In a eutectic reaction, when a liquid solution of fixed composition, solidifies at a constant temperature, forms a mixture of two or more solid phases without an intermediate pasty stages. This process reverses on heating. In a eutectic system, there is always a specific alloy, known as ‘eutectic composition’ that freezes at a lower temperature than all other compositions.
At ‘eutectic temperature’, two solids form simultaneously from a single liquid phase. The eutectic temperature and composition determine a point on the phase diagram known as ‘eutectic point’.
Binary alloy eutectic system may be classed as follows:
1. One in which, two metals are completely soluble in the liquid state but are insoluble in each other in the solid state.
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2. The other in which two metals are completely soluble in the liquid state but are partly soluble in each other in the solid state.
System # 1. Two Metals (or Components) Completely Soluble in the Liquid State but Completely Insoluble in the Solid State:
No two metals are completely insoluble in each other technically. However, in some cases the solubility is so restricted that for practical purposes they may be considered insoluble, e.g., Tin-zinc or Cadmium-bismuth.
Fig. 2.10 illustrates the equilibrium diagram of Cadmium-bismuth system. If a small amount of cadmium is added to molten bismuth, the freezing point of the resulting alloy is lowered, as indicated by the line BE. On the other hand, if a small amount of bismuth is added to molten cadmium, the freezing point of the resulting alloy is lowered.
From this, it is apparent that each metal lowers the freezing point of the other; the lines connecting these freezing points must intersect at some point. This point is E as shown in the diagram and represents 60% bismuth and 40% cadmium. This meeting point, which is called the eutectic point, is of great importance. The alloy whose composition is represented by the eutectic is called the eutectic alloy.
We shall now consider the cooling characteristics of an alloy of 20% bismuth and 80% cadmium. As the temperature declines the line A1C1 indicating this specific composition strikes the line AE at A1 as shown in the Fig. 2.10. Pure cadmium crystals start separating and the alloy gets poorer in cadmium and richer in bismuth.
As the temperature further falls, more crystals of cadmium separate. This process will continue until the composition of the residual bath attains the eutectic composition, i.e., 60% bismuth and 40% cadmium. At this stage, the eutectic will solidify at the temperature of 140°C.
The solidification will be just like the solidification of a pure metal, i.e., at a specified temperature. Similarly, if we consider the solidification characteristics of an alloy containing 80% bismuth and 20% cadmium, pure bismuth will separate and will continue to crystallise until the composition of the residual liquid bath assumes the eutectic composition, i.e., 60% bismuth and 40% cadmium. At this juncture, the eutectic so formed will solidify at a temperature of 140°C.
Eutectic consists of alternate layers of cadmium and bismuth which form at the eutectic temperature (140°C in this case).
System # 2. Two Metals Completely Soluble in the Liquid State, but only Partly Soluble in the Solid State:
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Fig. 2.11 shows the equilibrium diagram of a system of components that have complete mutual solubility in the liquid state and limited solubility in the solid state.
I. The lines ae and be are the liquidous. Crystals of a solid solution of metal B in metal A (α) begin to precipitate from the liquid alloy along the line ae; the solid solution of metal A in metal B (γ) precipitates along line be.
II. The lines ad, dc and bc correspond to solidus. The point d corresponds to maximum solubility of metal B in metal A at the eutectic temperature (te). Point e is the same for metal A and metal B.
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III. Points k and f indicate the solubilities of metal A in metal B, and B in A, respectively, at normal room temperatures. The lines dk and fc thus show the variation in solubility with the temperature.
IV. At point e, the solid solutions a and p simultaneously separate from the liquid phase to form the eutectic (α + β).
V. The solidification of Alloy 1 begins at temperature t1 and finishes at t2. The solid alloy contains only crystals of solid solution α and no further phase transformations occur.
VI. The solidification of Alloy 2 begins at temperature t3 and finishes at t4. The composition of the liquid phase varies during solidification along the liquidous; that of the solid phase varies along solidus. Thus at the temperature t3 the points p and q determine the compositions of the liquid phase and the solid solution crystals, respectively.
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When it is completely solid, alloy 2 will consist only of a solid solution crystals. Upon further cooling to temperature t5, the solid solution will become supersaturated and, at lower temperatures, it will decompose and the surplus component B will be separated as β crystals whose amount increases as the temperature falls.
Therefore at temperatures below t5 the alloy will consist of two phases α + β. The composition of the α and β solutions varies along line dk for α crystals and along if for β crystals upon a fall in temperature. For example, at temperature t’, the composition of the α-phase is determined by point δ and that of the β-phase by point γ.
The quantitative relation between the weights of the α- and β-phases conforms to the lever rule:
VII. The solidification of Alloy 3 begins at temperature t6 and is completely solidified at temperature te.
During solidification, the composition of the liquid part of the alloy varies continuously along the liquids, approaching the eutectic composition (point e); the composition of the solid phase varies along the solidus moving towards the maximum solubility (point d).
The relation between the liquid and solid phases may be found at any temperature by the lever rule. At temperature te, the liquid phase reaches the eutectic composition and the alloy completely solidifies. In this, alpha and beta solid solutions, forming the grains of the eutectic, are simultaneously precipitated from the liquid phase.
VIII. An alloy of the composition corresponding to point e (60% B) will contain only grains of the eutectic (α + β) after solidifying.
IX. Fig. 2.12 illustrates the equilibrium diagram for Lead-tin alloys which are an example of complete mutual solubility in liquid state and limited solubility in the solid state.
