Preparation of α-Al2O3/NiAl multilayer coatings on GH3535 superalloy surface by pack cementation and subsequent in-situ oxidation

Abstract

Tritium would be produced in the Thorium-based Molten Salt Reactor and then quickly penetrate through the structural material of GH3535 superalloy into environment at elevated temperatures. To suppress the permeation of tritium, the α-Al2O3/NiAl multilayer coatings were prepared on GH3535 by pack aluminizing (PCA) and subsequent low vacuum oxidation as tritium permeation barriers. Herein, the effects of PCA process parameters on the composition and structure of the Ni–Al coatings were characterized, simultaneously, the influence of oxidation temperature on the composition and structure of Al2O3 films were analyzed in detail. Experimental results demonstrated that the main phase of the Ni–Al layers is Ni2Al3 (or NiAl) doped with Al86Cr14. Significant amounts of Mo-aluminides were found on the surface of the samples aluminized at 1050 °C. The Ni–Al layers growth kinetic equation under 650–850 °C was empirically obtained as h = 7.07•104W1/2t1/2exp[-7.53•104/RT]. After low vacuum heat-treatment, the α-alumina films with a thickness of approximately 1 μm were formed at 1050 °C for 2 h, densely and non-porously. Meanwhile, the brittle Ni2Al3 in the transition layer was transformed into the tough NiAl, providing a self-healing capability as well as alleviating the thermal mismatch between the coating and the substrate.