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When
a lead-tin alloy with the eutectic composition is cooled slowly through the eutectic
temperature it develops the microstructure shown in the photograph.
The material is polycrystalline, however, each crystal grain has a lamellar structure
of a- and b-phases. The a-phase
is the dark layers.
The diagram illustrates how the layered structure
of each grain happens. At the eutectic temperature the a-phase contains 19.2 wt % Sn and the b-phase 97.5 wt % tin. The liquid composition has only
61.9 wt % tin and, therefore, it is necessary to separate tin and lead atoms
to different regions during the solidification process. It is an advantage
to minimize the distance that they have to be moved at the interface between
the two solids and the liquid and this consideration favors the plates (lamellae)
of a- and b-phase seen in the photograph. |
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