Research On Hot Deformation Behavior Of FGH4096 Nickel-based Powder Superalloy

Nickel-based powder superalloys are highly alloyed and have defects such as poor hot workability and narrow hot work window,which can affect the forming as well as the performance of the material.Therefore,it is of great significance to investigate the hot deformation behavior,hot working window,and microstructure evolution of nickel-based powder superalloys for optimizing its hot working processes.In this paper,the hot deformation behavior and microstructure evolution in different states of hot isostatically pressed（HIPed）and hot extruded（HEXed）nickel-based powder superalloys 4096（FGH4096）were investigated by hot compression.The friction-temperature correction was performed on the collected flow stress curves,and the corrected flow stresses were used to construct the constitutive equations and hot working maps.The microstructure evolution under different deformation conditions was observed and analyzed.The established hot working maps were combined with the microstructure analysis to verify the rationality of the hot working maps and to obtain the optimal hot working window.This research provides a theoretical basis for selecting the hot working window and controlling the microstructure of FGH4096 powder superalloy during actual hot working process.The research results show that:（1）The initial microstructure of HIPed-FGH4096 powder superalloy exhibits obvious prior particle boundaries and non-uniform grain size.The initial microstructure of HEXed-FGH4096 superalloy exhibits small grains with random orientation distribution,and the prior particle boundaries disappear completely after HEX compared with HIPed-FGH4096powder superalloy.（2）The true stress-strain curves of both HIPed and HEXed FGH4096 powder superalloys sharply rise to a peak and then decrease or maintain,showing obvious dynamic recrystallization（DRX）or dynamic recovery（DRV）characteristics.The flow stress is closely related to the deformation conditions.At the same temperature,the stress increases with increasing strain rate,and conversely,the stress decreases with increasing deformation temperature at the same strain rate,indicating that FGH4096 powder superalloy is a positive strain rate-sensitive material.（3）The corrected flow stresses were used to construct the constitutive equations of HIPed and HEXed FGH4096 powder superalloys,and the hot deformation activation energy Q was calculated as 951.4259 KJ/mol and 1023.3067 KJ/mol,respectively.The average absolute relative errors（AARE）were 6.66%and 9.46%,and the goodness of fit（R²）was0.9809 and 0.9817,respectively,indicating good prediction accuracy.The optimum hot working window for HIPed-FGH4096 powder superalloy was:1080～1107℃,10^-0.2～10^-1.3s^-1.The optimum hot working window for HEXed-FGH4096 powder superalloy was:1102～1130℃,10^-1.26～10^-0 s^-1.Wedge cracks,voids,and intergranular cracks were the main causes of flow instability.（4）The average grain size of HIPed and HEXed FGH4096 powder superalloys was measured using Image J software,and it was found that the grain size was related to both deformation temperature and strain rate.The grain size increased with increasing deformation temperature at a constant strain rate and increased with decreasing strain rate at a constant deformation temperature,except for a strain rate of 10 s^-1.Different DRX mechanisms dominated in different hot working regions,which are strongly influenced by the deformation temperature.At low deformation temperatures,continuous dynamic recrystallization occurs simultaneously with discontinuous dynamic recrystallization,while only discontinuous dynamic recrystallization is activated at high deformation temperatures.（5）The geometrically necessary dislocation density of HEXed-FGH4096 powder superalloy was determined using the kernel average misorientation.It was found that the degree of DRX was inversely proportional to the geometrically necessary dislocation density.The relationship models between the hot working parameter Zener-Hollomon（Z）,flow stressσ,and DRXed grain size D were established based on the measured grain size.A more intuitive new size model was obtained by extending the model based on parameter Z.