Study On Microstructural Stability And Stress-rupture Properties Of A Nickel-based Single Crystal Superalloy

Nickel-based single crystal superalloys have become an irreplaceable structural material for military and aero-engines and high-temperature components of gas turbines because of its excellent comprehensive mechanical properties.Great efforts have been made to improve the high-temperature creep properties of superalloys resulted in the development of several generations of these alloys.However,microstructural stability at elevated temperatures has been a concern for the third generation alloys.In this paper,a third generation single crystal superalloy under development was used as experimental material.The effects of long-term aging and adjusting Al element content on the structural stabilities and stress-rupture properties were studied by means of optical microscopy(OM),scanning electron microscopy(SEM),transmission electron microscopy(TEM),differential thermal analysis(DTA),electron probe microanalysis(EPMA)and X-ray diffraction(XRD).It is found that the coarsening of ?? and the precipitate ? phase occur in the alloy during long-term aging at 1100 oC.The coarsening degree of ?? and the amount of precipitation of ? phase increase with the aging time.There are three forms of ? phase in this paper: needle,rod and block.The rupture life of the alloy decreases with the long-term aging time,and the samples life after long-term aging for 1000 h are almost half of that of the fully heat treatment.In order to improve the microstructural stability of the single crystal alloy,the content of Al in the alloy was reduced by 0.8wt% under the premise that the content of other elements remained unchanged.The results showed that the incipient melting temperature and liquidus temperature of the alloy were slightly increased and the solid temperature was reduced by decreasing the content of Al in the as-cast alloy.Under the condition of fully heat treatment,the decrease of Al content increases the size of the ?? phase and the width of the matrix channel,decreases the volume fraction of the ?? phase and the absolute value of misfit at room temperature.In the long-term aging process,the decrease of Al content reduces the coarsening rate of the ?? phase and greatly inhibits the precipitation of TCP phase.In addition,the reduction of Al content reduces the topological inversion during stress-rupture test.The application of external stress promotes the precipitation of TCP phase,and the existence of a small amount of TCP phase has little effect on the stress-rupture properties of the alloy.