Microstructures and Thermal Properties of Rapidly Solidified Aluminum-Rare Earth Alloys
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MICROSTRUCTURES AND THERMAL PROPERTIES OF RAPIDLY SOLIDIFIED ALUMINUM-RARE EARTH ALLOYS
D. ELIEZER*, S. J. SAVAGE**, Y. R. MAHAJAN, and F. H. FROES Air Force Wright Aeronautical Laboratories, Materials Laboratory, AFWAL/MLLS, Wright-Patterson Air Force Base, OH 45433 *Sabbatical leave from the Ben-Gurion University of the Negev, Beer Sheva, Israel **Now with the Swedish Institute for Metals Research, S-114 28 Stockholm, Sweden
ABSTRACT Rapidly solidified (RS) Al-Er, Al-Nd, and Al-Gd ribbons were prepared by pendant drop melt extraction (PDME). The microstructures were studied in detail by optical and transmission electron microscopy. A wide range of microstructures are observed, including "conventional" RS type zones, a cellular structure surrounded by a degenerate eutectic and coupled (lamellar) eutectic structures. A bimodal precipitate distribution is seen in many areas of the ribbons. A series of isochronal heat treatments was conducted to determine the relative thermal stability of these microstructures, which were evaluated by Knoop microhardness measurements.
INTRODUCTION Aluminum alloys containing additions of dispersoid forming elements can show attractive mechanical properties at elevated temperatures if the dispersed phase is stable and is present as a fine, homogeneous distribution in the matrix [1]. Examination of the equilibrium phase diagrams of Al-lanthanide alloy systems [2] indicates that such a dispersion may be produced by rapid solidification followed by an appropriate heat treatment. For example, all the phase diagrams show a high liquid solubility (10-30 wt%) and a very low solid solubility (:0.05 wt%) of the rare earth in aluminum. The rare earths also have low duffusivity and form stable, high melting intermetallic compounds. In this paper some preliminary results of microstructural characterization and thermal stability of Al-10.3%Gd, Al-10.8%Er, Al-9.8%Ce, and Al-1O.7%Nd alloys are presented; the work is part of a continuing program at the authors laboratory to characterize the Al-lanthanide alloys.
EXPERIMENTAL Rapidly solidified ribbons of this alloy were prepared by the pendant drop melt extraction (PDME) method [3, 4]. Ribbons produced were typically 80 pm thick and 2 mm wide. Samples were characterized by optical and transmission electron microscopy (TEM) in the as-solidified state and after heat treatment for 1 hr at temperatures between 100'C and 500'C. TEM foils were prepared by electropolishing from the top surface of the ribbon in order to study the chill zone at the wheel side. The microhardness data were obtained at room temperature using a Knoop indenter with a 5 gm load and 15 second dwell time. Measurements were restricted to the chill zone. As a standard, the microhardness of a sample of as-cast 99.999% Al was measured.
Mat. Res. Soc. Symp. Proc. Vol. 58.
1986 Materials Research Society
294
RESULTS AND DISCUSSION Figure la shows a representative longitudinal section optical micrograph of the as-cast A1-10.8Er PDME ribbon. Three zones are evident. Beginning with th
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