COOLING OF AN ALLOY CONSISTING OF 80 BISMUTH AND 20 CADMIUM A HYPO EUTECTIC

Cooling of an alloy consisting of 80 bismuth and 20

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COOLING OF AN ALLOY CONSISTING OF 80% BISMUTH AND 20% CADMIUM (A HYPO-EUTECTIC ALLOY) Similarly for an alloy of 80% bismuth and 20% cadmium (hypo-eutectic), the amount of cadmium present in solution compared with the amount of bismuth present in solution will gradually increase as crystals of pure bismuth precipitate out until the eutectic composition is reached. Thus, in this instance the composition of the solid alloy will consist of crystals of pure bismuth in a matrix of crystals of eutectic composition. COOLING OF AN ALLOY CONSISTING OF 60% BISMUTH AND 40% CADMIUM (A EUTECTIC ALLOY) Finally, for an alloy of 60% bismuth and 40% cadmium only crystals of eutectic composition will be present. The solid alloy compositions discussed above are graphically presented in Fig. 4 below.
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~ Page 73 of 79 ~ Fig. 5 below (next page) graphically summarizes the above discussion.
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~ Page 74 of 79 ~ 2. THE SOLID SOLUTION TYPE As you might already know, copper and nickel are not only mutually soluble in the liquid (molten) state, they are also mutually soluble in the solid state and they form a substitutional solid solution. The phase equilibrium diagram for copper-nickel alloys is shown in Fig. 6 below. Again, the line marked liquidus joins the points where solidification commences, whilst the line marked solidus joins the points where solidification is complete. This time there is no eutectic composition. o For 100% copper and 0% nickel (pure copper) there is a single solidification temperature of 1084 ˚C. This is to be expected since, for a pure metal (in fact for any pure crystalline substance), the transition from liquid to solid takes place at a constant temperature. o As Fig. 6 above shows, for an alloy of 80% copper and 20% nickel, solidification starts at 1190 ˚C and is complete at 1135 ˚C. Between the solidus and the liquidus is a solution of molten copper and nickel together with crystals of a solid solution of copper and nickel. o As Fig. 6 above also shows, for an alloy of 80% nickel and 20% copper, solidification starts at 1410 ˚C and is complete by 1380 ˚C. o Finally, Fig. 6 shows that for 100% nickel and 0% copper (pure nickel) solidification occurs at the single temperature of 1455 ˚C. o Below the solidus the alloy consists entirely of crystals of copper and nickel in solid solution.
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~ Page 75 of 79 ~ 3. THE COMBINATION TYPE Many metals and non-metals are neither completely soluble in each other in the solid state, nor are they completely insoluble. Therefore they form a phase equilibrium diagram of the type shown in Fig. 7. In this system there are two solid solutions labelled ⍺ and β. The use of the Greek letters ⍺, β, γ, etc., in phase equilibrium diagrams may be defined, in general, as follows: o A solid solution of one component A in an excess of another component B, such that A is the solute and B is the solvent, is referred to as solid solution ⍺. o A solid solution of one component B in an excess of the component A, so that B now becomes the solute and A becomes the solvent, is referred to as solid solution β. o In a more complex alloy, any further solid solutions or intermetallic compounds which may be formed would be referred to by the subsequent letters of the Greek alphabet.
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