A theory of diffusion in the Ll(2) structure .2. Tracer and chemical diffusion

In the present study a six frequency model is considered for describing diffusion in binary A(3)B ordered alloys taking the L1(2) structure. The model supposes that the sublattices exist a priori. Three vacancy exchange frequencies describe A jumps from the alpha sublattice to the beta sublattice and its reverse, and intra alpha sublattice jumps. A further three exchange frequencies describe B jumps from the beta sublattice to the alpha sublattice and its reverse, and again intra alpha sublattice jumps. Explicit expressions are derived for the average atomic jump frequencies for both atomic species. The thermodynamic factor for chemical diffusion is calculated by adapting the Widom J. Chem. Phys. 39, 2808 (1963), formalism. The collective correlation factors were calculated using the Manning relations and the tracer correlation factors calculated earlier for this structure by Belova and Murch J. Phys. Chem. Solids (in press). The former were compared with computer simulation results which are also presented in this study. Good agreement was found. Analytical expressions for the tracer and intrinsic diffusion coefficients were determined. The present formalism was then used to provide limits for the exchange frequencies in Ni3Al and Ni3Ge. (C) 1997 Elsevier Science Ltd. All rights reserved.