Application of Ir-base alloys to novel oxidation resistant bond-coatings

High temperature oxidation and hot corrosion properties of Ir-Ta, Ir-Pt coated and aluminized coatings are presented. An Ir-Ta binary alloy, proposed as a novel metallic bond coat material, was coated on a Ni-base single crystal superalloy TMS-75 using electron beam physical vapor deposition (EB-PVD), followed by a conventional Al pack cementation process. Cyclic oxidation tests and hot corrosion tests revealed that these Ir-Ta coated and aluminized specimens showed reasonably good oxidation and hot corrosion resistance. In addition, the formation of TCP phases is suppressed by the presence of the Ir-Ta enriched layer. These results indicated that the Ir-Ta alloy system is promising as a new metallic bond coat for high temperature structural materials. On the other hand, substitution of Pt with Ir in Pt-Al coatings is of interest for cost reduction and strengtheneing purposes. Cyclic oxidation properties of (Ir, Pt)-coatings, which were successfully electrodeposited on TMS-75, followed by aluminization, were also investigated. Cyclic oxidation tests at 1373 K in air revealed that materials having better thermal cyclic oxidation resistance can be arranged in the following order: Pt-Al = (36at%Ir, Pt)-Al > Ir-Al ≫ simply aluminized. The presence of Ir and/or Pt may promote the formation of dense and adherent oxide scale and thus retard the growth of thermally grown oxides. Ir addition is thus promising, not just in terms of cost reduction, but also expected solid solution strengthening.