Effect Of Minor Re Adjustment On Microtructure And Mechanical Properties Of The Second Generation Ni-based Single Crystal Superalloy

With the development of Chain’s aero-engines technology,It is urgent to put high-grade nickel-base single-crystal(SC)superalloys with higher temperature-bearing capacity and better organizational stability into mass production and service.Starting from the second-generation single-crystal superalloys,the addition of rare metal Re significantly improves the high-temperature mechanical properties of single crystal alloys.However,the addition of Re element greatly increases the cost of SC alloy and severely restricts the application of alloy,it is particularly important to study the influence of trace adjustment of Re element on the microstructure and properties of SC alloy.In this study,the second-generation nickel-based SC superalloys with broad application prospects were used to simulate the micro-adjustment of Re content by thermodynamics.The effects of trace adjustment of Re content on the microstructure and mechanical properties of SC alloys were studied.The effects of micro-adjusted Re content on the microstructure,precipitation phase behavior and element segregation of single crystal alloy were investigated by means of OM,SEM,DSC,EDS and EPMA.The effect of micro-adjusted Re content on the mechanical properties of single crystal alloy was studied by high-temperature tensile at 870℃ and stress rupture life at 1093℃/158 MPa.The thermodynamic equilibrium calculation and DSC analysis show that the increase of Re content increases the content of γ’ phase,lowers the solidus temperature of the alloy,increases the liquidus temperature of the alloy,and enlarges the temperature range of the mushy zone.The as-cast microstructure of SC superalloy is characterized by typical dendritic features.The interdendritic are distributed with sunflower-like γ/γ’ eutectic structure and block-like and skeleton-like MC-type carbides.As the temperature interval of the mushy zone increases,the primary dendritic spacing increases in the as-cast alloy,the length of the secondary dendrite arm increases,and the growth of the tertiary dendritic arms is promoted.Due to the lower diffusion coefficient,Re inhibits the diffusion of other elements,and the size of γ’ phase decreases.At the same time,the degree of segregation of the elements of Re,W,Co and Mo in the dendrites,Al and Ta elements between dendrites is increased,resulting in an increase in the number of eutectic between dendrites and an increase in size.Re increases the mismatch of γ/γ’ two phases,increasing the degree of γ’ cubicization and increasing the volume fraction.The micro-adjusted of Re content has little effect on the high temperature tensile properties of single crystal superalloy at 870℃,but has a great influence on the stress rupture life at 1093℃/158 MPa.With the increase of Re content,the tensile strength of the alloy increased from 990 MPa to 1040 MPa,the yield strength increased from 817 MPa to 845 MPa,the percentage elongation decreased from 23.0%to 21.5%.Fracture is a mixed fracture mode with both slip surface fracture and dimple fracture;the stress rupture life of the alloy increased from 35.2h to 49.3h,and the increase rate reached 40.0%.The elongation and fracture shrinkage after fracture showed the first increase and then decrease.After micro-adjusted of the Re content,the comprehensive mechanical properties of the three alloys meet the design requirements of single crystal turbine blades,and they meet different pursuit goals of cost and performance.