Investigation on freckle formation of nickel-based single crystal superalloy specimens with suddenly reduced cross section

The formation and distribution of freckles in the third generation single crystal superalloy DD9 were investigated by a designed stepwise specimen with suddenly reduced cross section in this paper. The microstructural characteristics of freckles were analyzed by optical microscope(OM), scanning electron microscope(SEM) and electron back scattering diffraction(EBSD). Numerical simulation was used to analyze the thermal field and the shape of the mushy zone of the specimens during the directional solidification. The results show that freckles are composed of disorderly orientated dendrite fragments and γ/γ′ eutectic. Freckles only appeared on the surface of the specimens and are prone to occur in the larger parts and edges of the specimens. The morphology of the isotherm has an important influence on the formation of freckles. The presence of lateral temperature gradient increases the instability of segregated liquid in the mushy zone near the shell mould wall and makes the freckles tend to form on the higher position of the isotherm. The reduction of the melt volume in the mushy zone combining with the decrease in the tilt of the isotherm suppress the formation of freckles. Based on the experimental results and simulation results, a modified Rayleigh-number based criterion for freckle formation is proposed.