Study On Microstructure Stability And Mechanical Properties Of Novel Nickel-Based Single Crystal Superalloys

This paper is based on a fourth-generation single-crystal superalloy,the alloy exhibits good creep properties under high temperature and low stress conditions,nevertheless,the presence of TCP phase during long-term aging is common.Therefore,the purpose of this paper is to improve the microstructural stability of the alloy without affecting the mechanical properties of the alloy by changing the content of certain elements in the alloy.In this paper,six kinds of nickel-based single crystal superalloys were designed.The variables were Co?4Co,8Co,12Co,16Co,?and Mo?1Mo,2Mo?.Scanning electron microscopy?SEM?,differential thermal analysis?DTA?,electron probe?EPMA?and X-ray diffractometry were used to study the effects of key elements such as Mo and Co on the microstructure and creep properties of nickel-based single crystal superalloys.High temperature oxidation behavior of nickel-based single crystal superalloys at 1100°C and 1140°C was studied by means of constant temperature oxidation using X-ray diffraction?XRD?,and energy dispersive spectroscopy?EDX?.Experimental results are as follow:In the as-cast microstructure of several alloys,the eutectic contents,the initial melting temperature and the liquidus temperature first increased and then decreased with the increase of Co content,and reached the highest at 12Co.For specimen after standard heat treatments,as Co content increased,the size of the ? phase in both interdendritic and dendritic regions as well as the width of the ? matrix channel first decreased and then increased.Moreover,the volume fraction of the ? phase first increased and then decreased.However,the increase of Mo result in the reduce of the eutectic content in the as-cast structure and the size of the?phase in both interdendritic and dendritic regions.In addtion,the volume fraction of the ? phase increase with increasing Mo content.There is no obvious reverse distribution of alloying elements.The averageg ?/?' lattice misfits of several alloys at room temperature is negative.The alloy has a large number of TCP precipitates after isothermal long-term aging at 1100°C,and the microstructure stability has not been improved.Therefore,the content of refractory elements is adjusted based on the original alloy composition,and no TCP phase was observed at 100 and 500 h for long-term aging.After 1000 h,a small amount of TCP phase precipitated,and the single crystal superalloy with good structure and properties was obtained.Under the condition of high temperature and low stress of 1140°C/137 MPa,the creep properties of the alloy first increased and then decreased with the increase of Co content.The performance at 12Co was the best,while the increase of Mo significantly improves the creep properties of the alloy under high temperature and low stress.This is because the change in Co and Mo significantly reduces the minimum creep rate of the alloy and increases the proportion of the creep steady state.The addition of Mo can reduce the interface dislocation network spacing,improve the stability of the interface dislocation network,and make the interface dislocation network distribution more uniform which is beneficial to reduce the minimum creep rate and prolong the duration of the second phase of creep,thereby improving the creep life of the alloy ultimately.The weight of the experimental alloy increased rapidly in the initial stage of oxidation,but with the oxidation time prolonging,the growth of the oxide film became slow.Due to the oxide layer deterioration during the oxidation process,the oxidation weight gain curve slightly deviates from the parabolic law oxidation.The oxide scale is roughly divided into three layers:the outer layer is composed of Cr₂O₃,and NiO,the middle layer is mainly dominated by Cr₂O₃,the inner layer is Al₂O₃.