Heat Treatment Optimization And High Temperature Performance Of Nickel-base Single Crystal Superalloy DD5

Nickel-based single crystal superalloy is the preferred material for turbine blades.Its design feature is to add a large amount of refractory elements and expensive rare earth elements to enhance the high-temperature performance of the alloy.However,the addition of these alloy elements also brings a series of problems,such as poor microstructure stability of single crystal,high density and high cost,which seriously restricts the application of Ni-based single crystal superalloy in industry.In order to exploit the performance potential of the existing low-cost Ni-based single crystal superalloys,the research group improved the original composition of Ni-based single crystal superalloy DD5,and added trace elements C(0.05%),B(0.04%),Y(0.078%).The heat treatment microstructure,high temperature tensile properties and fatigue properties were studied.It is of great significance to provide the basis and theoretical basis for the study of other mechanical properties of this single crystal alloy.In this paper,the high rate solidification technique(HRS)was used to prepare with the <001> orientation all test bars of Ni-base single crystal superalloys DD5.Through the optimization of its heat treatment process and microstructure,the results show that the optimum solution treatment is 1280 °C/2 h + 1315 °C/2 h + 1320 °C/2 h + 1325 °C/6 h,AC and the optimum aging treatment is 1120 °C/ 4 h,AC + 900 °C/24 h,AC.After the heat treatment,the shape is regular,the cube is high,the average size is about 0.50 ?m and the distribution is homogenous,and the ?/?' phase interface is clear.The total volume fraction of ?' phase is about 68.8%.In this research,the high temperature tensile properties of single crystal alloy DD5 in different heat treatment states were studied.The observation and analysis of the stress-strain curve,fracture morphology and ?' phase morphology of three single crystal samples showed that the single crystal alloy was optimized heat treatment,the high temperature tensile properties of single crystal alloys have been significantly improved.The regular distribution of the cubic ?' phase gives the alloy a uniform deformation structure,strengthens the ?' relative dislocation hindrance,improves the ?' phase strengthening effect,and gives the single crystal a high strength.The thermal fatigue behavior of the second orientation of single crystal alloy DD5 was investigated.Thermal fatigue experiments were carried out on samples of the second orientation [110] and [100] at 20-900 °C and 20-1000 °C.It is shown that the thermal fatigue resistance of the second orientation [100] specimens is superior to the orientation [110] ones.In the second orientation [100] and [110] samples of DD5 single crystal Ni-base superalloy,the initiation and propagation directions of thermal fatigue cracks are the same.For the samples of the second orientation [100] and [110],the thermal fatigue crack initiates at the 45° position and expands at an angle of 45° with the directional solidification direction.The residual eutectic structure,carbides,within the single crystal superalloy and the oxide layer around the hole accelerate the crack propagation,which is harmful to the fatigue properties of the superalloy.In this paper,the low cycle fatigue behavior of single crystal alloy DD5 was studied.Through observation and analysis of low cycle fatigue deformation behavior,fracture characteristics and microstructure,the results show that the single crystal DD5 has a cyclic softening in the process of cyclic loading at 760 ? and 850 ?.The fatigue crack originates from the surface or subsurface of the sample,the loose shrinkage defect inside the sample,and has multiple fatigue sources.The existence of river-like patterns and cleavage steps indicates that the fatigue fracture of single crystal alloy DD5 at 760 °C and 850 °C is brittle cleavage fracture along the {111} slip plane.