MICROSTRUCTURE SELECTION DURING RAPID SOLIDIFICATION OF AL-SI POWDER

摘要

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, while also presenting an opportunity to study the fundamental processes of nucleation and growth. Aluminum-silicon was chosen as a model for simple eutectic systems displaying a faceted/unfaceted interface. Fine droplets of Si-15Al and Si-18Al (wt. %) were produced by high-pressure gas atomization, using N_2 and He gas, respectively. Quantitative measurements of eutectic spacing were made from SEM images of cross-sectioned particles. The spacings were compared to those predicted by theoretical solidification models and a maximum undercooling was estimated for the particles. A variation in the selection of nucleation sites as a function of particle size was also observed. These results are analyzed to develop understanding about microstructure selection as a function of both composition and particle size.