In the general case of Fig 1 the two components present are referred to as

In the general case of fig 1 the two components

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In the general case of Fig. 1, the two components present are referred to as metal A and metal B. Although they are mutually soluble in the liquid state, both components retain their individual identities of crystals of A and crystals of B in the solid state.
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~ Page 71 of 79 ~ If you refer to Fig. 1 again, you can see that the line joining the points where solidification begins is referred to as the liquidus and that joining the points where solidification is complete is referred to as the solidus. This type of equilibrium diagram gets its name from the fact that at one particular composition (E), the temperature at which solidification commences is a minimum for the alloying elements present. With this composition the liquidus and the solidus coincide at the same temperature, thus the liquid changes into a solid with both A crystals and B crystals forming instantaneously at the same temperature. This point on the diagram is called the eutectic, the temperature at which it occurs is the eutectic temperature, and the composition is the eutectic composition. In practice, few metal alloys form simple eutectic type phase equilibrium diagrams. Exceptions to this are the cadmium-bismuth alloys and the thermal equilibrium diagram for such alloys is shown in Fig. 2 below. You can see that the eutectic composition occurs when the alloy consists of 40% cadmium and 60% bismuth. For this composition solidification occurs at just over 140 ˚C with both metals crystallizing out of solution simultaneously. The eutectic structure is usually lamellar in form, as shown in Fig. 3. In this instance there are alternate layers or ‘laminations’ of cadmium and b ismuth.
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~ Page 72 of 79 ~ COOLING OF AN ALLOY CONSISTING OF 80% CADMIUM AND 20% BISMUTH (A HYPER-EUTECTIC ALLOY) o Above the liquidus there is a liquid solution of molten bismuth and molten cadmium. o As the solution cools to the liquidus temperature, for the alloy under consideration, crystals of pure cadmium precipitate out (point A, Fig. 2). This increases the concentration of bismuth and reduces the concentration of cadmium present in the remaining solution. Thus the solidification temperature is reduced to that appropriate for this new ratio of cadmium and bismuth, and further crystals precipitate out. This again reduces the percentage of cadmium present in the remaining solution and the solidification temperature is further reduced with more pure cadmium crystals being precipitated out. This process repeats itself until the eutectic composition is reached (point B, Fig. 2). o At the eutectic composition, crystals of cadmium and bismuth precipitate out simultaneously to form lamellar eutectic crystals of the two metals as shown in Fig. 3. Thus the final composition of the solid alloy will consist of crystals of pure cadmium in a matrix of crystals of eutectic composition.
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