Molecular dynamics simulation studies of atomic-level structures in rapidly quenched Ag-Cu nonequilibrium alloys

Ag-Cu has been a classical example demonstrating the formation of a single-phase supersaturated solid solution by rapid quenching in a system immiscible in equilibrium at room temperature. This study examines, using molecular dynamics simulations, the local structures and homogeneity of these crystalline and amorphous Ag-Cu alloys produced by quenching from the liquid at different cooling rates (5 X 10¹⁰-2.5 X 10¹³ K/s). It is observed that the retention of amorphous structures requires extremely high quench rates. The amorphous alloys formed are chemically uniform in both long and short ranges. Meanwhile, topologically significant local icosahedral order was able to develop during quenching even at these extreme cooling rates. The Ag-Cu amorphous structures are discussed in comparison with those of Ag-Ni, a system immiscible even in the liquid state. At moderately reduced quench rates, homogeneous crystalline solutions form instead. With further decreasing quench rates, the chemical short-range-order parameter becomes increasingly more positive, suggesting spinodal decomposition. Our simulations point to the need for a careful experimental study of the ultrafine-scale composition modulations in the face-centered-cubic solid solution that is hitherto believed to be homogeneously supersaturated for a wide range of rapid-quench conditions.