Using the lead-tin phase diagram in Figure  6.3, determine the liquid and solid phase compositions for a nominal  composition of 90% Sn and 10% Pb at 204°C (400°F).  Part of the solution to this question is  provided in the e-textbook.  However, you need to (1) make marks and  label on the phase diagram to show how you obtain the readings, and (2)  write down your answer.

THE TIN–LEAD ALLOY SYSTEM

A more complicated phase diagram is the Sn–Pb system, shown in Figure 6.3. Tin–lead alloys have traditionally been used as solders for making electrical and mechanical connections (Section 30.2).1 The phase diagram exhibits several features not included in the previous Cu–Ni system. One feature is the presence of two solid phases, alpha (α) and beta (β). The α phase is a solid solution of tin in lead at the left side of the diagram, and the β phase is a solid solution of lead in tin that occurs only at elevated temperatures around 200°C (375°F) at the right side of the diagram. Between these solid solutions lies a mixture of the two solid phases, α + β.

Figure 6.3 Phase diagram for the tin–lead alloy system.

Another feature of interest in the tin–lead system is how melting differs for different compositions. Pure tin melts at 232°C (449°F), and pure lead melts at 327°C (621°F). Alloys of these elements melt at lower temperatures. The diagram shows two liquidus lines that begin at the melting points of the pure metals and meet at a composition of 61.9% Sn. This is the eutectic composition for the tin–lead system. In general, a eutectic alloy is a particular composition in an alloy system for which the solidus and liquidus are at the same temperature. The corresponding eutectic temperature, the melting point of the eutectic composition, is 183°C (362°F) in the present case. The eutectic temperature is always the lowest melting point for an alloy system (eutectic is derived from the Greek word eutektos, meaning “easily melted”).

Methods for determining the chemical analysis of the phases and the proportions of phases present can be readily applied to the Sn–Pb system just as in the Cu–Ni system. In fact, these methods are applicable in any region containing two phases, including two solid phases. Most alloy systems are characterized by the existence of multiple solid phases and eutectic compositions, and so the phase diagrams of these systems are often similar to the tin–lead diagram. Of course, many alloy systems are considerably more complex, for example, the iron–carbon system, considered next.