Effect Of Temperature Gradient On Microstructure And Mechanical Properties Of Ni₃Al Based Single Crystal Superalloy

Ni₃Al based single crystal superalloy has the advantages of low density,high strength,good oxidation and corrosion resistance.It can lower the manufacturing cost and reduce the weight of superalloy while ensuring excellent high temperature performance,so as to become a new material urgently needed for the manufacture of aeroengines with high-to-weight ratio.During the directional solidification of single crystal superalloys,the change of temperature gradient at the solid-liquid interface will affect the dendrite spacing and solidification microstructure of superalloy,thereby further affecting the heat treatment regime and mechanical properties of superalloy.Therefore,the effect of temperature gradient on the microstructure and mechanical properties of Ni₃Al based single crystal superalloy was studied,to provide technical reserves for improving and optimizing the preparation of Ni₃Al based single crystal superalloy.In this research,a low-cost Ni₃Al based single crystal superalloy developed by Beijing Institute of Aeronautical Materials was selected as the object.Two alloys with different temperature gradients were prepared by liquid metal cooling and rapid solidification method,respectively.The effects of temperature gradient on the microstructure of as-cast and heat treated alloys were studied.The tensile properties of two alloys with different temperature gradients were tested from room temperature to 900℃.The stress rupture life of two temperature gradient alloys was tested under different creep conditions.The deformation structures of two alloys with different temperature gradients after tensile and stress fracture were analyzed by SEM and TEM,and the effect of temperature gradient on related mechanism of on the microstructure and mechanical properties of Ni₃Al based single crystal superalloy were studied.The experimental results show that the as-cast microstructure of two temperature gradient alloys is typical dendritic morphology,and a few eutectic structure is observed in the interdendritic region.The primary and secondary dendrite spacing of the alloy with high temperature gradient are approximately 250μm and 75μm,respectively,and those of the alloy with low temperature gradient are approximately 360μm and 113μm,respectively.The morphology ofγ′phases in the interdendritic region of the alloy with high and low temperature gradient is irregular or nearly spherical,and the size ofγ′phase in dendrite arm region of the alloy with high temperature gradient is small.The segregation degree of the alloy with high temperature gradient is obviously lower than that of the alloy with low temperature gradient in as-cast state.After full heat treatment,the eutectic structure in two alloys with different temperature gradients are completely eliminated,and the morphology ofγ′phase in interdendritic regions and dendritic regions tends to be consistent.The segregation of each element in two alloys with different temperature gradients is alleviated.The yield strength of two alloys with different temperature gradients has the same variation with temperature,which increases first and then decreases,and has obvious anomalous yield behavior.The yield strength reaches the peak value at 800℃.The fracture modes of two alloys with different temperature gradients are basically same at different temperatures.In the temperature range of room temperature to 900℃,the fracture mode changes from pure shear fracture to microvoid accumulation fracture and the transition temperature is 760℃.It was found that the dislocations were mainly shearing intoγ′phases in the form of dislocation pairs at room temperature.As the temperature increased to 800℃,the dislocations began to climb over theγ′phase.The results of stress rupture tests at 980℃/186MPa and 1040℃/135MPa show that the stress rupture life of the alloy with high temperature gradient is slightly higher than that of the alloy with low temperature gradient.The fracture mode of two alloys with different temperature gradients are both microvoid accumulation fracture.In addition,a few of M₆C carbides were precipitated in two alloys during creep process and the interfacial dislocation networks formed at the interface.