Morphology at the reactive wetting front during spreading was first observed in situ at an atomic scale using a high resolution transmission electron microscope. Two typical nanomorphologies of an Ag-Cu-Ti molten alloy were observed on a SiC substrate depending on the nucleation position of the reaction product, TiC. In the case of the precursor atomic thin layer spreading ahead of the droplet of the molten alloy. the TiC nucleated in the thick region of the precursor, which was a few nm behind the tip of the precursor layer. In contrast. molten alloy spreading with TiC growing simultaneously at the tip was also observed. In this case. the precursor layer on SiC did not exist. Nucleation and growth rate of the reaction product were related to the tip morphology.
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