The competing crystalline and amorphous solid solutions in the Ag-Cu system

Upon nonequilibrium processing using vapor quenching or mechanical alloying, the supersaturated fcc solid solution predominates over the amorphous solution in the Ag-Cu system. The thermodynamic and kinetic origins of this phase selection are explored. The enthalpy of formation of both solutions has been determined as a function of composition using calorimetry measurements and molecular dynamics (MD) simulations. The enthalpy of the fcc solution is found to be lower than that of the competing amorphous phase. The preference for the fcc crystalline state is enhanced by the low kinetic barrier to crystallization of the amorphous solution, which occurred during quenching even when high cooling rates were employed or during annealing at low temperatures. Consequently, the retention of an amorphous Ag-Cu solution required kinetic trapping using ultrahigh quenching rates achievable only under stringent vapor deposition conditions or in MD simulations. However, transmission electron microscopy revealed the presence of local regions of amorphous Ag-Cu after cold rolling of multilayers, of elemental Ag and Cu foils at room temperature. This result of partial amorphization demonstrates the possibility of mechanically driven solid-state amorphization in a system with a positive heat of mixing. (C) 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.