The average ternary interdiffusion coefficients in Ni3Al with Ir additions were determined using solid-to-solid diffusion couples annealed at 1200C for 5h. Disc-shaped alloy specimens were prepared by vacuum arc melting to give compositions of Ni-24Al, Ni-25Al, Ni-26Al, Ni-23.5Al-1Ir, Ni-24.5Al-1Ir, Ni-23Al-2Ir, Ni-23Al-2Ir, Ni-24Al-2Ir and Ni-23Al-3at%Ir. The surfaces of the alloys were polished, and diffusion couples were assembled in Si3N4 jigs for initial bonding heat-treatment (1200C, 0.5h). Additional diffusion annealing was carried out at 1200C for 4.5h, outside of the Si3N4 jig, so that diffusion couples could be water-quenched. Concentration profiles of individual components were measured by electron probe microanalysis using pure standard of Ni, Al and Ir. The interdiffusion flux of an individual component was determined directly from the experimental concentration profiles, and the moments of interdiffusion flux were examined in order to calculate the average ternary interdiffusion coefficients, D̃̄ijk with either Al or Ni as the dependent component. The calculated interdiffusion coefficients suggested that Ir-alloyed Al2O3-forming oxidation resistant coatings would be beneficial in reducing the interdiffusion flux of Ni from superalloy substrates to the coating, and in reducing the interdiffusion flux of Al from the coating to the superalloy substrate.

Ternary Interdiffusion in L12-Ni3Al with Ir Alloying Addition. Ode, M., Garimella, N., Ikeda, M., Murakami, H., Sohn, Y.H.: Defect and Diffusion Forum, 2008, 273-276, 637-42

Figure 4

Diffusion of Al and Cr in a Ni75.5Al24.5/Ni77.5Al18.0Cr5.0 couple

White circles, i = Cr; black circles, i = Al