The microstructure of rapidly solidified Al 6 Mn
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I.
INTRODUCTION
THEmicrostructure of rapidly solidified binary aluminumtransition metal alloys was intensively investigated in the past. Most of the studies were performed on dilute solutions but some examined alloys of higher concentrations. ~-5 Rapidly solidified dilute solutions from supersaturated solid solutions and alloys of higher solute content form cellular microstructures. Beyond a certain concentration, characteristic of the alloy system, another phase forms. The phase is spherulitic in shape, and its size ranges from O. 1 p~m to a few/xm. Although the solidification as separate spherulites is not usual in rapid solidification, it is common in rapidlysolidified concentrated aluminum-transition metal alloys. Several examples will be given here. Binary aluminum chromium alloys, containing up to 5 wt pct Cr are characterized 2"3"4 by a supersaturated cell-free structure. At 5 wt pct Cr, a cellular microstructure that contains separate spherulites forms. The volume fraction of the spherulites increases with the chromium concentration of the alloy and reaches a homogeneous high density in the 15 wt pct Cr alloy. 4 The aluminum-iron system also contains spherulites in a cellular aluminum matrix when the iron concentration exceeds 10 wt pct. 5 The microstructure of rapidly-solidified binary aluminummanganese alloy ribbons containing up to 15 wt pct Mn has been studied. ~The ribbons were found to be supersaturated and cell-free up to manganese concentrations of 5 wt pct. Alloys of 9 to 15 wt pct concentration are cellular. The cell boundaries in these alloys contain fine precipitates but the cell interiors are mostly free from precipitation. In this paper we report on a spherulite phase found in rapidly solidified aluminum manganese alloys with composition exceeding 15 wt pct Mn. II.
EXPERIMENTAL
Alloy buttons with Mn concentrations of 18, 22, and 25.3 wt pct Mn were prepared by arc melting, using 99.999 pct Mn. Small pieces cut from these buttons were induction heated in zirconia-coated quartz tubes, as inputs for melt spinning. Immediately upon melting, the metal was
D. SHECHTMAN is with the Department of Materials Engineering, Technion, Haifa, Israel. I.A. BLECH is with Zoran Corporation, 710 Lakeway Drive, Sunnyvale, CA 94086. Manuscript submitted October 2, 1984. METALLURGICALTRANSACTIONS A
Table I. Heat Treatment Times and Temperatures of Rapidly Solidified AI-25.3 Wt Pct Mn
T [~ t [hi
300
350
350
400
400
2.5
1
6
1
6
squirted onto a copper melt-spinning wheel, 10 cm in diameter, which rotated at 6800 rpm. The process was carried out under helium at 0.9 atmospheric pressure. The material obtained had the shape of small flakes, about 40/~m thick and 3 mm in diameter. For annealing, specimens were sealed in borosilicate glass ampules with 0.5 atmosphere of helium, and heat treatments were carried out at temperatures and durations given in Table I. Microstructural studies were performed in a 120 kV scanning transmission electron microscope. The specimens were thinned by jet electropolishing i
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