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In order to improve our current understanding of the microscopic structure of metastable alloys of immiscible elements such as Ag-Cu and Co-Cu, the Helmholtz free energy of several microstructures based on an fcc unit cell has been calculated and compared with that of a reference state. The microstructures considered for the free energy calculations at fixed volume are ͑1͒ a structure formed by alternating layers of fixed thickness of metal 1 and metal 2 separated by coherent interfaces; ͑2͒ an atomically disordered solid solution; ͑3͒ a structure comprising a random distribution of elemental cubic grains separated by coherent interfaces. Numerical results show that the Helmholtz free energy of structure ͑3͒ decreases with increasing grain size and that its value calculated for a sufficiently large grain size approaches the free energy of structure ͑1͒. Further molecular-dynamics simulations for the Ag-Cu system have allowed the calculation of the enthalpy at the equilibrium volume of several microstructures including some of those listed above. A comparison of the calculated values of the enthalpy with the heat release observed experimentally allows the advancement of an hypothesis concerning the reaction path and the structure of the equiatomic Ag-Cu alloy obtained by ball milling. ͓S0163-1829͑97͒08901-7͔