The Effect of an Applied External Tensile Stress on the Oxidation Behavior of a Nickel-Base Alloy with a Re-base Diffusion-barrier-coating

A diffusion-barrier-coating system with a duplex layer structure comprised of an inner Re-base alloy layer and an outer β-NiAl layer was formed on the Ni–Mo alloy, Hastelloy-X. Alloy specimens with and without the coating were oxidized at 970 °C in air for up to 200 h with an imposed tensile stress of 22.5 MPa. The oxidation behavior under the stress-free condition was also investigated for comparison purposes. Strain rates of the specimens with a diffusion-barrier-coating system decreased rapidly for about 5 h, followed by a slow creep-deformation with a strain of 3.5% and strain rates of (0.7–0.2) × 10⁻⁷/s for 200 h. There was little change in both the coating structure and the composition (at%) of the inner Re-base alloy layer. Considering the creep behavior of the uncoated alloy, as well as the fact that there were few cracks and flaws in the Re-base alloy layer, it was concluded that this inner layer was subject to creep-deformation along with the alloy substrate. The external scale on the coated alloy consisted mainly of θ-Al₂O₃ at the early stage of the oxidation/deformation, and with further oxidation the surface scale formed a duplex layer structure consisting of outer plate-like θ-Al₂O₃ and inner equi-axed Al₂O₃. There was exfoliation of the outer θ-Al₂O₃ scale during the creep deformation. After the 200 h oxidation the outer β-NiAl contained (40–50)% Al, while the alloy substrate near the inner layer had less than 1 at% Al. It was found that the Re-base alloy layer acted an effective barrier against inward Al diffusion and outward diffusion of alloying elements.