High Temperature Oxidation, Hot Corrosion And Thermal Fatigue Behavior Of GH4698 Superalloy

GH4698 is a structural engineering material used in advanced gas-tubines. It is important to study the behavior of the oxidation, hot corrosion and thermal fatigue resistance of this alloy. High-temperature oxidation, hot corrosion and thermal fatigue behavior of the nickel-base wrought superalloy were investigated by optical metallography(OM), X-ray diffraction(XRD), scanning electron microscopy (SEM/EDX), transmission electron microscopy (TEM)and Chemistry analysis.The isothermal oxidation kinetics of GH4698 alloy followed logarithm law at 650～750℃while followed cubic law at 800℃.The composition of protective scales mainly consisted of Cr2O3. Internal oxidation of Al was due to the depletion of chromium at scale/matrix interface and larger amount of TiO2 in the scale at 800℃and the oxidation resistance of the alloy was degreased to some extent. The logarithm kinetics of the alloy can be explained by Davies et al.'s model related to grain-boundary diffusion in the oxide layers. Carbides and grain boundaries are all important spots where oxidation occurred seriously.The dissolution and precipitation ofγ′phases occurred in the course of oxidation.The hot corrosion behavior of GH4698 alloy with sodium sulfate(75%wt) and sodium chloride(25%wt) deposits at 650~750℃was developed by employing completely-immersion crucible test. The corrosion kinetics (determined by weight loss) can be divided in two stages: incubation and propagation stage.In incubation, selective oxidation of Cr appeared on surface of samples In propagation stage of corrosion, the molten salt penetrated the scales and reached the matrix of the alloy, the rapid corrosion rates were due to Ni-Ni3S2 low melting eutectic appearing in corrosion layers. The hot corrosion of the alloy involved a combination of various mechanisms, including fluxing of the oxide scale as well as mixed sulfidation-oxidation attack.The origin and growth of thermal cracks were influenced by the upper limit temperature elevating. The experimental result shows that the thermal property decreases with the elevation of upper limit temperature and cracks were influenced by...