Eutectic halt

Note the breaks (abrupt change in slope) at temperatures corresponding to points a, b, and bi. Also note the eutectic halts at az and >3. 

A break in the cooling curve (Figure 6.13(b)) occ forming a eutectic halt occurs at point ei 20°C... 

When the system point reaches the eutectic temperature at point g, cooling halts until all of the liquid freezes. Solid B freezes out as well as solid A. During this eutectic halt, there are at first three phases liquid with the eutectic composition, solid A, and solid B. 

The duration of the temperature halt at the eutectic temperature depends on the amount of heat evolved at the eutectic crystallization. It is evident from the phase rule that the amount of eutectic melt increases from the pure component to the eutectic composition and in the same manner also the amount of heat evolved at the eutectic crystallization increases. Plotting the time duration of the eutectic crystallization versus the composition, we get the so-called Tamman triangles (Figure 3.46). This procedure helps to detect the position of the eutectic point. 

Figure 3.46. The halts of temperature at eutectic crystallization form the Tamman triangles.

Temperature-composition phase diagrams such as this are often mapped out experimentally by observing the cooling curve (temperature as a function of time) along isopleths of various compositions. This procedure is thermal analysis. A break in the slope of a cooling curve at a particular temperature indicates the system point has moved from a one-phase liquid area to a two-phase area of liquid and solid. A temperature halt indicates the temperature is either the freezing point of the liquid to form a solid of the same composition, or else a eutectic temperature. 

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