why upon solidification, an alloy of eutectic composition forms a microstructure consisting of alternating layers of the two solid phases. ?
Eutectic microstructure forms as a result of the diffusional partitioning of elements. According to the diffusional ledge mechanism, the partitioning of elements occurs through the shortest path of diffusion. The alternating layers structure fulfills the shortest diffusion path.
Not all eutectic systems produce lamellar structures. Only if the interfacial energy between the two solids (S1/S2; S1 and S2 denote Solid 1 and Solid 2) is lower than those of S1/Liquid and S2/Liquid a lamellar eutectic structure is formed.
Eutectic microstructure forms as a result of the diffusional partitioning of elements. According to the diffusional ledge mechanism, the partitioning of elements occurs through the shortest path of diffusion. The alternating layers structure fulfills the shortest diffusion path.
I agree with the replies above. The forms depend on the free energy of phases, the diffusion of elements, the liquid flow rate etc. at the solid–liquid interface. Some papers are interesting:
https://core.ac.uk/download/pdf/27267169.pdf
https://doi.org/10.1016/j.commatsci.2020.109523
Please see the below attached link. This might be helpful for you.
https://www.youtube.com/watch?v=NMwFuw1weO4&t=2593s
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