Eutectic line

The phase diagram for aluminum/silicon (Fig. 4.5) is a typical example of a system of two components that form neither solid solutions (except for very low concentrations) nor a compound with one another, but are miscible in the liquid state. As a special feature an acute minimum is observed in the diagram, the eutectic point. It marks the melting point of the eutectic mixture, which is the mixture which has a lower melting point than either of the pure components or any other mixture. The eutectic line is the horizontal line that passes through the eutectic point. The area underneath is a region in which both components coexist as solids, i.e. in two phases. 

On the Eutectic Composition A eutectic system is best represented by the Ag-Cu phase diagram as shown in Fig 16. As explained earlier, the eutectic point is the point at which two liquidus curves meet. And the horizontal line intersecting the eutectic point is known as eutectic line. Normally, the eutectic composition of 28.1 at. % Cu below eutectic line is left open [29] as shown in Fig. 16-(a). However in a strict sense this is not correct and in my view as well as in the view of a few other scientists [30], the eutectic composition should be indicated with a straight vertical solid or dotted line as shown in Fig.l6-(b). We shall now examine in detail the reason for the correctness of such view i.e., indicating the eutectic composition by a solid or dotted vertical line as shown in Fig. 16-(b). 

This process will continue till the temperature ultimately reaches the eutectic line. According to the phase diagram in Fig. 16-(a) below eutectic line temperature, assuming at 700° C solid state will consist two phases a and 3 whose proportionality will follow the lever rule that is, ab/ac = (3/a. 

The effect of pressure on the position of the eutectic point may be examined by considering the differential equations obeyed by the eutectic line, at all points along which the affinity of transfer of the two components from the solid to solution is zero. The affinity is given by (18.19) which may be applied separately to each of the two components ... 

These equations were given by van Laar. The quantities which figure on the right hand side of equations (22.25) and (22.26) refer to the eutectic line. Equation (22.25) is analogous to the Clausius-Clapeyron equation (14.4). 

In the Mn rich region (where no experimental data exist) calculation [1990Hua] predicts the existence of the only invariant reaction (U type) L + (6Mn) (pMn) + 7 and of a maximum on the eutectic line L (pMn) -I- 7. Unfortunately, neither a temperatirre nor exact compositions of the participating phases are provided. 

Figure 8.6 Predicted SLE diagram and eutectic lines for the ternary system o-xylene-m-xylene-p-xylene assuming ideal behavior.

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