PHENOMENOLOGICAL COEFFICIENTS FOR MATTER TRANSPORT BY THE DUMBBELL MECHANISM IN A CONCENTRATED FCC ALLOY

The phenomenological coefficients for matter transport are calculated for a model of transport by dilute dumbbell defects in a concentrated f.c.c. alloy of two components; the defects and substitutional atoms m ix randomly and different binding energies for dumbbells of different atom compositions are allowed. The kinetic theory calculations use the same theoretical framework as in earlier studies of the Manning rando m alloy model for vacancy migration; terms of highest order in fluctua tions in the occupancy variables for the substitutional atoms are negl ected. The final expressions contain Brillouin zone integrals which ar e functions of the alloy composition and defect jump frequencies; thes e integrals must be calculated numerically for each thermodynamic stat e of interest. Moderate agreement with earlier Monte Carlo simulations is found when the ratio of largest to smallest jump frequency is 10 t o 1. The difficulties in formulating a more accurate and self-consiste nt theory based on the kinetic equations, comparable with the Manning theory for vacancy transport, are indicated.