Two Metals Completely Soluble in the Liquid State and Completely Insoluble in the solid state

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1 Two Metals Completely Soluble in the Liquid State and Completely Insoluble in the solid state Technically, no two metals are completely insoluble in each other. However, in some cases the solubility is so restricted that for practical purpose they may be considered insoluble. The series of cooling curves for the pure metals and various alloys are shown in the figure given below. the cooling curves for the pure metals A and B show a single horizontal line at their freezing points. As B is added to A, the temperature for the beginning of solidification is 60B. It can be seen that over a wide range of compositions, a portion of the cooling curve that shows the end of solidification occurs at a fixed temperature. The lower horizontal line at TE, shown dotted in the above figure, is known as eutectic temperature. In one alloy, the eutectic composition 40A-60B, complete solidification occurs at a single temperature, the eutectic temperature.

2 The melting points of the two pure metals, points M and N, are plotted on the vertical lines that represent the pure metals A and B. For an alloy having composition 80A-20B, the points showing the beginning of solidification T1 and end of solidification TE are plotted as shown. The same procedure is followed for all other alloys. The upper lines on the phase diagram connecting the points M, E and N is the liquidus line and shows the beginning of solidification. The point, at which the liquidus lines intersect, the minimum point E, is known as the eutectic point. TE is called the eutectic temperature and 40A-60B the eutectic composition. As solidus line is a continuous line connecting the melting points of pure metals, the complete solidus line is MFGN.

3 It is common practice to consider alloys to the left of the eutectic composition ashypoeutectic alloys and those to the right as hypereutectic. The way solidification takes place and resulting microstructures at various stages can be studied by following the slow cooling of different alloys. The process is explained by study of slow cooling of Alloy 1 (eutectic composition), Alloy 2 (hypoeutectic alloys) and Alloy 3 (hypereutectic alloys) in the figure given below Eutictic composition Alloy 1 is the eutectic composition 40A-60B. As it is cooled from temperature T0, it remains a uniform liquid solution until it reaches point E, on the horizontal eutectic temperature line. the liquid must now start to solidify and the temperature cannot drop until the alloy is completely solid. The liquid will solidify into a mixture of two phases as alternate of pure metals (A&B).The reaction below show this transformation :

4 To understand how the eutectic solidifications occur let us assume solidify asmall amount of pure metal A remaining liquid is enriched with B this lead to tends the composition has shifted to the right. To restore the liquid composition to equilibrium layer of B will solidify. If slightly too much B is solidified, the liquid composition will have shifted to the left, requiringlayer of A to solidify to restore equilibrium. Therefore, at constant temperature, the liquid solidifies alternately layers from pure A and pure B, resulting in extremely fine mixture usually visible only under the microscope. This is known as eutectic mixture shown at (4) in the below figure In the above figure at (1), the alloy is in liquid state. At (2), as the alloy starts to solidify, it forms alternate layers of pure A and pure B. This layered microstructure is known as lamellar microstructure. The reason that a eutectic alloy forms in this way has to do with the diffusion times required to form the solid The grains grow by adding A to A and B to B until they encounter another grain as shown at (3). Further nucleation sites will also continue to form within the liquid parts of the mixture and solidification completes as shown at (4). The change of this liquid of composition E into two solids at constant temperature is known as the eutectic reaction and may be written as Hypoeutectic alloys Let consider the solidification of an Alloy 2 of composition of(80%a,20%b) at point 1 liquid solid solution is exist.when temperature falls to the T 1 crystal nuclei of A begin to form (The temperature horizontal or tie-line, T 1, cuts the liquidus at the chosen composition, L 1,and the other phase boundary is the 100% A ordinate) since pure A is deposited it follows.

5 that the liquid which remains becomes correspondingly richer in B.Therefore the composition of the liquid moves to the right no further deposition of A takesplace until the temperature has fallen tot2 at point (3 ). When this happens more A is deposited, and dendrites begin to develop from the nucleiwhich have already formedthe amount of A and L2 can be calculated using the Lever Rule as under. A (percent) = (x2l2 / T2L2) x 100 = (10 / 30) x 100 = 33 percent L2 (percent) = (T2x2 / T2L2) x 100 = (20 / 30) x 100 = 63 percent The growth of the A dendrites, on the one hand, and the consequent enrichment of the remaining liquid in B,on the other, continues until the temperature has fallen to T E.The remaining liquid then contains 40% A and 60%, ie the eutectic point E has been reached at point (4) Then a structure composed entirely of eutectic would be formed as outlined above. Hypereutectic Alloys Alloy 3, a hypereutectic alloy composed of 10A-90B, undergoes the same cooling process as alloy 2 except that when the liquidus line is reached at temperature T3 the liquid deposits crystals of pure B instead of A as shown at (2). As the temperature is decreased, more and more of B will solidify, leaving the liquid richer in A. The amount of liquid gradually decreases, and its composition gradually moves down and to the left along the liquidus line until the point E is reached at the eutectic temperature. The remaining liquid now solidifies into the eutectic (A + B) mixture as shown at (5). After solidification the alloy will consist of 75 percent grains of primary B and 25 percent eutectic (A + B) mixture.the area below the solidus line and to the left of the eutectic composition is labeled Solid A + Eutectic mixture and that to the right, Solid B + Eutectic mixture.figure below show the microstructure of hypereutectic alloy.

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