Hot Deformation Behavior And Microstructure Evolution Of Incoloy825 Superalloy

Nickel base superalloy is an austenitic alloy based on nickel,which has excellent corrosion resistance and high temperature resistance.It has a very broad application prospect in the fields of automobile,ship and aviation.Due to the high alloying rate of nickel base superalloys,cracking,segregation,coarse and uneven grains often occur in the processing process.It seriously affects the mechanical properties of parts,such as strength,plasticity,hardness,toughness and fatigue strength.Therefore,it is necessary to systematically study the high temperature deformation mechanism,microstructure refinement and homogenization law of nickel base superalloy,which is conducive to the accurate control of workpiece quality in the machining process.Taking Incoloy825 superalloy as the research object,this paper uses Gleeble-3800 thermal simulation testing machine to carry out thermal compression experiments at different temperatures,different strain rates and different strains.The deformation parameters are controlled in real time through the integrated module.According to the stress-strain curve of thermal compression,the constitutive relationship model,hot working diagram and dynamic recrystallization model of Incoloy825 superalloy are established.The microstructure evolution mechanism of Incoloy825 superalloy was studied by means of optical microscope（OM）and electron backscatter diffraction（EBSD）,and the relationship between dynamic recrystallization nucleation mechanism,dynamic recrystallization percentage,new grain size and deformation parameters was analyzed.The purpose is to provide theoretical guidance for the hot deformation process of Incoloy825 superalloy.First,the single pass thermal compression experiment of Incoloy825superalloy was carried out by Gleeble-3800 thermal simulation testing machine,and the stress-strain curves under different parameters were obtained.The rheological behavior of Incoloy825 superalloy was analyzed.It was found that Incoloy825 superalloy was sensitive to deformation temperature and strain rate,and the rheological stress decreased with the increase of deformation temperature and the decrease of strain rate.The rheological curve shows two trends:one is that the rheological stress decreases after reaching the peak stress,showing typical dynamic recrystallization characteristics;The other is that the flow stress maintains a stable trend after reaching the peak stress,showing typical dynamic recovery characteristics.This is closely related to the evolution of microstructure.According to the stress-strain curve,the constitutive model and hot working diagram of Incoloy825 superalloy are established.The constitutive relationship model expresses the relationship between stress and deformation parameters in a functional way,which intuitively reflects the relationship between variables and dependent variables.The hot working diagram shows the processing window of the alloy,which can avoid the instability zone.The hot working window of Incoloy825 superalloy is 1000℃～1200℃,the strain is 0.69～0.92 and the strain rate is 0.005～0.5s^-1.Secondly,the dynamic recrystallization model was established according to the rheological curve of hot deformation experiment.The effects of hot deformation temperature,strain and strain rate on the critical point and percentage of dynamic recrystallization of Incoloy825 superalloy were studied by means of mathematical model.The critical strain point of dynamic recrystallization increases with the increase of strain rate or the decrease of temperature.At the same strain rate,the growth rate of dynamic recrystallization increases with the increase of deformation temperature;At the same temperature,the growth rate of dynamic recrystallization decreases with the increase of strain rate（both before complete dynamic recrystallization）.Then,the tissues were observed and analyzed by optical microscope,and the samples with appropriate parameters were selected for EBSD analysis.In the process of hot deformation,with the increase of strain,the original grains are gradually replaced by new grains,and the grain size of new grains increases with the increase of strain.It is found that the complete dynamic recrystallization of Incoloy825 superalloy occurs for the first time between 1100～1150℃and0.001～0.01s^-1 at the deformation temperature of 950～1150℃,strain of 0.92 and strain rate of 0.001～1s^-1;When the temperature is 1050℃,the strain rate is 0.01s^-1 and the strain is 0.22～0.92,the complete DRX occurs between 0.69～0.92,and the average grain size is about 32.5μm.It has reached a relatively stable size state.For Incoloy825 superalloy with low stacking fault energy,DDRX is the main nucleation mechanism and CDRX is the auxiliary mechanism.At the same time,DRX is also the main softening mechanism of Incoloy825 superalloy.