3- Cooling courves -1 - Cooling curve for pure metal...

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Cooling below freezing point is called under cooling. Difference between freezing point and temperature at which crystallization begins is called degree of under cooling. After the metal has been transformed completely to solid state, temperature drops and solid is cooled again to room temperature. Cooling curve for pure metal
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Cooling curve for a binary alloy of solid solution type. At point ‘a’ freezing begins and ends at the point ‘b’. The temperature corresponding these points ‘a’ and ‘b’ are known as liquidus and solidus temperatures respectively. These points depends upon the composition of an alloy. Cooling curve for alloy Eutectic type alloy Solid solution type
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For eutectic type alloy the solidification range is indicated by aa’b (Fig. b), where in the horizontal line (a’b) indicates the eutectic reaction i.e. transformation of liquid to solid at constant temperature, lowest for a given system. It may be noted that the curve ‘aa’ is analogous to the heat obtained in the cooling curve of the solid solution type (Fig. a), while a’b is the same as ‘ab’ in the cooling curve of a pure metal (Fig. b). (b) Eutectic Type Alloy
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Construction of phase diagram a)Cooling curve b)phase diagram
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Figure shows the construction of binary (two component system) phase diagram (solid solution type) with corresponding cooling curves. Points ‘ta’ and ‘tb’ represent the melting point of pure metals A and B respectively. For successive compositions, solidification commences at a1, a2, and a3 and completes at b1, b2 and b3 etc. The line joining the points a1, a2 and a3 is called liquidus line; similarly line joining the points b1, b2 and b3 is called solidus line. Above the liquidus line the alloy exist in liquid state; while below the solidus line the alloy exist in solid state. Between liquidus and solidus both liquid and solid solutions are co-exist in equilibrium.
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Application of Lever Rule The point ‘O’ (intersection of vertical and horizontal lines) may be considered as fulcrum of a lever, the relative lengths of the lever arms multiplied by the amount of phases present must be balance.
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Common Types of Phase Diagrams 1. Isomorphous systems – Two components are completely soluble in each other in both liquid and solid states, and exist in a single phase for all composition of the alloy. 2. Eutectic systems – Two components are completely soluble in liquid state, but either partially or completely insoluble in the solid state. 3. Peritectic system – Melting points of two metals differ considerably, and a liquid phase combined with solid phase to produce new solid phase. 4. Monotectic system – In this system, one liquid transforms into another liquid and one solid . 5. Eutectoid systems – Phase diagrams in which transformation will take place in solid state i.e. one solid decomposes into two different solids.
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In the solid state two metals can dissolve in each other in all proportions and exist in a single phase.
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