A free energy diagram of the Ag–Mo system with a very positive heat of formation of +57 kJ/mol was constructed, with special consideration to the interfacial free energy in the multilayered films for ion mixing. It was found that amorphization was possible to achieve within two separate composition ranges toward Ag and Mo ends, respectively, if 12 layers, or about 13% of the interfacial atoms were included in the films. Accordingly, ion mixing of the Ag–Mo multilayered films was conducted at liquid nitrogen temperature by 190 keV xenon ions and the formation of amorphous phases was in good agreement with the calculation. Besides, a new metastable fcc phase was formed and, more interestingly, it transformed into an amorphous state upon aging, which can also be interpreted by a related free energy calculation. © 1995 American Institute of Physics.