The liquidus surface for the Al- Li- Si system from 0.20 wt Pct Li and 0.20 Wt pct Si
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nation: surface preparation was for the most part, standard, but for alloys containing more than ~4 wt pct Li it was necessary to polish with an oil lubricant and to spray the freshly prepared surfaces with clear varnish to prevent atmospheric corrosion. Temary alloy compositions were based on weighed quantities of the binary alloys which gave thermal arrests in close agreement with the published binary phase diagrams. ~.3.4In the A1-Li system there occurs the eutectic reaction, liquid (10 wt pct Li)~A1 (4 wt pct Li) + AILi (20.8 wt pct Li), at 600 ~ while in the A1-Si system there is the reaction, liquid (12.6 wt pct Si)~--A1 (1.65 wt pct Si) + Si at 578 ~ In the binary A1-Li system, hypereutectic alloys contain primary, nonfaceted dendrites of the compound A1Li and the eutectic structure is typically nonfaceted, while in the A1-Si system the eutectic has the well-known faceted/nonfaceted structure (e.g., 5). The liquidus surface of Figure 1 is based on thermal analyses and microstructures of some 40 alloys. The binary A1-A1Li eutectic is depressed by silicon to a ternary eutectic point, E.I, at 592 ~ (10.6 wt pct Li, 1.1 wt pct Si) ~ A1 (~4 wt pet Li) + A1Li + a faceted phase which appears to be a temary compound approximating to A1LiSi, see below; this is the light grey phase in Figures 2(b) and 3(b). The binary A1-Si eutectic is also depressed, by lithium, to a ternary eutectic, E. 2, at 575 ~ (1.4 wt pct Li, 13.8 wt pct Si)~-A1 (1.65 wt pct Si) + A1LiSi + Si; this is the fine structure of Figure 3(b). Figures 2(a) and 3(a) show the cooling curves for two examples of ternary alloys with primary, 1, secondary, 2, and tertiary, 3, arrests while the corresponding microstructures are shown in Figures 2(b) and 3(b). For the
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Fig. 1--The liquidus surface. M.D. HANNA, formerly Postdoctoral Research Associate, Michigan Technological University, is now with General Motors Research Center, Warren, MI. A. HELLAWELL, Professor, is with the Department of Metallurgical Engineering, Michigan Technological University, Houghton, MI 49931. Manuscript submitted August 5, 1983. METALLURGICALTRANSACTIONS A
first case, the first arrest corresponds to the A1 liquidus (refer to Figure 1), the second to the binary eutectic valley, liquid~,~A1 + A1LiSi, and the third to the ternary eutectic, E. 1. For the second example (again refer to Figure 1), the first arrest is for the steeply sloping A1LiSi liquidus, the second for the binary monovariant valley, liquid~,-~A1 + VOLUME 15A, MARCH 1984--595
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Fig. 2--Cooling curve (a) and microstmcture (b) for Al-6. l wt pet Li, 1.4 wt pct Si, showing first large arrest, l, corresponding to primary aluminum, second small arrest, 2, for binary eutcctir Al (white) + LiAISi (light gray), and arrest, 3, for ternary eutectir Al + LiAISi + LiAl (dark). Unetched,
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