Fig. 3. This diagram shows the liquidus surface for alumi- num-rich alloys in the ternary Al-Zn-Mg system. (Tempera- tures are given in C.)
图3：这个图表显示 了在Al-Zn-Mg三元 合金系统中关于富Al 合金的液相线的表面 （温度：℃）
the interest here is in aluminum-rich alloys, the top portion of the triangle (corresponding to magnesium-rich composi- tions) 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 additional 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. Tere is a second ternary eutectic in the lower right-hand side of the diagram at a temperature of 887F (475C). A similar procedure shows this eutectic contains approximately 61% zinc, 13% magnesium and 26% aluminum. Phase diagrams are useful to show what phases form during
solidification and the relationship between the phases. Under- standing what happens with multiple-element alloy systems will help the metalcaster derive practical conclusions about selecting, feeding, pouring and heat treating commercial castings.
Dendrites’ Clues for Castability
A book on snowflakes at the Carnegie Library in Pitts- burgh depicts many photographs of individual snowflakes. In the introduction, the author claims each snowflake is unique, and no two crystals are alike. Tis claim may be true, in spite of the incred- ibly large number of snowflakes that form each winter. Te variety
Fig. 4. These are dendrites found in Al-20% Cu liquid. (Pictures were taken (a) 110 seconds, (b) 139 seconds and (c) 360 seconds after the first grains appeared.)
晶图。
三元共晶与二元共晶类似。但是，根据相律，加入了 新组分增加了另一个自由度数。三元共晶出现仅仅伴随 这个反应：
液相→固相1+固相2+固相3
共晶点时三个固相的形成意味着反应在固定温度和固 定成分下发生。
三元相图的顶端靠左部分有三元共晶点（447℃）。 把共晶点拉出一条线平行于左边倾斜边的直线，这条线 按照比例相交到底端是13%的Zn。如果画一条与底边平 行的水平线，其与左边相交在31%的Mg。这样，三元 共晶组织的成分是13%Zn、31%Mg、56%Al。 在图中下部右手边的温度值475℃的点是第二个三 元共晶点。类似的过程可以得到三元共晶的成分是 61%Zn、13%Mg、26%Al。
相图对于阐明凝固过程中哪些相形成以及它们直接的 关系很有帮助。理解关于多元合金系统将发生什么，将 有助于铸造工作者获得有关选择、充填、浇注以及热处 理商业铸件的实践结论。
树枝晶为铸造性能研究提供线索
在匹兹堡的卡内基图书馆关于雪花的一本书描绘了许 多单个雪花的照片。在引言中，作者认为每个雪花都是 独一无二的，没有两个一样的冰晶。尽管每个冬天会有 无数的雪花形成，但这种说法可 能是正确的。书中展示的多样化 的雪花令人惊奇。
在铸型内每次的金属凝固过程 与上述相似。液固相的转变过程
图4：在Al-20% Cu液相中发现的树 枝晶（分别于第一个晶粒形核后的 110秒、139秒、360秒拍照）
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FOUNDRY-PLANET.COM | MODERN CASTING | CHINA FOUNDRY ASSOCIATION June 2014
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