| K4169 alloy is widely used to make high-temperature alloy castings such as turbine disk and blade used for aviation and space,since a good overall performance in intermediate and low temperature.In recent years, with the rapid development of the aero-engine and especially the parts of the ground gas turbine,K4169 alloy is made higher demands to expect to work at a higher temperature.In the thesis,based on previous research about the modified version of IN718 alloy, the K4169 alloys are designed by adding different content of Al and a small amount of Co.The microstructural,compressive properties and hardness are investigated by means of optical microscope,SEM,TEM, EDX, X-ray diffraction and mechanical property test,and the high temperature oxidation resistance was studied.From those investigations the following conclusions can be drawn:1.After increasing the countent of Al and Co in K4169 alloy, the microstructure of as-cast alloys are the same to the as-cast master alloy K4169:dendritic y matrix,main strengthening phasesγ",dispersed y'phase and blocky MC carbides and Laves phase on grain boundary and in interdendritic regions.With the content of Al increasing, the microsegregation of elements such as Nb,Mo,Ti and Fe is alleviated,the precipitated phases in interdendritic regions are increased markedly, and the secondary dendrite arms become longer, while the effect of increase of Co on the microstructure of as-cast alloys is hardly clear.When increasing Al, the yield strength and the hardness of as-cast alloys are increased and then decreased.Meanwhile,the yield strength and the hardness are increased since a small amount of Co added.2.All the alloys with different content of Al and Co by standard heat treatment contain precipitations on grain boundary such asδphase,MC carbide phase,Laves phase,M7C3 carbide andσphase. With increasing Al, the precipitated phases on grain boundary of alloys after standard heat treatment are increased gradually:rod-likeδphase is decreased, while the vast amount of Laves phases are increased,and M7C3 carbide andσphase are also increased gradually. Meanwhile,the precipitates ofγ"andγ' are formed from a relatively independent of precipitation to compact precipitation ofγ"/γ'.The effect of Co on the grain boundary precipitates was not observed when Al is the same,and the effect of Al and Co on the yield strength and hardness of alloys are similar to the as-cast alloys.When Al 1.24% and Co 0.6%,the No.5 alloys have better yield strength and hardness.3.After long aging at 680℃temperature,the the precipitates of grain boundary and the crystal grow significantly, especially the Laves phase grow more significantly and Laves phase increases.When the content of Al is higher, the compact precipitation ofγ"/γ' grow more slowly, but the grain boundary precipitates grow rapidly. The relatively independent precipitated Spheroidal y'and disk-shapedγ" phases of alloy No.0 have grown and y" shape becomes flaky,γ" is decreased sinceγ" transformed intoδ.The precipitates of alloys grow more slowly than alloy No.0.As the aging time extension,this trend grew more pronounced.4.The alloys with different content of Al and Co were oxidized at 950℃for 50h,and the results indicate that oxidation kinetics obey parabolic law.The surface of oxide film is all grayish.The oxide films of the alloys added different content of Al and Co are more compact than alloy No.0,and oxide particles are also smaller than alloy No.0.The oxidized films of alloys broadly were divided into three layers:the outer layer is the mixed layer which consisted of mainly Cr2O3,TiO,NiO,and such a small amount of NiCr2O4 phase;intermediate layer Cr2O3;as well as discrete internal oxides Al2O3.With the Al increasing,the high-temperature anti-oxidation is improved,but the high-temperature anti-oxidation is weakened with Co increasing.The effect of Al on alloys is greater than Co.when Al 1.45% and Co 0.3%,the alloy performs better high-temperature anti-oxidation,while alloy No.0 was poor.
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