to prevent die soldering.
• The elements below manganese have a value of k greater than one. This means there is a “negative” segrega- tion—the equilibrium concentration in the solid is greater than that in the liquid. As a result, the melting point of aluminum increases. When another element is added to a
binary alloy, there is a ternary (three ele- ment) system. It is somewhat more com- plicated to read ternary phase diagrams, but it is often useful to consult them. Figure 3 shows the liquidus surface
for aluminum-rich alloys in the ter- nary Al-Zn-Mg system. Tis diagram is similar to a topographic map used for hiking or hunting outdoors. Te contours show the temperature (C) at which solid aluminum begins to form during solidification. A full ternary diagram is an equi-
lateral triangle, but since the interest here is in aluminum-rich alloys, the top portion of the triangle (corresponding to magnesium-rich compositions) has been
removed. Te key to ternary diagrams is reading the composition coordinates. Te diagram shows two ternary eutectics which will be used for instruction in the correct procedure. Ternary eutectics are similar to the
binary eutectics. However, the addi- tional component adds another degree of freedom according to the phase rule. Tus, a ternary eutectic occurs only with this reaction:
Liquid→Solid1 +Solid2 +Solid3
Te formation of three solid phases in the eutectic means the reaction occurs at a fixed temperature and composition. At the top left of the phase
diagram there is a ternary eutectic at 837F (447C). If a line is drawn from this point parallel to the sloping left edge, this line intersects the scale at the bottom at about 13% zinc. If a horizontal line is drawn parallel to the bottom edge, it intersects the left edge
at about 31% magnesium. Tus, this ternary eutectic contains 13% zinc, 31% magnesium and (by difference) 56% aluminum. There is a second ternary eutec- tic in the lower right-hand side of the diagram at a temperature of 887F (475C). A similar proce- dure shows this eutectic contains approximately 61% zinc, 13% magnesium and 26% aluminum. Phase diagrams are useful to show
what phases form during solidifica- tion and the relationship between the phases. Understanding what happens with multiple-element alloy systems will help the metalcaster derive practical conclusions about selecting, feeding, pouring and heat treating commercial castings.
The next article in this series on solidif ication in aluminum castings will focus on dendritic solidif ication. The paper this article is based on was originally presented at the 117th Metalcasting Congress.
April 2014 MODERN CASTING | 37
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