Alloying in an Immiscible Cu-Nb System upon Solid-State Reaction

Abstract

In an immiscible Cu-Nb system, an amorphous alloy and two metastable crystalline phases were obtained by solid-state reaction of Cu-Nb multilayered films, and the formation of the alloy phases was found to be quite sensitive to the average composition of the films. At Nb concentration of 75at%, amorphization was achieved by 250°C annealing for 50 min, while in the films with compositions of 70 and 80 at% Nb, a simple cubic (a=0.405±0.005nm) and an orthorhombic phase (a=0.421, b=0.334, c=0.291±0.005nm) were observed, respectively. Thermodynamic calculation was conducted for the Nb-Cu alloy phases and the energetic state of the multilayers, which consisted of 9 Cu/Nb bilayers. It turned out that the excess free energy originating from the interfacial atoms could raise the multilayers to an energy level being higher than that of the amorphous or/and metastable crystalline phases both with a convex shape, and thus provided a major driving force for alloy phase formation in such immiscible system.