Fig. 2. Detail from the aluminum- silicon phase diagram is shown. The composition and temperature of both liquid and solid phases follow the arrows.
图2：展示了Al—Si相图的细 节。液相和固相的曲线和温度都 如箭头所示。
conducted at Alcoa. Foundry alloys are grouped into
three classes based upon silicon content. Hypoeutectic alloys—Tese
alloys have a silicon content less than the eutectic composition. Most of the common alloys have between 5% and 10%. Tese alloys are designed primarily for high strength applications where good ductility is also required. Eutectic alloys—Tese alloys
have between 10% and 13% silicon, and consist mainly of Al-Si eutectic in the cast structure. Tey have a narrow freezing range, excellent fluidity and are easy to cast. Tey also have good wear resistance and are quite ductile when not alloyed and heat treated to high strength. Hypereutectic alloys—Tese alloys have between 15%
and 20% silicon, so their cast structure is composed of primary silicon particles imbedded in a matrix of Al-Si eutectic. Tese materials have remarkable wear resistance and are used where this characteristic is desired. Tey also have good high tem- perature strength, but are difficult to machine. A more detailed look at the Al-Si phase diagram provides a better understanding of what these characteristics mean in practice. Te most important portion of the Al-Si phase diagram for the metalcaster is shown in Fig. 2. Consideration is given to the solidification of a typical
hypoeutectic alloy, containing 7% silicon. Te molten metal alloy is taken from a furnace held at 1,400F (760C). Tis metal cools in the mold to a temperature of about 1,139F (615C). At this temperature the first solid forms in the shape of aluminum crystals containing 1% silicon. As solidification continues, the silicon concentration in the
liquid portion of the casting increases. Silicon segregates to and accumulates in the liquid phase. Tis segregation during solidifi- cation is best described by a distribution coefficient: Te phase diagram tells us that, at equilibrium, the silicon
content in solid aluminum is 13% of that found in the sur- rounding liquid. Te other 87% remains in the liquid, where it accumulates. And as the silicon content increases in the liquid, its melting point decreases. Hence, the composition and temperature of both solid and liquid phases follow the arrows in Fig. 2. Tis segregation continues until the liquid contains 12.6% Si and cools to the eutectic temperature. At this point, a eutectic mixture of solid Al and Si forms. Another important factor that can be determined from
the phase diagram is the depression of the melting point of aluminum. Tis is defined by the slope of the liquidus curve and by this equation for the Al-Si system:
司）的研究所得。
铸造用的合金根据硅含量划 分为以下三类：
亚共晶合金——这些合金的 硅含量低于共晶点的含量。大 部分在5%—10%。此类合金 设计主要用于强度高、韧性好 的使用需求。
共晶合金——这些合金的硅 含量在10%-13%，铸态组织主
要是Al-Si共晶体。其凝固区间窄，流动性好、铸造性 能佳。具有良好的耐磨性以及韧性，但是不能通过合 金化和热处理来强化。
过共晶合金——这些合金的含硅量在15%—20%， 所以其铸态组织是嵌入在Al-Si共晶体基体中的初生硅 颗粒形成的组织。此类材料耐磨性能优异并用于需要 高耐磨性的场合。其也具有良好的高温强度，但是机 加工性较差。
详细解读Al-Si相图，可对相图特性在实际中的意义 有更好的理解。对于铸造者而言，Al-Si相图最重要的 部分见图2。 请看含硅7%的亚共晶铝硅合金的凝固过程。熔融金 属合金从保温760℃的炉内倒入铸型，在金属型中冷 却到约615℃。在此温度时初始固态组织形成含1%Si 的Al晶体。
当凝固过程持续进行，铸件液态组分中的硅浓度增 大。硅产生偏析并聚集在液相。凝固过程中的偏析可 以用分配系数很好地描述。
相图告诉我们，在平衡态时，此时固态铝内的硅含 量周围是液态中的13%。剩余的87%的硅残留在液态 并聚集在液态内。并且随着液态内的硅含量增加，其 熔点降低。从此，液固两相的成分和温度沿图2的箭头 变化。偏析持续进行，直到液相硅含量达到12.6%，
同时温度冷却到共晶温度。在这点获得共晶组织。 从相图得到的另一个重要参数是铝熔点的降低幅
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FOUNDRY-PLANET.COM | MODERN CASTING | CHINA FOUNDRY ASSOCIATION June 2014
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