Hot Deformation Behavior And Microstructure-properties Control Of Hot-pressed Sintered Ep741np Ni-based Alloy

EP741NP alloy is a nickel-based superalloy with precipitation strengthened throughγ’（Ni₃（Al,Ti））phase in a disordered face-centered cubic matrix because of its excellent oxidation resistance,fatigue,crack growth resistance and corrosion resistance play an important role in the aerospace industry,especially in the preparation of high-performance rotating components for aero-engines.Although EP741NP alloy has the above advantages,there are a large number of solid solution strengthening elements such as W,Mo,Cr and Co in the alloy,which increases the difficulty of hot processing and limits its application in the aerospace field.In addition,the powder metallurgy EP741NP superalloy successfully avoids the complex problems such as shrinkage cavities,shrinkage porosity and alloy segregation during the preparation of traditional metallurgical methods on the basis of meeting the material performance requirements of aero-engines.At present,the hot pressing sintering process and densification mechanism,high temperature deformation behavior and dynamic softening mechanism of the alloy,the selection of a reasonable hot working window and the influence of hot processing parameters on the microstructure of the alloy still need to be clarified and further studied.Therefore,in this paper,following investigations were carried out.EP741NP pre-alloyed powder was used as raw material,and EP741NP alloy material with dense and uniform structure was prepared by vacuum hot pressing sintering process.The densification mechanism in the hot-pressing sintering process,the influence of sintering temperature on microstructure evolution,the density and tensile properties of the alloy were explored,and the optimal hot-pressing sintering parameter was determined,and the high temperature deformation behavior of the alloy was investigated,the constitutive equation and the processing map including the dynamic recrystallization model was discussed,the optimal hot working windows were obtained.The dynamic softening mechanism of EP741NP alloy was revealed,and the effect of hot processing parameters on microstructure,γ’phase and dynamic softening mechanisms.The analysis results of the hot pressing sintering densification curves,microstructure and properties of EP741NP pre-alloyed powders in the temperature range of 1030-1230oC show that the densification mechanism of the material in the heating and pressing stage is mainly plastic deformation,and the grain boundary diffusion is dominant in the heat and pressure preservation stage.There were obvious powder particle boundaries（PPB）in the hot-pressed sintered samples,the precipitates along the PPB were discontinuous,and carbides were formed during hot pressing sintering of powders.The hot-pressed sintered samples obtained at 1180oC/50MPa/1h had higher comprehensive mechanical properties at room temperature and 650oC.The hot temperature deformation behavior of hot pressed EP741NP alloy was analyzed by thermal simulation compression test,and the flow stress curves obtained after thermal deformation were corrected by friction and adiabatic temperature.According to the corrected stress-strain curves,the DRX critical model and kinetics model were established,and the constitutive model of the alloy was obtained by combining the temperature compensation strain rate factor.The constitutive model was evaluated by microstructure and statistical error analysis.It was found that the hot deformation activation energy of the alloy was Q=577.45k J·mol^-1.The hot processing map of EP741NP alloy was constructed.The researches show that the optimal processing window of the alloy is D₁(1160-1190°C/0.05-0.819s^-1),D₂(1130-1165°C/0.0025-0.05s^-1)和D₃(1055-1085°C/0.011-0.135s^-1).The DRX volume fraction of the material is positively correlated with the deformation temperature and negatively corrected with the strain rate.There are more smooth on the boundary ofγ’phase at low strain rate,but with the increase of strain rate,the edge ofγ’phase in the alloy gradually becomes smooth.By comparing the microstructures under different strains with strain rate of 0.01s^-1and deformation temperature of 1080oC and 1140oC,it is found that the dynamic recrystallization volume fraction increases with the increase of the strain,the content of LAGBs is the highest at the strain of 0.5,and with the increase of strain,the contents of LAGBs decreased and the contents of TBs increased,the LAGBs gradually transformed into HAGBs,and the value of the average orientation angle increased.In addition,discontinuous dynamic recrystallization（DDRX）is the dominant deformation mechanism at the strain rate of 0.01s^-1 and the deformation temperature of 1080oC and1140oC,and nucleation is mainly through grain boundary bowing,but continuous dynamic recrystallization（CDRX）is indeed activated during the deformation process,but at high temperature,DDRX always dominates the microstructure evolution,which relatively weakens the effect of CDRX.