MOLECULAR-DYNAMICS CALCULATIONS OF THERMODYNAMIC PROPERTIES OF METASTABLE ALLOYS

In order to improve our current understanding of the microscopic struc ture of metastable alloys of immiscible elements such as Ag-Cu and Co- Cu, the Helmholtz free energy of several microstructures based on an f cc unit cell has been calculated and compared with that of a reference state. The microstructures considered for the free energy calculation s at fixed volume are (1) a structure formed by alternating layers of fixed thickness of metal 1 and metal 2 separated by coherent interface s; (2) an atomically disordered solid solution; (3) a structure compri sing a random distribution of elemental cubic grains separated by cohe rent interfaces. Numerical results show that the Helmholtz free energy of structure (3) decreases with increasing grain size and that its va lue calculated for a sufficiently large grain size approaches the free energy of structure (1). Further molecular-dynamics simulations for t he Ag-Cu system have allowed the calculation of the enthalpy at the eq uilibrium volume of several microstructures including some of those li sted 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 equi atomic Ag-Cu alloy obtained by ball mining.