Abstract
Freckles are detrimental solidification defects in Ni-based single-crystal superalloy castings that are formed during directional solidification. However, the evolution of freckles into sliver defects has rarely been reported, and the mechanism of this evolution is unclear. In this study, the evolution of freckles into slivers in the extended zone of casting was established to be the deformation of dendrites induced by the thermal contraction stress in the freckle channel. The morphology of the mushy zone was simulated using ProCAST, which indicated that freckles were formed at the highest position of the mushy zone. During the initiation of the slivers, deformation occurred at the freckle channel, indicating that the thermal contraction stress was a result of the solidification shrinkage of solute-rich liquid. The misorientation from the initial freckle grains to the slivers ranged from 15° to 57°. In addition, the orientation and ability of dendrite branching are crucial for the extension of the sliver dendrites.
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