Hot Deformation Behavior and Mechanism of DP-GH4169 Alloy during Delta Process
Ni-based Superalloy GH4169 is a precipitation strengthened alloy.The metastable body-centered tetragonal coherent pre-cipitate γ"(Ni3Nb)phase and the face-centered cubic coherent precipitate γ'(Ni3Al)phase are strengthening phases,and the γ"phase is the major strengthening phase.The equilibrium phase corresponding to γ"phase is the orthorhombic incoherent 8(Ni,Nb)phase.GH4169 alloy is an important material used for aviation and aerospace engines.The mechanical properties of GH4169 alloy are sensitive to microstructure.In order to improve the safety and reliability of engines,it is crucial to obtain the forging with a uniform and fine microstructure.Generally,the forgings with large size and complex shape,such as turbine disks and engine shafts,are manufac-tured by multi-stage hot working processes.In addition,the microstructure of the alloy is sensitive to the hot deformation parameters.Therefore,the defects of coarse grain and duplex grain always appear in the forgings.Based on the MinigrainTM processing and the strong pinning effect of 8 phase on the grain boundaries in GH4169 alloy,Delta process(DP)has been applied to the forging of GH4169 alloy,which uses an intentional 8 phase precipitation cycle and subsequent thermomechanical processing to obtain the forg-ing with uniform fine grains.The processing maps have been developed on the basis of dynamic material model(DMM).In this model the workpiece under hot working is considered to be a dissipater of power.The processing mapis composed of the power dissipation map and the instability map.The variation of the efficiency of power dissipation(η)with deformation temperature and strain rate con-stitutes the power dissipation maps.The various domains in the map are correlated with specific microstructural evolution processes.Therefore,the processing maps have been widely applied to understand the workability of materials and optimize the hot working pro-cess.The effect of 8 phase on the deformation behavior and the microstructure evolution of GH4169 alloy during the hot working have been extensively studied.In DP process,to make a full use of the pinning effect of 8 phase on the grain boundaries,the plate-like 8 phase has been pre-precipitated in GH4169(DP-GH4169)alloy through an intentional 8 phase precipitation cycle to obtain the Wid-manstätten δ microstructure.But the morphology of 8 phase precipitated in the current references is short sticks or particles.In addi-tion,to ensure the development of dynamic recrystallization(DRX)sufficient in the subsequent hot deformation process of DP pro-cess,the deformation temperature must be controlled between the dissolution temperature of δ phase and DRX temperature,and the deformation rate must be low enough.In the present work,the objective was to study the hot deformation behavior and mechanisms of DP-GH4169 alloy during DP process using the isothermal compression tests,and optimize the deformation parameters in DP process using the power dissipation maps.To ensure only δ phase precipitating in the alloy,the alloy was first solution treated at 1040 ℃ for 45 min,followed by water quenching,and then the solution treated samples were aging treated at 915 ℃ for 24 h,followed by water quenching.The isothermal compression tests were carried out on a Gleeble-3800 thermomechanical simulator.For DRX temperature of GH4169 alloy was about 850 ℃,and the dissolution of δ phase was 1020-1040 ℃.Therefore,the deformation temperatures were ar-ranged from 954 to 996 ℃ with an interval of 14 ℃,the strain rates were 0.001,0.005,0.01 and 0.1 s-1,and the height reductions were 30％and 50％.The specimens were heated to the test temperature at a rate of 5 ℃·s-1 and held for 180 s to ensure a uniform tem-perature in the specimens.To retain the deformation microstructure,the deformed specimens were quenched by water as soon as the compression tests were completed.The deformed specimens were sectioned parallel to the compression axis for microstructure analysis.Based on the flow stress data obtained from the isothermal compression tests,the activation energy of deformation,strain rate sensitivi-ty exponent and power dissipation map were generated for DP-GH4169 alloy.The deformation mechanisms and microstructure evolu-tion during DP process of DP-GH4169 alloy were investigated by using optical microscope(OM),transmission electron microscope(TEM)and high-resolution electron backscatter diffraction(EBSD).The results indicated that the main microstructure evolutions were dynamic recrystallization and spheroidization of plate-like δ phase,and the activation energy of deformation was 332.276 kJ·mol-1.The interaction effect between the spheroidization of plate-like δ phase and grain boundary sliding(GBS)mechanisms resulted in the different variation in strain rate sensitivity exponent(mn)in the isothermal compression tests.During the hot deformation of DP-GH4169 alloy in the strain rate ranging from 0.001 to 0.1 s-1,GBS mechanisms transferred from diffusion flow to dislocation movement.Mean-while,during the hot deformation,the spheroidization of the plate-like 8 phase provided the favorable geometric morphology for GBS,and the dissolution of δ phase resulted in the decrease in the number of δ/γ phase boundaries.According to the power dissipation map,as the deformation temperature was higher than 980 ℃,the alloy with finer grain had been obtained easily at higher strain rate.As de-formed at the temperature ranging from 957 to 971 ℃ and strain rate ranging from 0.001 to 0.004 s-1,the superplasticity deformation of DP-GH4169 alloy had been occurred.
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