The figure above is a sketch of different microstructures that form during the solidification of an alloy that forms a solid solution down to room temperature. When the liquid cools to the liquidus temperature, small "chunks" of solid begin to form. These "chunks" increase in number and size as cooling continues until there is no liquid left in the mixture at the solidus temperature. As cooling continues, there are no further phase changes indicated on the pase diagram, so no additional phases form.
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This figure depicts the solidification of an alloy that passes through a solid solution region during cooling, but decomposes into a two phase mixture at lower temperatures. Solidification initiall begins the same as the figure to the left, but as cooling continues below the solidous line, the solvus line is crossed. Upon crossing the solvus line, the second phase forms in the material, but since the material is solid, and therefore diffusion is slow relative to diffusion in a liquid, very small dispersed particles of the second phase form. These particles are called "precipitates."
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This figure depicts solidification of an alloy exactly at the eutectic composition. Unlike the two scenarios above, this alloy does not intersect the liquidous line. All of the liquid phase will solidify at the eutectic temperature. This leads to a layered, or "lamellar," structure. The layered structure is due to diffusion of the atomic species. This is explained more below.
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This figure shows an alloy whose composition is not exactly the eutectic temperature, but upon cooling, the alloy still crosses the eutectic line. Above the eutectic temperature, solidification occurs very similarly to the solidification in both of the figures above, when the temperature of those alloys is above the solidous line. "Chunks" of solid begint to form and grow in the liquid. These "chunks" are referred to as pro-eutectic, since they formed above the eutectic temperature. When the temperature reaches the eutectic temperature, however, all of the liquid remaining in the mixture solidifies like the eutectic composition alloy to the left. The resulting microstrucutre is a mix of the pro-eutectic "chunks" and the lameller structure.
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