Segregation kinetics of immiscible alloying elements for understanding phase separation in multicomponent alloys

The diffusion and segregation rates of immiscible alloying elements in Cu-based alloys with one (V) or five refractory metals (V, Nb, Mo, Ta and W) at 300 to 500 °C were investigated for understanding the kinetics of phase separation in multicomponent alloys. At a comparable grain size and a similar influence of grain boundary diffusion (more dominant at 300 °C), the summed rate constant of segregation of the five alloying elements was higher than that of the one element, attributable to the larger entropy change for vacancy formation and atom exchange. The rate constant and time exponent of segregation depended on the melting temperature of the alloying element (W > Ta > Mo > Nb > V), while the activation energy of diffusion, 5.1 to 12.3 kJ/mole (W < Mo < V < Nb < Ta), was inversely related to the delta mixing enthalpy (to Cu) or electronegativity.