The effects of a DC current on the solidified microstructure, segregation and mechanical property of a single crystal superalloy are inverstigated experimentally in this thesis. the calculation for temperature profile of the directional solidification under the DC current, the CAFD simulation for the microstructure and segregation, and a phase current model for the precipitation are presented respectively.
The experiment shows that with the increase of current, the microsegregation decreases and the dendrite arm spacing is reduced, the size of γ'phase at interdendritic region reduces remarkably, the volume of γ/γ'eutectic decrease. The subgrains reduce and their breath decrease.
The eletromigration of the ions depends on not only their valency but also their atomic weight, which can be evaluated by the parameter Wt/Z*. When the direction of eletromigration is opposite with the diffusion caused by the chemical potential, the microsegregation of the alloying elements decreases.
The yield strength of the sample solidified with the DC current is notably improved and the ductility is reduced at room temperature, but both the rupture life and the ductility at high temperature are remarkably raised. The DC current can also improve the mechanical behavior of the single crystal superalloy even after heat-treatment.
A CAFD model is developed to describe the dentrite growth with the DC current, The simulation results indicate that the Lorenz force caused by the DC current and the induced magnetic field can drive the melt to flow, which accelerates the dendrite growth. But the electromagnetic flow usually can be ignored in the solidification with the DC current. Both the Joule heat and the eletromigration can delay the dentrite growth.
The morphology of the γ'phase tends to be spherical to reduce the interfacial energy at early stage of the process when the particles are small. With the average size increase, the γ'phase changes to cubical under the increscent coherent elastic energy.
The rise of mismatch decelerates the precipitation process and reduces the volume fraction of the phase; The simulation also proved that the growth law of R3 ~ t is practicable only when the volume fraction is very low. Both the growth rate and the volume fraction of γ'phase increase during the precipitation with DC current.
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