The Hunt-Jackson Theory of Lamella Spacing

The orientation of the lamella tends to be along the direction of heat flow. In polycrystalline grains solidified from a melt of eutectic composition, the orientation is more or less random but the spacing of the lamella is indicative of the rate the material was solidified. However, if a eutectic composition is directionally solidified, the lamella will be aligned along the solidification direction to form what is known as an in situ composite. If the eutectic system happens to be a low volume fraction eutectic (the eutectic composition is such that A B or vice versa), the minority phase will form a rod-like structure rather than the lamella structure described above. The spacing of the rods will still be governed by the 

Schematic of the mechanism by which a melt of the eutectic composition solidifies into alternating a and p lamellas as described by the Jackson-Hunt theory. 

Hunt-Jackson relation described above. In either case, slightly more kinetic imdercooling is required to drive the solidification of a eutectic because of the higher interfacial energy. 

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