Binary Alloy System (or) The Simple Eutectic System

BINARY ALLOY SYSTEM (Or) THE SIMPLE EUTECTIC SYSTEM

1. The Lead-Silver system

Since the system is studied at constant pressure, the vapour phase is ignored and the condensed phase rule is used.

F' = C – P + 1

The phase diagram of lead-silver system is shown in Fig. 3.5. It contains lines, areas and the eutectic point.

1. Curve AO

The curve AO is known as freezing point curve of silver. Point A is the melting point of pure Ag (961°C). The curve AO shows the melting point depression of Ag by the successive addition of Pb. Along this curve AO, solid Ag and the melt are in equilibrium.

Solid Ag ⇌ Melt

According to reduced phase rule equation.

F' = C – P + 1; F' = 2 - 2 + 1; F' = 1

The system is univariant.

2. Curve BO

The curve BO is known as freezing point curve of lead. Point B is the melting point of pure lead (327°C). The curve BO shows the melting point depression of ‘Pb' by the successive addition of ‘Ag?. Along this curve ‘BO', solid ‘Pb' and the melt are in equilibrium.

Fig. 3.5 Phase diagram of Lead - Silver system

Solid Pb ⇌ Melt

According to reduced phase rule equation.

F' = C – P + 1; F' = 2 - 2 + 1; F' = 1

The system is univariant

3. Point 'O' (Eutectic point)

The curves AO and BO meet at point “O’at a temperature of 303°C, where three phases (solid Ag, solid Pb and their liquid melt) are in equilibrium.

Solid Pb + Solid Ag ⇌ Melt

According to reduced phase rule equation.

F' = C – P + 1; F' = 2 – 3 + 1; F' = 0

The system is non-variant.

The point 'O' is called eutectic point or eutectic temperature and its corresponding composition, 97.4%Pb + 2.6%Ag, is called eutectic composition. Below this point the eutectic compound and the metal solidify.

4. Areas

The area above the line AOB has a single phase (molten Pb + Ag). According to reduced phase rule equation.

F' = C – P + 1; F' = 2 - 1 + 1; F' = 2

The system is bivariant.

Both the temperature and composition have to be specified to define the system completely.

The area below the line AO (solid Ag + liquid melt), below the line BO (solid Pb + liquid melt) and below the point 'O' (Eutectic compound + solid Ag or solid Pb) have two phases and hence the system is univariant

F' = C - P + 1; F' = 2 - 2 + 1; F' = 1.

Application of Pattinson's process for the desilverisation of Argentiferous lead

The argentiferous lead, consisting of a very small amount of silver (say 0.1%), is heated to a temperature above its melting point, so that the system consisting of only the liquid phase represented by the point 'p' in the Figure 3.5. It is then allowed to cool. The temperature falls down along the line ‘pq'. As soon as the point 'q' is reached, Pb is crystallised out and the solution will contain relatively increasing amount of ‘Ag'. On further cooling, more and more ‘Pb' is separated along the line ‘BO'the melt continues to be richer and richer in silver until the point O is reached, where the percentage of Ag rises to 2.6%.

Thus, the process of raising the relative proportion of Ag in the alloy is known as Pattinson's process.

Uses of Eutectic system

1. Suitable alloy composition can be predicted with the help of eutectic systems.

2. Eutectic systems are used in preparing solders, used for joining two metal pieces together.