Development of an iridium–tantalum modified aluminide coating as a diffusion barrier on nickel-base single crystal superalloy TMS-75

An iridium–tantalum modified aluminized (Ir–Ta–Al) coating obtained by a two-step process is investigated. Ir–71 at.% Ta alloy was first coated on a nickel-base single-crystal superalloy TMS-75 by an electron beam physical vapor deposition (EB-PVD), and the resultant Ir–Ta alloy coating on the TMS-75 was treated by a conventional low-activity pack-cementation aluminizing process. It is found that a triplex layer structure, Ir–Ta alloy layer/β-NiAl/Al-diffusion zone, is formed in the order from the surface. In order to assess the oxidation resistance of the coated material, cyclic oxidation tests were carried out at 1373 K in air. It is found that Ir–Ta–Al coated TMS-75 had better thermal cyclic oxidation resistance than simply aluminized TMS-75. The microstructural evolution caused by the interdiffusion of solute elements shows remarkable difference on the precipitation of detrimental phases such as topologically close-packed (TCP) phase. The precipitation of TCP phase in the Ir–Ta–Al-coated TMS-75 was limited only within 180 μm in depth from the surface, whereas in the simply aluminized TMS-75, such detrimental phases were precipitated to the depth of more than 300 μm from the surface, indicating that the Ir–Ta–Al coating functions as an effective diffusion barrier for the alloying elements.