The microstructure of the unstable FCC-Fe50Cu50 alloy prepared by mechanical alloying has been studied by transmission electron microscopy (TEM) and Mossbauer spectroscopy. The lamellar structure was found to have a range of lamellar thicknesses from about 10 nm to 200 nm. The composition and the structure of each layer were determined to be about 50 at.% iron and 50 at.% copper and FCC structure, respectively. The crystallite size in the as-milled FCC-Fe50Cu50 alloy varied from a few nm to a few tens of nm, From Mossbauer measurement it was found that the Fe and Cu atoms can occupy any site and the lattice can be considered as FCC with a 'random' atom at each lattice point. Nearest-neighbour atomic configuration follows that given by the binomial distribution. The driving force for the alloy formation of the system with a positive heat of mixing can be provided by the energy stored in the grain boundaries and by the configuration entropy of randomly distributed atoms in the solid solution.

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