Gas atomization techniques provide access to regions of extreme undercooling in metal alloys. The rapid solidification which occurs at such extremes often produces unique and desirable microstructures, as well as presents an opportunity to study the fundamental processes of nucleation and growth. Aluminum-silicon was chosen as a model system for studying simple eutectic systems displaying a faceted/unfaceted, coupled growth interface. Fine droplets of Al-15Si and Al-18Si (wt. %) were produced using high-pressure gas atomization. After size classification, the particles were deep-etched and the microstructure observed using both optical and scanning electron microscopy. The results correlated well with a microstructure map proposed in previous work. Contrary to initial assumptions, no critical size for solidification of solute partitionless single crystal particles was found, however particles with extremely fine coupled growth surrounding small, primary silicon crystals were observed.
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