Fig. 2. Detail of the Al-Cu phase diagram is shown.
Al-Cu二元合金相图细节（应 该是富铝角的附近部分）
Figure 1 shows the com- plete Al-Cu phase diagram. A number of intermetallic compounds form in this system. Aluminum casting facilities are concerned with the aluminum-rich por- tion of the figure and the formation of the ϴ (Al2Cu) phase. Te relevant portion of the figure is shown in Fig. 2.
If an alloy contain- ing 4.5% copper is poured and held near the eutectic temperature, the casting will be in the single phase field corresponding to solid aluminum. Tis is indicated by the red box in Fig. 2. According to the phase diagram, this alloy should be a single phase—aluminum with copper in solid solution. Looking at a sample from a casting of this alloy, it is
found to contain a significant amount of eutectic Al2Cu phase. Tis eutectic should not be present, according to the phase diagram. So, it is called a non-equilibrium eutectic. Also, if the distribution of copper in a sample of the
casting is studied by microprobe analysis in a SEM, one finds that the copper content in the aluminum phase varies. In the center of dendrite arms, which corresponds to the first solid, the copper content is low. Moving toward the outside of the arms, which corresponds to metal freezing later, the copper content increases. An example of this type of mea- surement is shown in Fig. 3. Solidification scientists have studied this phenomenon for many years and have offered
1，即铝铜二元相图）注富 铝部分和θ相（Al2Cu）的 形成。上图中相关部分如 图2所示。
如果含铜4.5％的合金 浇注后在共晶温度附近保 温，铸件将处在单相区得 到相对应的铝固溶体。如 图2的红色方块所示。根据 相图，该合金应为单一相 的铜溶于铝固溶体。 观察该合金铸件的样 品时，发现含有大量的 Al2Cu共晶相。根据相 图，此共晶不应该存在。
因此，它被称为非平衡共晶。
此外，如果用扫描电子显微镜的显微探针分析对铜在 铸件样品中的分布进行研究，发现铜的含量在铝相会变 化。在枝晶臂中心，相对应于凝固初期，铜的含量低。 沿着晶臂的向外移动，对应于金属凝固后期，铜含量增 加。某一显微探针分析的实例如图 3所示。 凝固科学家多年研究这种现象，已经建立数学模型 来解释微观偏析。人们现在可以根据布罗迪-弗莱明斯 提出的模型（B-F）计算非平衡共晶的量。B-F方程可以 用于计算凝固过程中液相的 成分，和形成的共晶固相分 数。B-F方程非常适用于除极
Fig. 3. This diagram shows the distribution of copper in Al-4-5% Cu alloy. (Local solidification time is 750 seconds.)
Al-4.5%Cu合金的铜分布图（局 部凝固实践为750s）
September 2014
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