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The freckle formation and evolution in directionally solidified single-crystal specimens with different thicknesses and abruptly expanding rectangular cross-section were investigated at different withdrawal rates. The microstructure was studied by optical microscope (OP) and scanning electron microscope (SEM), and the crystallographic orientation in freckle areas was characterized by electron backscattered diffraction (EBSD). The experimental results showed that the freckle was the most severe in specimens with a thickness of 10 mm at the same withdrawal rate, and the freckle chain was observed to form below an abruptly expanding cross-section ended at the expanding step. It was found that freckles formed exclusively on the edges of casting facing the central sprue of the shell mould. Findings reveal that freckles tend to form at the highest position of the mushy zone, mainly due to the thermosolutal convection and the radial flow caused by feeding behavior in the mushy zone. The freckle formation is pronounced at a low withdrawal rate, and can further generate stray grains. • Freckle formation is more pronounced at a low withdrawal rate of 1 mm/min. • Freckles form exclusively on the edges facing the central sprue of the shell mould. • Radial flow and feeding behavior play a major role in freckle position. • Freckle grains can evolve into stray grains at a low withdrawal rate of 1 mm/min. [ABSTRACT FROM AUTHOR]

• The misorientation between two corners of triangular GCs was increased as the increase of flare angle and withdrawal rate. • The freckle chains formed at the inside corner of triangular GCs with a low flare angle under a low withdrawal rate. • The freckle defect formed at the start position of thick triangular GCs with high flare angle under a low withdrawal rate. • Under 3 mm/min withdrawal rate, 2 mm triangular GCs with 46–68° flare angle were expected to prepare high quality casting. The evolution of the crystallographic orientation and the microstructures in triangular adapters of grain continuators (GCs) with different flare angles and thicknesses prepared by a 3rd-generation single crystal (SX) superalloy were investigated under different withdrawal rates. The microstructures were observed by an optical microscope and the crystallographic orientation of the GCs was measured by electron backscatter diffraction (EBSD). The directional solidification process was simulated by a commercial simulation software ProCAST. The experimental results showed that the [001] misorientation between both sides of the GCs at top cross section increased with the increasing of the flare angle, the thickness of specimens, and the withdrawal rate. These phenomena were attributed to the changing of the isotherm morphology in the mushy zones and the velocity of the solidification front. As for a low flare angle, the freckle defect formed at one side of specimens under the withdrawal rate of 1 mm/min. With the increasing of flare angle, the freckle defect cannot be found at the same position of the specimens. The formation mechanisms of freckle defect in the triangular adapters of GCs were further discussed. [ABSTRACT FROM AUTHOR]