Grain refined microstructure is often obtained spontaneously in the solidification of metals and semiconductors from the undercooled melt without any external forces. Although it has been reported that the grain refinement is mainly caused by the fragmentation of the dendrites, the dynamic process of the fragmentation of dendrites has not been fully understood because only the microstructure after the solidification has been analyzed. Here, we present a time-resolved two-dimensional x-ray diffraction experiment on the solidification of Si from the undercooled melt. The number of diffraction spots observed at low undercoolings (ΔT< 100 K) did not increase at the plateau stage, while the diffraction pattern at medium undercoolings (100 K<ΔT<200 K) changed from the spots with the tail to rings with the lapse of time. Both this result and high speed video imaging suggested that the high-order arms of the dendrites mostly detached from the main stems because nucleation could not be expected at the melting point after recalescence. The several spots observed at low undercoolings drastically changed to rings at high undercoolings (ΔT>200 K), which indicated the complete fragmentation of dendrite main stem as well as high-order arms. This complete fragmentation resulted in the grain refined microstructure.
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