Let us consider the solidification of an alloy of composition x1, containing about 80 % Cd and 20 % Bi when the temperature falls to T1, crystal nuclei of pure Cd begin to form. The temperature horizontal, T1 cats the liquids at the chosen composition, x1 since pure Cd is deposited, it follows that the liquid which remains becomes correspondingly richer in (Bi). As the temperature has fallen, more Cd is deposited, and dendrites begin to develop from the nuclei which have already formed.
The growth of Cd dendrites, on the one hand, and the consequent enrichment of the remaining liquid in Bi, on the other, continues until the temperature has fallen to 140?C. The remaining liquid then contains 40 % Cd and 60 % Bi, the eutectic point E has been reached.
If the original liquid contained exactly 40 % Cd, 60 % Bi at the outset, then no solidification whatever would occur until the temperature had fallen to 140?C. Then a structure composed of eutectic structure. The eutectic part of the structure will be of constant composition and will always contain 40 % Cd, 60 % Bi
It is important to realize that there is no question of solid existing in any way in the final structure, whatever its composition with aid of microscope, we can see the two pure metals Cd and Bi as separate constituents in the microstructure. In other words, this is a case of completely insolubility in the solid state.
Example
Write the eutectic reaction (from Fig -5- )
Example
Draw the cooling curve for Cd – Bi alloy at concentration (20% Cd + 80 % Bi) (see Fig -5-)
Fig -7- Cooling curve for Cd – Bi alloy at concentration 20% Cd + 80 % Bi
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