Research On Hot Deformation Behavior And Microstructure-properties Control Of Gh4151 Alloy

GH4151 alloy,a novel hard-to-deform nickel-based superalloy,is regarded as a potential material for the next generation superalloy for turbine disks due to its excellent comprehensive properties and higher service temperature.The alloy contains about 35 wt.%of solid solution strengthening elements(Co,Cr,W,Mo)and up to 10 wt.%of precipitation strengthening elements(Al,Ti,Nb),which is a cast&wrought superalloy for turbine discs with the highest content of precipitation strengthening elements.On the one hand,the high alloying characteristics of GH4151 alloy can increase the service temperature and high temperature strength,but inevitably reduce the hot workability of the alloy.On the other hand,the elements segregation in the alloy and high fraction of 52%γ’ strengthening phase 52%lead to the complicated microstructure evolution of GH4151 alloy under hot working conditions.And it is extremely difficult to control the microstructure and properties.As a novel nickel-based superalloy,the basic research on hot deformation and heat treatment is rarely conducted,especially the lack of systematic and in-depth research on hot deformation behavior and dynamic softening mechanism,microstructure-properties control under different heat treatments.In response to the above problems,the following works has been carried out in this research:(1)Thermophysical simulation experiments were carried out to study the hot deformation behavior of the hot-extruded GH4151 alloy.The hyperbolic sine-based Arrhenius constitutive relationship model and processing map based on the dynamic material model(DMM)were established.The optimal hot working interval for billet and disc die-forging were determined.Studies have shown that the best hot working parameters for die forging of disc forgings are:1045-1125℃,strain rate ε<10-2.5 s-1.(2)In order to verify the effectiveness of the superplasticity of the hot-extruded alloy,superplastic tensile behavior was studied in this research.The tensile tests were carried out at 1060℃,1080℃ and 1100℃ for 10-4 s-1,5×10-4 s-1 and 10-3 s,respectively.The strain rate sensitivity coefficients at different temperatures are m 1060℃=0.69,m1080℃=0.73,m1100℃=0.73.The deformation activation energy at different strain rates are Q104=192kJ/mol,Q5×104=258kJ/mol,Q10-3=342kJ/mol.When the strain rate ε=10-4 s-1,the supedplastic deformation is mainly attribute to the grain boundary sliding mechanism controlled by grain boundary diffusion.However,when ε=5×10-4 s-1 and ε=10-3 s-1,the grain boundary slip mechanism controlled by lattice diffusion is dominant.(3)The microstructure evolution of the hot-extruded GH4151 alloy during hot deformation is studied,the dynamic recrystallization mechanisms of GH4151 alloy under the experimental conditions is clarified,and the optimal process interval is detennined.The results indicate that a new dynamic recrystallization mechanism occurs during hot deformation of the hot-extruded GH4151 alloy.The nucleation mechanism can be described as such a feature,that is a primary γ’(Ni3(Al,Ti,Nb))precipitate embedded in a recrystallized grain existed the same crystallographic orientation,which is defined as heteroepitaxial dynamic reciystallization(HDRX).Meanwhile,the conventional DRX mechanisms,such as the discontinuous dynamic recrystallization(DDRX)characterized by bulging grain boundary and continuous dynamic recrystallization(CDRX)operated through progressive sub-grain merging and rotation,also take place during the hot deformation of the hot-extruded GH4151 alloy.HDRX mechanism can be deemed as a new explanation for the role of γ’Ⅰphase in the dynamic recrystallization process a new explanation.(4)In this research,it is systematacially studied that the primary γ’phase(γ’Ⅰ)re-dissolution at the grain boundary and grain growth behavior under sub-solvus conditions.The Sellars grain growth kinetic model with time index m is established,which reveals the correlation between γ’Ⅰ re-dissolution and grain growth.The single-slope cooling and dual-slope control of slow cooling and then rapid cooling after sub-solvus treatment are studied.The control of the bimodal distribution of γ’phase can be achieved by dual-slope cooling control path.Accordingly,the standard heat treatment of GH4151 alloy is determined.Studies have shown that standard heat-treated alloys have excellent mechanical properties:tensile strength,yield strength at room temperature,and high temperature durability at 750℃/650MPa are 1666.5MPa,1306.5MPa and 173.35 h,respectively.(5)The evolution characteristics of the γ’Ⅱ phase during the long-term thermal exposure experiment of the alloy were studied.The results show that the γ’Ⅱ phase did not grow up significantly after exposure at 750℃ for 5000h.However,at 800℃,the evolution of the γ’Ⅱ phase followed the Ostwald ripening process controlled by lattice diffusion.The evolution of γ’Ⅱ phase is onsistent with the classical LSW theory.The value of coarselilg rate constant k is 690.5 nm3/h.It is concluded thatγ’ phase of GH4151 alloy has long-term stability at service temperature γ’ phase at 750℃-800℃,and the deterioration of mechanical properties is mainly related to the coarsening of grain boundaries and the precipitation of μ phase.In conclusion,it is illuminated the relationship between the process parameters-microstructure-mechanical properties of the novel difficult-to-deform superalloy GH4151 at high temperature by studying the hot deformation behavior,microstructure evolution and mechanical properties of the alloy.