Development of Re-based diffusion barrier coatings on nickel based superalloys

A diffusion barrier type coating with a duplex layer structure, an inner sigma-(Re, W, Cr, Ni) as a diffusion barrier and outer Ni-aluminide as an Al reservoir, was formed on a Nickel based, single crystal, superalloy (TMS-82 +) and on Hastelloy X. Oxidation properties of both the alloys with or without the diffusion barrier coating were investigated in air under thermal cycling between room temperature and 1423 K for up to 360 ks. The inner sigma layer with a composition (at%) of (35-40) Re, (15-20) W, (15-25) Cr and (15-25) Ni was produced by electrodeposition of Ni-70Re and Ni-20W films from aqueous solutions followed by Cr-pack cementation at temperatures between 1473 and 1573 K, and the outer Ni-aluminides of beta-(Ni,Cr)Al + gamma'-(Ni,Cr)(3)Al was formed by electrodeposition of a Ni film, followed by Al pack cementation. After the 360 ks oxidation it was found that the structure and composition of both a layer and alloy substrate were retained with little change. Furthermore, there was little At in the a layer. It could be concluded that the Re-based alloys such as sigma (Re(W),CrNi) are very promising candidates as a diffusion barrier between the outer Al-reservoir layer and alloy substrate at temperature of 1423 K. It was found that the Re(W)-Cr-Ni acts as a diffusion barrier for both inward diffusion of Al and outward diffusion of alloying elements in the alloy substrate.