The occurrence of aligned microstructures in directionally solidified aluminum-bismuth alloys
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(a)
(b) Fig. l n P u r e binary A1-3.4 wt pct Bi, solidified at 2.5 ~ m s-~ with a mean temperature gradient ~15 K mm -~, (a) transverse section showing Bi fibers in Al matrix with a matrix grain boundary delineated by larger Bi fibers, optical micrograph, unetched, and (b) longitudinal section after deep etching of AI matrix, SEM.
ISSN 0360-2133/82/0311-0493500.75/0 AMERICAN SOCIETY FOR METALS AND THE METALLURGICALSOCIETY OF AIME
VOLUME 13A, MARCH 1982 - - 493
Fig. 3 - - B i and Fe analyses across the quenched front of Figure 2 from Edax counts taken from 2 m m line scans parallel to growth front.
Fig. 2 - - A I - 3 . 4 wt pct Bi 0.2 wt pct Fe, longitudinal section through quenched front with growth conditions as for Figure 1, optical micrograph, unetched.
Both the iron and bismuth concentrations were followed across the quench from X-ray counts in a S. E.M. The method used was to take counts from successive line scans of 2 mm lengths, each taken parallel to the plane of the growth front, 4 giving mean concentrations at intervals in the slowly grown section and in the quenched liquid. The results are plotted in Figure 3, in which each data point corresponds to a 2 mm line scan at right angles to the direction of growth. The iron content in the slowly grown material and in the bulk chilled liquid was consistent with the synthetic composition of 0.2 wt pct Fe. Within - 2 mm of the quenched front iron content was higher and fell from - 1 . 5 wt pct Fe immediately ahead of the quench in what would have probably been of exponential form, except that the inhomogeneous cored structure of the chilled liquid gave uneven analyses from place to place. The iron was present interdendritically in the chilled liquid structure as very fine eutectic, presumably as AI6Fe, 5 and as occasional pockets of A1-AI3 Fe eutectic in the slowly grown section (Figure 2). Bismuth was present as fine globules in the chilled liquid, and, given the inhomogeneity of the chilled liquid, the concentration was close to that of the monotectic composition of 3.4 wt pct Bi, but in the as-grown section it appeared to be very low because the bismuth occurred there as a few widely spaced rods or globules which only occasionally coincided with the longitudinal section. Taking the constitutional supercooling expression 6 for steady state conditions, with data for the A1-Fe system 7 and a prevailing average gradient, G, of "~ 15 K mm -~, and assuming a liquid diffusion coefficient, D , of 5 • 10-1 m 2 s-1, one would expect a binary A1-0.2 wt pct Fe alloy to undergo a planarcellular transitional at a growth rate of - 4 / x m s-t, and this 494 - - VOLUME 13A, MARCH 1982
is consistent with our observation within the mfirgin of uncertainty about D and G. The alloy therefore solidifies as would a binary A1-Fe alloy, and the immiscible liquid bismuth does not affect the microstructure except in as much as it collects at cell boundaries or interdendritically. With regard to the phase orientations, the aluminum matrix was examined by X-ray back reflection
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