For the immiscible Cu-Ta and Cu-W systems, realistic n-body potentials are derived under an embedded-atom method through fitting the cross potentials to some physical properties obtained from ab initio calculations for a few possible metastable Cu-Ta and Cu-W crystalline phases, respectively. Based on the derived potentials, molecular dynamics simulations reveal that in the Cu-Ta system, 30 at. % of Ta in Cu is the critical composition for the crystal-to-amorphous transition in the Cu-rich Cu-Ta solid solutions, and that in the Cu-W system, amorphous alloys can be formed within the composition range of 20–65 at. % of W. Interestingly, amorphous alloys are indeed obtained by ion-beam mixing in properly designed Cu70Ta 30, Cu65Ta35, Cu60Ta 40, and Cu50Ta 50 multilayered films, while crystalline Cu and Ta remain in Cu75Ta25 multilayered sample, which matches well with the critical composition of 30 at. % of Ta predicted by simulation. Moreover, there have been experimental data, which are in support of the predicted composition range of the Cu-W system by simulations.