Since the amount of salt which can be held in solution with water varies with

Since the amount of salt which can be held in

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Since the amount of salt which can be held in solution with water varies with temperature, water separates out as ice crystals between the arrest points of hypo-eutectic solutions as the temperature falls, therefore the remaining solution is always of a constant concentration and has a constant arrest point temperature. This concentration is the eutectic composition. The fact that excess water or salt is rejected from solution so that eutec tic ‘balance’ or equilibrium’ is always ultimately achieved, results in the diagram formed from the cooling curves (Fig. 3) being referred to as a phase equilibrium diagram (also referred as a ‘thermal equilibrium diagram’ in some older texts).
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~ Page 69 of 79 ~ EXERCISE 1. With aid of sketches, explain what is meant by a ‘family’ of cooling curves and how these curves are used to construct phase equilibrium diagrams. 2. With the aid of a diagram, explain the difference between the terms ‘hypo - eutectic solution’ and ‘h yper- eutectic solution’. 3. Briefly explain what is meant by the following terms: (a) solvent; (b) solute; (c) solution; and (d) saturated. 4. Explain what is meant by the following terms as applied to the solidification of metals: (a) a cooling curve; (b) a ‘phase’.
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~ Page 70 of 79 ~ PHASE EQUILIBRIUM DIAGRAMS In contrast to pure metals, which solidify at a constant temperature - freezing point, alloys solidify over a range of temperature, depending on the alloy components and their concentrations. In course of solidification and subsequent cooling of solid alloy processes of phase transformations take place. The phases, compositions and their quantities change with the temperature. Phase diagrams are used for quantitative description of the phase transformation and changes. Phase diagram of an alloy system is a graphical presentation of the relationships between the phases, compositions and their relative amounts at any given temperature and under equilibrium conditions. Despite the fact that in real metallurgical processes, especially in the processes occurring in solid state, the equilibrium conditions are not reached, phase diagram is a very useful instrument of analysis and quantitative evaluations of the alloy behavior. Phase diagram of an alloy system consisting of two components is called binary phase diagram . There are many other examples of binary alloys which could be quoted, but the three examples considered below and their combinations cover the three most common types of phase equilibrium diagrams and the behavior of most of binary alloys. 1. THE EUTECTIC TYPE Fig. 1 below shows a eutectic-type of phase equilibrium diagram and it can be seen that it is identical with the ‘family’ of cooling curves diagram for a sodium chloride and water solution which indicates total solubility of the salt water in the liquid state and total insolubility (crystals of ice and separate crystals of salt) in the solid state.
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