Research On High-temperature Oxidation Property Of Calorized Coatings On GH907 Superalloys

GH907 alloy, a precipitation-hardening austenitic superalloy based on Iron-cobalt-nickel, is a new low expansion superalloy modelled on the Incoloy907 developed by American International Nickel Company. The alloy can be used to make ring parts and turbine cases to maintain the clearances tight of all kinds of aeroengine below 650℃. Because of the lack of antioxidative element, the alloy must be applied protective coating for long-term use at high temperatures (700℃).The aluminized coating on GH907 superalloy was prepared by pack cementation. According to oxidation weight gain method by Chinese standerd, to meet the actual operation conditions and requirements of the alloy, the isothermal oxidation testing at 650℃in air for 200 hours was carried out. By electronic balance, metallographic microscope, scanning electron microscope (SEM) and energy dispersive spectroscope (EDS), the oxidation kinetics law, oxidation thermodynamics, the surface and cross-sections of the oxides formed on the surface, the rules of elemental diffusion and the degradation mechanism of the coating were discussed.The surface of the calorized coating is composed of particles which are accumulated and stacked. The particles of all. sizes are not close-connected, and there is significant space between the particles. The particles are comprised of the iron aluminides intermetallics. From the microstructure of the coating section, the aluminizing layer has two layers:the outer diffusion layer and the inner diffusion layer, whose thicknesses are 17μm and 10μm respectively, and second-phase particles is dispersed in the matrix alloy.The results show that the Antioxidant levels of calorized coating is completely antioxidant, and the oxidation kinetics curve does not obeys the parabolic law. The calculated Oxidation kinetics equation indicates that the reaction between oxygen and the surface of coating controll the reaction. The protective scale ofγ-Al2O3 grows whole and compact and has excellent resistance of oxidation. The continuous diffusion of aluminum ion lead to phase change, Fe2Al5â†'FeAl+3Al3+. Al2O3 scale spalls off the alloy due to the stress and internal cavities with the time extended. Yet, the high content of aluminum and its self-repair capability allow the regeneration of Al2O3 scale and continuous protection for matrix alloy.The antioxidant levels of GH907 alloy is antioxidant and the oxidation kinetics curve of the GH907 alloy obeys the parabolic law. On the basis of oxidation thermodynamics, the Gibbs free energy of the metal oxide is worked out as follows,â–³GFe2O3<â–³GFeO<â–³GFe3O4·Thus, Fe2O3 is formed preferentially. To reach a stable state, CoO makes a compound react with Fe2O3 and a protective spinel structure CoFe2O4 is formed, which is the principal agent of the antioxidation of the alloy. However, with time extending, vacancy and microvoid are formed during the elemental diffusion, and then crack, wrinkling and spalling generate on sample surface. In the late oxidation, with the decrease of cobalt element contents, Fe2O3 and FeO are formed in the peeling area of CoFe2O4, and the oxidation resistance declines dramatically.