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96ةلحرولاةيناثلاةيسدنه داوه2017-20117ةيهلاسلاا ةعهاجلافييكتلاو ديربتلا تاينقت ةسدنه نسقم .مديجولادبع دوحه ءاوضأ .Lecture No.17 Binary Phase Diagrams 17.1Binary Eutectic System In this case, the two metals are soluble in each other and form a single homogeneous liquid in all proportions when they are melted together, but only partially soluble in each other in the solid state; this is known as a binary eutectic phase diagram. As might be expected, very many alloy systems fall into this category. An example of this type of system is afforded by the leadtin (Pb-Sn) alloy series as shown in Figure 17.1. This alloy system is the basis for the most common alloys used for soldering.
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07ةلحرولاةيناثلاةيسدنه داوه2017-20117ةيهلاسلاا ةعهاجلافييكتلاو ديربتلا تاينقت ةسدنه نسقم .مديجولادبع دوحه ءاوضأ .Figure 17.1 The leadtin phase diagram.A number of features of this phase diagram are important and worth noting. First, three single-phase regions are found on the diagram: , ,and liquid, The phase is a solid solution rich in lead; it has tin as the solute component. The -phase is also a solid solution, but lead is the solute. Pure lead and pure tin are also considered to be and phases, respectively. Thus, the solubility in each of these solid phases is limited, in that at any temperature below line BEG only a limited concentration of tin will dissolve in lead (for the phase), and similarly for lead in tin (for the phase). The solubility limit for the phase corresponds to the boundary line, labeled CBA, between the /(+ ) and /(+ L) phase regions; it increases
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07ةلحرولاةيناثلاةيسدنه داوه2017-20117ةيهلاسلاا ةعهاجلافييكتلاو ديربتلا تاينقت ةسدنه نسقم .مديجولادبع دوحه ءاوضأ .with temperature to a maximum (117.3 wt% Sn at 1173C) at point B, and decreases back to zero at the melting temperature of pure lead, point A (327C). At temperatures below 1173C, the solid solubility limit line separating the and + phase regions is termed a solvus line; the boundary AB between the and + L fields is the solidus line, as indicated in Figure 17.1. For the phase, both solvus and solidus lines also exist, HG and GF, respectively, as shown. The maximum solubility of lead in the phase, point G(2.2 wt% Pb), also occurs at 1173C. This horizontal line BEG, which is parallel to the composition axis and extends between these maximum solubility positions, may also be considered a solidus line; it represents the lowest temperature at which a liquid phase may exist for any leadtin alloy that is at equilibrium.
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