Experimental Method of Construction of A Simple Eutectic Phase Diagram

EXPERIMENTAL METHOD OF CONSTRUCTION OF A SIMPLE EUTECTIC PHASE DIAGRAM

1. Thermal analysis (or) cooling curves

Thermal analysis is a method involving a study of the cooling curves of various compositions of a system during solidification. The shapes of the freezing point curves for any system (involving metals) can be determined by thermal analysis. The form of the cooling curve indicates the composition of the solid.

Example 1 : Cooling curve for a pure solid

A pure substance in the fused state is allowed to cool slowly and the temperature is noted at different time interval. Then graph is plotted between temperature and time (Fig. 3.2).

Initially the rate of cooling is continuous. When it reaches the point ‘b' solid begins to appear, now the temperature remains constant until the liquid melt is completely solidified. Solidification completes at the point ‘c'. The horizontal line ‘bc' represents the equilibrium between the solid and liquid melt. After the point 'c', temperature of the solid begins to decrease along the curve 'cd'.

Example 2 : Cooling curve for a mixture

If a mixture of two substances (say A and B) in the fused state is allowed to cool slowly, the cooling curve is obtained in a similar manner (Fig. 3.3).

Initially the rate of cooling is continuous. When it reaches the point b'one substance (either A or B) begins to solidify out of the melt, which is indicated by a break and the rate of cooling is different. On further cooling at the break point 'c'the second compound also begins to solidify. Now the temperature remains constant until the liquid melt is completely solidified, which forms the eutectic mixture (line cd). After the break point d'cooling of solid mass begins. The temperature of horizontal line 'cd' gives the eutectic temperature.

The experiment is repeated for different compositions of A and B and the various cooling curves are recorded. From the cooling curves of various compositions, the main phase

Fig. 3.4 Cooling curve of various compositions of two solids

diagram can be drawn by taking composition in X-axis and the temperature in Y-axis. (Fig. 3.4)

Uses of cooling curves

1. Melting point and eutectic temperature can be noted from the cooling curve.

2. Percentage purity of the compounds can be noted from the cooling curve.

3. The behaviour of the compounds can be clearly understood from the cooling curve.

4. The composition corresponding to its freezing point yields the composition of the alloy.

5. The procedure of thermal analysis can be used to derive the phase diagram of any two component system. company