Nanophases of binary and multicomponent alloys

I present a theory which explains the appearance of an amorphous-like, disordered phase in nanoparticles of binary alloys. The theory claims that this phase represents a transition state between two bulk phases of the binary system. While the transition state is completely unstable in the conditions of an open system, where exchange of the species between the particle and its surrounding is not limited, it may become stabilized in the closed system where the species exchange is prohibited. I derive the material-parameters criterion, which is the condition for the stabilization. The transition state represents a shallow minimum of the molar Helmholtz free energy as a function of the order parameter and is not significant in a large system because the global optimum there is delivered by the heterogeneous mixture of the two bulk phases connected by the common-tangent construction. However, in a particle of the size below the critical, the transition state becomes the global optimizer because of the prohibitively large energy “cost” of the phase separating interface. Thus the theory explains the effect as due to the free energy of the phase separating interface, not the energy of the free surface of the particle. The theory sheds light on the structure of the critical nuclei in the process of nucleation of one phase from another one. New experiments to verify the theory are suggested.