The heat treatment of Ni-based single crystal superalloy requires high temperature and long time. Recently, heat treatment controlled by the external electric field has been developed and it has been accepted that using the electric field during heat treatment of alloys is an effective way to enhance the dissolution of the second phase, accelerate the homogeneous process and improve the properties of the alloys. However, there are a few researches concerned with the effect of the electric field on heat treatment of superalloys.This thesis studies the dissolution of Gamma prime phase and Gamma/Gamma prime eutectic phase, elements distribution, and the precipitation of Gamma prime phase of a Ni-based single crystal superalloy under the action of, and the effect of electric fields heat treatment on the mechanical properties of the auperalloy. DC electric current and DC electrostatic field heat treatment of a Ni-based single crystal superalloy are developed in this thesis.
Under the action of different electric currents the superalloy was solution treated at different temperature. The experiment results show that the electric current enhances the dissolution of Gamma prime phase and Gamma/Gamma prime eutectic phase during the solution treatment process of the superalloy; both the size and volume fraction of un-dissolved Gamma prime phase and Gamma/Gamma prime eutectic phase decrease with the increase of electric current density. The dissolved time of Gamma prime phase was reduced by electric current. It is considered that the electromigration effect of electric current accelerates the diffusion of atoms during solution heat treatment of the superalloy, which enhances the dissolution of Gamma prime phase and Gamma/Gamma prime eutectic phase.
The precipitation of Gamma prime phase is influenced by electric current and cooling rate. High cooling rate attributes to the precipitation of fin g¢ particles. However, the Gamma prime phase is bigger and more uniform under the action of electric current than that without electric current at the same cooling condition. The size and volume fraction of Gamma prime phase increase with the increase of electric current density, while the planar density of Gamma prime phase decreases during the process of air cooling. It is considered that the Joule effect of the electric current reduces the cooling rate, which speeds up the growth of Gamma prime phase.
The electric current accelerates the homogeneous process of the superalloy. Under the action of electric current, the distributions of the alloying elements in the alloy are more homogeneous than those without electric current. In particular, the segregation of W element, which is difficult to reach uniform during solution heat treatment, is remarkably reduced by the electric current.
The electric current enhances the growth of Gamma prime phase in the superalloy during the process of aging treatment. The experimental results show that the size and volume fraction of Gamma prime phase increase with the increase of current density. According to the theory of Ostwald coarsening, the activation energy for the Gamma prime phase growth, was calculated to be 208 kJ/mol for the condition without the electric current, and 189 kJ/mol for the condition with the electric current.
The experiment results for the effects of electrostatic field on the solution and aging treatment of the alloy show that the electric field enhances the dissolution of Gamma prime phase, improves the microsegregation of elements. With an appropriate intensity of electric field, heating temperature and holding time, uniform distribution of Gamma prime phase and narrow sub-grain can be obtained. The activation energy for the Gamma prime phase growth is reduced by the electric field, which accelerates the growth of Gamma prime phase. The attraction between the negative charge of vacancy and positive charges in the surface would lead to the increase of element diffusion, which maybe the key influence of the field.
The electric current enhances the yield strength at room temperature and the high temperature stress rupture life of the superalloy. The homogeneous distribution of elements, the size of Gamma prime phase and narrow sub-grain attribute to the increase of the stress rupture life of the alloy.
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