Proposed Definition of Microchemical Inhomogeneity and Application To Characterize Some Selected Miscible/Immiscible Binary Metal Systems

A new parameter is proposed to quantify the microchemical inhomogeneity (MCI) for a general multicomponent system and is applied to characterize some selected binary metal systems, i.e., the Ag−Ru, Ag−Co, Ni−Ru, and Ni−Hf systems, through molecular dynamics simulations. For the equilibrium immiscible Ag−Ru, Ag−Co, and Ni−Ru systems, ab initio calculations are performed to acquire some physical properties for fitting the respective n -body potentials. On the basis of the derived potentials, simulations reveal that in the highly immiscible Ag−Ru system, when the solute concentration is less than one of the critical values, i.e., either 6 atom % of Ru or 10 atom % of Ag, in the Ag-based and Ru-based solid solutions, respectively, the MCI is a small positive number and that when the solute concentration exceeds the critical values, i.e., falls in a composition range of 6−90 atom % of Ag, the MCI rises sharply to a large positive value ranging from 0.8 to 0.95, suggesting an incomplete-phase-separation tendency, which may result in forming a nanometer/subnanometer scaled structure. Similar simulations reveal that for the medium immiscible Ag−Co system, the MCI is about +0.7 within a composition range of 10−85 atom % of Ag and that for the Ni−Ru system, characterized by a nearly zero heat of formation, the MCI is nearly zero over the entire composition range, suggesting a possible metastable isomorphous phase diagram for the system, in which amorphous alloy, namely metallic glass, is hardly formed. In the contrast, for the miscible Ni−Hf system featuring a large negative heat of formation, the MCI is negative and is about −0.17 within a composition range of 25−80 atom % Ni, in which a crystalline solid solution prefers to turn into an amorphous state, corresponding almost exactly to the glass-forming range known from experiments as well as from theoretical prediction.