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.