Amorphization was achieved by room-temperature 190 keV xenon ion mixing of multilayered films in the Y - Nb system, which has a positive enthalpy of formation exceeding . To give further insight into the singularity of the amorphization behaviour of the system, a Gibbs free-energy diagram of the system was constructed by calculating the free energies of the amorphous phase and those of the as-deposited Y - Nb multilayers, in which the interfacial free energy was estimated and added. The established diagram can explain the irradiation-induced amorphization behaviour in the Y - Nb system well.