Research On Thermoplasticity Of FGH4096 Ni-based Powder Metallurgy Superalloy

Powder metallurgy（P/M）superalloys are often challenging to process,making research on their thermoplasticity particularly important.In this study,hot extruded（HEXed）FGH4096 nickel-based powder superalloys were used as the research subject.A sub-solvus temperature holding followed by extremely slow cooling（S-ESC）method was explored to control the size and distribution of theγ′phase,significantly improving the thermoplasticity of FGH4096 and resulting in successful superplastic deformation.Thermal compression experiments were conducted on both HEXed and S-ESCed samples to compare the deformation behavior and microstructural evolution.Additionally,uniaxial tensile experiments were performed on both samples to compare their thermal tensile deformation behavior,fracture characteristics,and microstructural evolution.（1）The concentration ofγ′precipitates in FGH4096 samples gradually decreases as the sub-solvus solution temperature increases.The preferential dissolution ofγ′precipitates occurs within the grain.An increase in holding time leads to the coarsening ofγ′precipitates,but extended holding times result in significant growth of theγmatrix grains.The growth ofγ′precipitates in FGH4096 samples is promoted by reducing the cooling rate.A slower cooling rate inhibits the re-precipitation ofγ′precipitates within the grain and promotes full growth ofγ′precipitates at the grain boundary.The end temperature of the slow cooling process has a considerable impact on the size ofγ′precipitates,with negligible growth observed during cooling to room temperature at 500°C.The final confirmed heat treatment process involves holding at sub-solvus temperature for some time followed by cooling to500°C at an extremely slow rate,which is referred to as the S-ESC process.After S-ESC processing,the dispersed fineγ′precipitates inside the grains and at grain boundaries exhibit significant growth.（2）Under most deformation conditions,the S-ESCed samples exhibit smaller flow stress and larger strain rate sensitive index（m）values than the HEXed samples,especially the peak value of m is 0.75 vs.0.32.This is because fineγ′precipitates limit deformation by pinning dislocations,and coarseγ′precipitates promote dynamic recrystallization（DRX）.（3）Under the high strain rate of 1 s^-1,the DRX degree of the HEXed sample is greatly affected by temperature,and both continuous dynamic recrystallization（CDRX）and discontinuous dynamic recrystallization（DDRX）characteristics are observed at 950℃and1050℃,while the S-ESCed samples maintain a high DRX degree at different temperatures.At the high temperature of 1100℃,the S-ESCed samples exhibit smaller grain sizes and higher DRX degrees than the HEXed sample under low strain rates,where the DRX mechanism is DDRX.（4）Under all tensile test conditions,the S-ESCed samples exhibited lower flow stress and higher elongation and reduction of area compared to the HEXed samples.The S-ESCed sample achieved superplasticity at 1090℃and 10^-3 s^-1,with a remarkable elongation of248.4%,significantly surpassing the 141.8%of the HEXed sample.The improved thermoplasticity of the S-ESCed samples is primarily attributed to the promotion of DRX behavior by the coarseγ′precipitates.（5）At low temperatures,the coarseγ′precipitates accelerate the DRX of theγmatrix grains through participation in deformation.At high temperatures,the coarseγ′precipitates promote DRX and control grain size by pinning grain boundaries.The elongation of the two material samples depends not only on DRX,but also on the crack condition.At 950°C,although the S-ESCed sample has a higher degree of DRX than the HEXed sample,the coarseγ′precipitates lead to severe cracking,thus limiting the increase in elongation.However,at 1090°C,the size of theγ′precipitates decreases and the influence of cracking is reduced,leading to a significant increase in elongation.