Numerical analysis of the rapid solidification of gas-atomized Al-8 Wt Pct Fe droplets
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I.
INTRODUCTION
PHYSICAL properties of materials are strongly dependent on their microstructures, and there have been many efforts to improve them. Rapid solidification is a typical method of developing new materials by controlling the microstructures. With this technology, the very fine solidification microstructures can be obtained with desirable phases through large undercooling and rapid heat removal. Rapidly solidified AI-8 wt pct Fe alloy and some ternary or quaternary alloys based on this have attracted attention as very promising high-temperature alloys, and the last decade has seen much experimental and theoretical research on microstructural evolution in the melt-spun ribbons and atomized powders of AI-8 wt pct Fe alloy, t~-81 The most pronounced characteristic feature in the microstructures of the rapidly solidified AI-8 wt pct Fe ribbons or powder is that there are two distinct regions: zones A and B. tlj Zone A, showing a slight response to the chemical etching, has been identified as a microcellular structure of ot-Al by transmission electron microscopy and believed to be formed during the recalescence stage with a large initial undercooling, tSJ Zone B, highly sensitive to chemical etching, has the microstructure formed after recalescence, which is mainly cellular a-A1 if the dimensions of ribbon or powder are small and a primary AI3Fe or a eutectic of a-Al and A16Fe, otherwise, tSj Besides these main phases, various kinds of intermetallic compounds have been found in the intercellular region, tg,~~ l] Thus, in a rapidly SEONG-GYOON KIM, Associate Professor, is with the Rapidly Solidified Materials Research Center in CNU and the Department of Materials Science and Engineering, Kunsan National University, Kunsan, 573-360 Korea. SUNG-HO SHIN, Senior Researcher, is with the Central Research Institute, Sammi Steel Corp. Changwon, 641-370 Korea. TOSHIO SUZUKI, Associate Professor, and TAKATERU UMEDA, Professor, are with the Department of Metallurgy, University of Tokyo, Tokyo, 113 Japan. Manuscript submitted October 28, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS A
solidified AI-8 wt pct Fe alloy, various types of microstructures and phases can appear, depending on the specimen's dimensions and thermal conditions of the processes, and these play a critical role in the mechanical properties. 12] The prediction of the microstructural evolution in this alloy, especially the relative volume fractions of zones A and B, has thus been the subject of s e v e r a l works. [5,6,11,121 Boettinger e t al. tS~ proposed a simplified onedimensional (l-D) model for the atomized droplet solidification using the Newtonian heat transfer and the dendritic growth model proposed by Lipton e t al. 113~ They discussed the phase selection conditions and the origin of two-zone microstructure. Chu and Grangerc6j calculated the volume fraction of zone A in the meltspun ribbon by a 1-D Newtonian model with the dendritic growth model of Lipton e t al. t~4J Recently, Gremaud e t al. tl~j and Carrard e t al. t~21 performed extensi
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