An analysis of the microstructure of rapidly solidified Al-8 wt pct Fe powder
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INTRODUCTION
A L U M I N U M alloys containing - 8 wt pct Fe and smaller additions of Ce, Mo, V, or Zr are currently the focus of several high temperature alloy development programs using rapid solidification technology. These programs are an outgrowth of research on A1-Fe binary alloys prepared by splat cooling or surface melting. 1-~ This research has recently been reviewed by Skinner, Okazaki, and Adam. 5 The present paper is concerned with the microstructural characterization of atomized powder of A1-8 wt pct Fe and the analysis of the solidification mechanisms which produce these structures. The phase diagram including metastable phase boundaries has been evaluated by Murray 6 and is shown in Figure 1. A prominent feature is the metastable phase A16Fe and its metastable eutectic with a-A1 at --650 ~ and - 3 . 2 wt pct Fe. The extended a-Al liquidus and solidus as well as the To curve for the partitionless solidification of a-A1 has also been estimated. One can see that for A18 wt pct Fe, the solidification of several metastable structures is thermodynamically possible depending on the transformation temperature. In fact, hypereutectic aluminum-iron alloys are well known to exhibit a wide range of microstructures depending on the solidification velocity. Alloys with compositions up to 4 wt pct Fe were studied by Adam and Hogan, 7 and alloys up to 6.1 wt pct Fe were studied by Hughes and Jones 8 using directional solidification. This work showed that for alloys above 3 wt pct Fe three distinct microstructures were observed for solidification velocities in the range between 0.1 cm/s and 1 cm/s. Near 0.1 cm/s the structure consists of primary intermetallic faceted dendrites of A13Fe with a eutectic of a-A1 and AI6Fe between the primary dendrites. Near 0.5 cm/s the structure consists of a fully eutectic structure of a-A1 and A16Fe. Near 1 cm/s the structure consists W.J. BOETTINGER and J . G . EARLY are Metallurgists with the Metallurgy Division, National Bureau of Standards, Gaithersburg, MD 20899. L. BENDERSKY is with the Center for Materials Science, The Johns Hopkins University, Baltimore, MD 21218 and is also a Guest Worker at the National Bureau of Standards. Manuscript submitted May 13, 1985. METALLURGICAL TRANSACTIONS A
of cells of a-A1 with a eutectic of a-A1 + A16Fe between the cells. The precise transition velocities depend on composition and are understood by an analysis of the asymmetric coupled zone for this alloy, s Similar microstructural variations are found in the present work on atomized powder: When a liquid alloy is broken up into a large number of fine droplets, as occurs in atomization, the most potent nucleation sites can become dispersed such that large liquid undercoolings below the equilibrium liquidus may be achieved prior to nucleation. Nucleation studies performed at slow cooling rates by the droplet-emulsion technique by Perepezko, LeBeau, Mueller, and Hildeman 9 on A1-Fe alloys show the possibility of large liquid undercoolings prior to solidification. A1-5 wt pct Fe powders wi
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