Study On Mechanism Of Microstructure Evolution Of GH4169 Superalloy By Electric Field Treatment

Superalloy is a typical metal material for national technology development and national defense ability, it is always paid attention by the scientists of material field. With developing of society and national defense industry, the performance of superalloy is expected to be improved constantly. Thus, the electric field application with the advantages of high energy density, low application cost, environment friendly and controllable becomes a popular topic to improve the microstructure and property of the alloy. However, the mechanism of microstructure evolution of superalloy has not been claified yet. As the experimental material, GH4169 nickel-base superalloy for the application of aerospace engineering of China was employed in this study. The effects of electrostatic field on microstructure evolution and deformation behavior were studied by the analysis methods of scanning electron microscope (SEM), transmission electron microscope (TEM), internal friction, and positron ammohilation. The mechanism of the effects was also discussed in order to provide the theoretic basis for achieving the microstructure controlling and strengthening of superalloy under electric field.The main conclusions in this study as follow:(1) Electric field application enhances the atomic vibration in GH4169 superalloy during aging, and it leads to the increasing of both quantity and average size of vacancies.(2) The vacancy flow consisting of monovacancies generates induced, and monovacancies transform to vacancy clusters9 under the action of EFT during aging, which causes the coarsening of precipitions in GH416. Aged at 1023 K and 1073 K with 6 kV/cm electric field, the coarsening behavior ofy’and y" phases obeys the Lifshit, Slyozov, and Wagmer (LSW) duffusion controlling theory, the growth activation energies of γ’and γ" phases are determined to 115.6 kJ·mol-1 and 198.1 kJ-mol"1, which decrease by 52.2% and 30.6% compared with those of the alloy without EFT. Meanwhile, the short-distance diffusion of Al and Nb atoms is accelerated by EFT, resulting in the increasing of the diffusion coefficients of these atoms, and which are about 1.6-5.0 times larger than those in the alloy aged without EFT(3) Elecric field applied on the sheet specimen of GH4169 alloy, the intensities of the induced electic fields achieve equilibrium at layer D, that is to say that two vacancy flows form and migrate from layer D towards the charged surfaces. The segregation of Fe and Cr atoms can thus be caused. The lattice distortion occurs in the matrix caused by the segregation of Fe and Cr atoms and the alloy is hardened at lattice distortion region.(4) An 8 kV/cm electric field applied on the standard heating treated GH4169 alloy as aged at 798 K for 10 h, the quantity of vacancies and defect energy of the alloy increase. Tensile test was operated at room temperature in the strain rates of 10-4 s-1 and 10-2 s-1, the alloy strength increase obviously because the dislocation motion is hindered by the increased interstitial atoms induced by EFT. At necking region, grain refinement takes place in the EFTed alloy, and the energy for the atomic arrangement is provided by the increased strain energy and initial defect energy.(5) The inhibition of the increased interstitial atoms to dislocation motion is closely related with the motion rate of dislocation. Lower the motion rate of dislocation, the inhibition effect is more remarkable, and the strain rate sensitivity index of the EFTed alloy in the strain rates of 10-4 s-1 and 10"2 s-1 is-0.011. As the higher strain rates employed, the inhibition effect of the increased interstitial atoms to dislocation motion is weak, but the dislocation density gradually increases due to the more slip systems being activated in the high strain rates, leading to the increasing of the strain rate sensitivity index of the EFTed alloy.(6) The increased interstitial atoms induced by EFT enhance the interstitial atoms pinning to dislocations. Therefore, the B type serrated yielding with higher formation activation energy of the dynamic strain aging of the EFTed alloy emerges even if tensile test operated at 923 K in the strain rate of 10-2 s-1. As the alloy deformation at high temperature, the climb and annihilation of dislocations can be hindered by the large numbers of vacancies and interstitial atoms, and dislocation density increases, causing the decrease in the stress diffence and waiting time for finishing single serrated yielding.