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Fig. 1. Shown is the phase diagram for the aluminum-silicon system.


图1:Al—Si系相图


as the pressure is fixed, the melt- ing point is fixed. According to the phase rule, when a second element is dis- solved in aluminum, we have an additional degree of freedom. In this case, the melting point can change. Tose who live, or have lived, in cold climates are familiar with the practice of adding salt to icy sidewalks and driveways to melt ice in the winter. Salt dis- solves in water, lowering its melt- ing point. Tis makes it easier to remove the ice, as long as the temperature is not far below the freezing point of water. Te same thing happens in aluminum. Adding a second element to pure aluminum usu- ally lowers the melting point, as illustrated in the aluminum- silicon system. Silicon lowers the melting point of aluminum, but aluminum also lowers the melting point of silicon. Te two curves for the melting of aluminum and silicon meet at a eutectic at a composition of 12.6 weight percent silicon and a temperature of 1,071F (577C) (Fig. 1). At the eutectic composition and temperature, the


solidification phase transformation occurs when the liquid aluminum-silicon alloy transforms into solid aluminum and solid silicon. Tis transformation occurs at a single, constant temperature, as anticipated by the phase rule: f = n + 2 - p = 2 + 2 - 3 = 1. At constant pressure, three phases can coexist in a binary


(two element) system only at a single temperature and at a sin- gle composition, so the eutectic temperature is fixed (constant). An appreciable amount of silicon dissolves in solid alumi- num at higher temperature. Te maximum solubility is seen to be 1.65 weight percent at the eutectic temperature. However, only a negligible amount of aluminum dissolves in silicon. Liquid aluminum and liquid silicon are completely soluble


in one another and form a single phase field represented by the “L” in Fig. 1. Using the standard terminology for this behavior, the two liquids are said to be miscible (mixable). At temperatures below the melting point of the pure met- als, but above the eutectic temperature, two phase fields of solid are in contact with liquid. Tese are labeled “L+S.” On the left-hand side, solid aluminum is in contact with liquid. On the right, solid silicon is in contact with aluminum. At temperatures below the eutectic temperature, there is another two-phase field containing two solids: aluminum and silicon. Phase diagrams for the Al-Si system proposed in litera-


ture over the years disagree about the exact eutectic composi- tion and temperature. Tis is because the formation of the Al-Si eutectic is sensitive to small amounts of impurities, especially potassium, sodium and other alkaline earth ele- ments. Te phase diagram in this article is based on a study


硅的百分含量


一自由度意味着只要压力固 定,熔点就是定值。 根据相律,当有第二种 元素溶解到铝中时,自由度 数增加。既然这样,熔点可 能变化。那些在寒冷气候下 居住着或者居住过的人们对 于冬天在人行道或者马路上 加盐以融化冰的做法都很熟 悉。盐在水中溶解,降低其 熔点。这样使除冰变得更容 易,只要温度不是远低于水 的凝固点温度。


在铝中,同样的情形发生了。在纯铝中加入第二 种元素通常会降低熔点,如图Al-Si体系所示。Si降 低了Al的熔点,但是Al也会降低Si的熔点。Al和Si的 熔化曲线在含Si质量分数12.6%、温度577℃(见图 1)时的共晶点相交。


在共晶成分和温度下,当液态Al-Si合金转变为 固定的Al及固态Si,凝固的相变发生。相变是在单 一、恒温下进行,按照之前的相律得知: 自由度数为1


在恒定压力下,仅在单一温度和单一组分时, 三相共存在二元系内。因此,共晶温度恒定(常 数)。


高温下固态铝可以溶解大量的硅。最大溶解度出现 在共晶温度,以质量比表示为1.65。然而,只有极少 量的铝溶解在硅中。


液态铝和液态硅完全互相溶解形成如图1所示单


一“L”相。用专业术语描述这种行为即两种液态完 全互溶。在温度低于纯金属的熔点、但高于共晶温度 时,两个固相区与液相连接。两者都标识为“L+S” 相。在相图左边,固相铝与液相接触。在相图右边, 固相硅与液相接触。当温度低于共晶点温度时,出现 了另一个包含铝和硅两种固态的两相区。 一直以来,文献对Al-Si 体系相图中的精确的共晶 成分和温度意见不一。这是因为Al-Si 共晶组织的形 成对微量杂质元素,尤其是钾、钠和其他碱土金属元 素非常敏感。本文的相图是基于Alcoa(美国铝业公


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