In the Co-Nb system, seven amorphous alloys were synthesized by room-temperature 200 keV xenon ion mixing of multilayered films with various compositions. The composition range favouring amorphization was determined to be from 23 to 80 at.% Co. In addition, two metastable crystalline (MX) phases were consecutively formed in the Co₂₃Nb₇₇ multilayered films, i.e. an HCP phase at a dose of 1*10¹⁵ Xe⁺ cm^-2 and a FCC phase at a dose of 6*10¹⁵ Xe⁺ cm^-2. The thermal stabilities of both ion-mixed amorphous alloys and the MX phases were examined by subsequent annealing. To obtain a deep insight concerning all the above observations, a free-energy diagram of the Co-Nb system was constructed by calculating the free-energy curves of the solid solutions, the amorphous phase, the equilibrium compound and the MX phases, which were considered as compound-like phases. To test the relevance of the constructed diagram in its outline, steady-state thermal annealing of the as-deposited Co₂₃Nb₇₇ multilayered films was performed and the results confirmed the calculated relative energy levels of the phases considered. The free-energy diagram can explain the amorphization range as well as the observed evolution of the ion-mixed phases.