Effect of Ta on microstructure and phase distribution in cyclic oxidized Ir–Ta modified aluminide coatings on nickel base single crystal superalloy

The effect of Ir–Ta coating (with varying amounts of Ta) on microstructure, composition variation and phase distribution in the modified aluminides on Ni-base single crystal superalloy, TMS-75 subjected to cyclic oxidation has been investigated. Ir–Ta coatings with Ta = 14.1, 23.1, 40.7 and 82.9 at.% deposited on 〈1 0 0〉 oriented nickel base single crystal superalloy, TMS-75 were aluminized and subjected to cyclic oxidation at 1373 K, for 600 h. The mass gain as a function of number of cycles data demonstrates an improved oxidation resistance with decreasing Ta content. In contrary to Ta-rich aluminides, Ir-rich aluminides show a large volume fraction of the β-phase, a less amount of γ′-Ni3Al phase near the surface of the coatings. Rapid decrease in the concentration of Al in the diffusion zone and formation of interconnected γ′-Ni3Al has been found to be responsible for the lower oxidation resistance of Ta-rich aluminides.