Characterization of Hypereutectic Al-Si Powders Solidified under Far-From Equilibrium Conditions
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Characterization of Hypereutectic Al-Si Powders Solidified under Far-From Equilibrium Conditions Y.E. KALAY, L.S. CHUMBLEY, I.E. ANDERSON, and R.E. NAPOLITANO The rapid solidification microstructure of gas-atomized Al-Si powders of 15, 18, 25, and 50 wt pct Si were examined using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). In order of increasing particle size, the powders exhibited microcellular Al, cellular/dendritic Al, eutectic Al, and primary Si growth morphologies. Interface velocity and undercooling were estimated from measured eutectic spacing based on the Trivedi–Magnin–Kurz (TMK) model, permitting a direct comparison with theoretical predictions of solidification morphology. Based on our observations, additional conditions for high-undercooling morphological transitions are proposed as an extension of coupled-zone predictions. I. INTRODUCTION
ATTRIBUTED primarily to its ability to achieve high cooling rates in a single process step for large quantities of material, gas atomization is, perhaps, the most industrially significant technique for rapid solidification, with over 50,000 tons of material produced by this method each year.[1] From a scientific standpoint, atomization methods provide experimental access to very high undercoolings in a containerless environment, presenting an opportunity to investigate the fundamentals of nucleation and growth in highly driven systems. In addition, the droplet size itself is a useful metric of the prevailing undercooling or cooling rate, and the atomization of a volume of liquid will typically produce a large range of droplet sizes, corresponding to a wide range of cooling rates. Thus, a quantity of atomized powder will exhibit a spectrum of solidification microstructures. Detracting from the scientific utility of the atomization method, however, is the chaotic nature of the process, which gives rise to considerable variation of microstructure, even for droplets of a particular size. Accordingly, employing gas atomization for the systematic study of solidification can be problematic. Numerous atomization experiments for fundamental investigation of microstructural evolution during rapid solidification have been reported.[2,3,4] A detailed investigation of the correlation between undercooling and microstructure was performed by Levi and Mehrabian,[2] who used a vacuum-electrohydrodynamic atomization process to produce submicron powders of Al-Si and Al-Cu alloys. The typical microstructure exhibited by their powders revealed that solidification occurred primarily in two stages, beginning with the planar growth of a supersaturated solid solution followed by a transition to a cellular morphology and the concomitant segregation pattern. This two-stage freezing behavior has been observed by others[5] and indicates a transition from the rapid cooling associated with the absorption of Y.E. KALAY, Graduate Student, and L.S. CHUMBLEY, I.E. ANDERSON, and R.E. NAPOLITANO, Professors, are with Metal and Ceramic Sciences, Ames Laboratory, United States Depar
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