Microstructures of rapidly solidified aluminum alloy submicron powders
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INTRODUCTION
R A P I D solidification processing (RSP) commonly results in progressive departures from the microstructures produced by conventional casting, generally in the direction of increased refinement and chemical homogeneity. These modifications are determined by the transport processes occurring at the liquid/solid interface, and hence, are critically dependent on the growth rate ( i . e . , local undercooling). It is now acknowledged that RSP effects may be achieved even under moderate or slow cooling conditions if the liquid is substantially undercooled prior to nucleation. 1'2 However, the benefits of this approach may be optimized only through a fundamental understanding of the solidification mechanisms and much needed characterization of the undercooled microstructures. Atomization is unique among the rapid solidification processes because the interactions with molds, substrates, or other particles are practically absent. Conceivably, then, most of the liquid droplets could achieve high undercoolings prior to nucleation, provided the purity level is adequate, because the active nucleation catalysts will be isolated into a small fraction of the total volume. This concept was proven experimentally by Turnbull et al 3'4'5 by slowly cooling emulsions of metal droplets. More recently, Perepezko et al 2'6 developed a refined version of Turnbull's technique and measured maximum undercoolings ranging between 0.26 TM for lead to 0.50 TM for gallium. In general, it was found that the achievable undercooling was a function of the surface coating (emulsifer) and the droplet size distribution. Furthermore, experiments in the Sn-Bi 6 and Cu-Ni 5 systems revealed that undercoolings comparable to those of the pure metals can be obtained in these alloys across the entire phase diagrams. C. G.LEVI, formerly a Research Associate at the University of Illinois, is now with the Metallurgical Process Engineering Department at ATISAATKINS, S.A. de C. V. in Mexico City, Mexico. R. MEHRABIAN is Chief of the Metallurgy Division, National Bureau of Standards, Washington, DC 20234. Manuscript submitted April 1, 1981. METALLURGICAL TRANSACTIONS A
Recent developments in vacuum atomization via electrohydrodynamics (EHD) have provided a tool for the controlled generation of powders below 1 /xm in diameter] These powders, while not of commercial importance, are especially suited for fundamental studies for two reasons. First, they are likely to undercool substantially due to their size and the minimization of surface films. Second, because they are electron-transparent - especially those of light metals like aluminum - it is possible to analyze the evolution of a complete microstructure from nucleation until the end of solidification without recourse to thinning procedures. This paper deals with the characterization of some novel microstructures typical of aluminum alloy submicron powders produced by the EHD process. The observations are coupled with thermodynamic, kinetic, and heat flow concepts to elucidate the thermal history
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