Ru.