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Determining the effect of casting geometric structure on freckle formation is important in designing the directional solidification parameter of single‐crystal superalloy castings. In this research, single‐crystal castings of a third‐generation nickel (Ni)‐based superalloy with different thicknesses and structures are prepared by conventional Bridgman high‐rate‐solidification technique at withdrawal rates of 1, 3, and 6 mm min–1. The experimental results show that the freckles in castings with expansion structures are severer than those with contraction structures at a withdrawal rate of 1 mm min–1, and vice versa at withdrawal rates of 3 and 6 mm min–1. It is found that the larger casting thickness tends to promote the freckle formation at the same withdrawal rate. Furthermore, a lower tendency is displayed at the withdrawal rate of 3 mm min–1 in freckle formation, whereas an increased tendency is displayed at the withdrawal rate of 6 mm min–1. The different withdrawal rates, thicknesses, and structures of casting cause differences in cooling rate, shrinkage flow, and mushy zone morphology, which are believed to be responsible for the tendency of freckle formation. [ABSTRACT FROM AUTHOR]