Effects Of Electric-field Treatment On The Microstructure Evolution, Deformation Behavior And Corrosion Properties Of Nickel-base Superalloys

Superalloy as one kind of the important metallic materials, it is said to be the marking of of industry level for a country. Therefore, research for the superalloys is considered as the key point work on materials science. In order to adapt the increasing requirements, more much alloying elements have to be added into the superalloys, it leads the poor deformability, the decrease of toughness and plasticity after long-term aging, and the poor surface durability and so on.Recently, the combination of materials science and electromagnetism theory have been developed into a new field, named Electromagnetic Processing of Materials (EPM). The advantages of electromagnetic field, as a powerful and controllable external field have been attentioned in the past years. Electric field is another important energy field except the temperature field and stress field, and it shows many special coupling effects when it is combinated with the other fields. As the experimental materials, the commercial nickel-base superalloys of GH4199, GH4586 and GH3625 in aerospace engineering of China were employed in present research. The effects of electrostatic field and electropulsing on microstructure evolutions, deformation behavior and corrosion properties were studied, and the mechanism of the effects were also discussed.Properties of a material are said to be related with the microstructure and defects (including vacancy, dislocation, subgrain boundary and grain boundary) of the material. Therefore, the improving properties of the material can be achieved by controlling the microstructure and defects. Microstructure evolution and tensile deformation behavior of nickel-base wrought superalloys (GH4199 and GH4586) after electrostatic field treatment were investigated, the mechanisms of the effects were also discussed. The results show that when the external electrostatic field was applied on the alloy at high temperature, the electrostatic field, current field and temperature field are coexist. The electrostatic field intensity is the main factor for accelerating the atom diffusion in the alloy. Both the number and size of carbides, and TCP phase increased with the increasing electrostatic field intensity. Under an optimum intensity and treatment temperature, the number of twins increased with increasing holding time. It is found that the direction of dislocation slip has been changed by the boundaries of annealing twins during deformation at lower strain rate. The direction of microcrack propagation has also been changed on crossing the boundaries of the annealing twins. It causes the increasing plastic deformation work and delays fracture, these are considered as the reasons of the increasing ductility of the alloy. With the increasing strain rate, the plasticity of the alloy increased with the increasing number of the active slip systems inside the grains. But, the weakening effects of the continuous carbides along the grain boundary became evidently, at the same time, the difference of the plasticity between the grain boundary and inside the grain increased. Thus the grain boundary was the main way to rupture and it led the decreasing of plasticity.With the rapid development of the aerospace engineering, the more excellent properties including the high-temperature strength and plasticity, oxidation existence, high-temperature corrosion resistance and irradiation resistance of the superalloy are required. In the present research, the microstructure and corrosion resistance of GH4199 alloy after an optimum electrostatic field treatment were studied, the effects and mechanisms of the electrostatic field on corrosion resistance of the alloy were also analyzed. The results show that the nucleation and growth of the annealing twins during the electrostatic field treatment cause the redistribution of alloying elements among the crossings between the original high angle grain boundaries and the annealing twins. The depletion of Cr and Mo elements at the grain boundaries have been aviaized. Therefore, the high angle grain boundaries are interrupted by thus low energy boundaries, so that lead corrosion ditches stop at such low energy boundaries. Consequently, the corrosion resistance of the superalloy is improved.The electropulsing was performed on nickel-base superalloy, the effects of high current density electropulsing on microstructure and deformation behavior of the alloy and responsible mechanisms were studied. The results show that the electropulsing accelerates the atomic thermal vibrations, and therefore decreases the activation energy for atomic transition during the high current density electropulsing. Both vacancy concentration and atom diffusion rate were increased with the thermal stress induced by the instant temperature rising in the early stage of electropulsing. It caused the changing of thermodynamics and dynamics of precipitation, and therefore coarsening ofγ'phase was promoted and carbides can be precipitated at lower temperature and rapid grown up. It is suggested that the coarsening ofγ' precipitates by high current density electropulsing follows with the L-S-W law. The value of activation energy forγ'coarsening is 89.86 kJ/mol, and it is about 64.31% lower than that during common aging.Recrystallization is said to be an important role in microstructure evolution and strengthening and toughening of polycrystalline materials. The electropulsing was performed on cold-rolled nickel-base superalloy in present work. The static recrystallization behavior of the alloy under electropulsing was examined and the mechanisms of the effect were also discussed. It is found that the movement and rearrangement of defects become easier and easier under the electropulsing treatment. The decreasing of diffusion activation energy and the increasing of diffusion rate of atom are considered the main factors for the recrystallization occurred within a short time at lower temperature. Comparing with the case of single temperature field, recrystallization process can be accelerated under electropulsing treatment.In summary, new effects of the electric field on microstructure evolution and behaviors of deformation and corrosion of nickel-base superalloys have been found in the present research. The influential mechanism of the electric field has been discussed. The results of present work will be helpful for developing of EPM. Based on the present research, a new mothed for improving the mechanics and corrosion resistance of wrought superalloy can be considered.