Development of Re-based diffusion barrier coatings on Nb-based alloys for high temperature applications

Niobium based alloys are candidate materials for gas turbines that can be used, uncooled, at temperatures between 1473 and 1773K, while they suffer from severe degradation due to high temperature oxidation. In the present investigation to protect Nb-5Mo-l5W alloy against high temperature oxidation a novel coating was developed with electroplating of a Re-Ni film, followed by Cr and Al pack cementation. The coating consisted of a duplex layer structure, an inner sigma(Re-Cr) or Re(Cr) layer and an outer alpha Cr(Al) or beta NiAl layer. The Re-Ni film containing more than 70at%Re, developed in the present investigation, is more useful than the conventional low Re-Ni film. The inner sigma(Re-Cr) and Re(Cr) act as a diffusion barrier between the Nb-5Mo-15W alloy substrate and the outer sigmaCr(Al) or beta NiAl layer, which forms a protective alpha-Al2O3 scale. The Re-based alloy layers were considered to be an outer a-phase in a Re-Cr(Ni) system and an inner chi-phase in a Re-Nb(Cr) system. The coated Nb-5Mo-15W alloy was oxidized in air at 1373 and 1473K for up to 360ks. It was found that due to a mutual reaction between the beta-NiAl and the alloy substrate, the Re-based alloy layers with the a-and X-phases significantly suppressed the decomposition rate of the outermost beta-NiAl in contrast to the rapid decomposition of conventional beta-NiAl coatings without a diffusion barrier layer. The Re-based sigma and chi phases are candidates for a diffusion barrier against Al inward diffusion, because they have very low solubility of Al. Diffusion couple studies shows that Nialuminides containing 12~50at%Al can be used as an Al-reservoir layer on the diffusion barrier layer of Re and Re-40Cr (sigma Re3Cr2). (AU)