DIFFUSIONAL BEHAVIOR IN B2 INTERMETALLIC COMPOUNDS

The Huntington-McCombie-Elcock (HME) and antistructure bridge (ASB) me chanisms propos ed for B2 intermetallic compounds are reviewed. By usi ng a thermodynamic argument, the HME mechanism is shown to be operativ e near the stoichiometric composition. On the other hand, the ASB mech anism is shown to be important at compositions away from stoichiometry . A combination of these two mechanisms is able to describe the compos itional dependences of the experimentally determined self-diffusion co efficients for several B2 intermetallic compounds. By using conclusion s drawn from these two mechanisms, it is further shown that the compos itional dependences of the activation energies and pre-exponential fac tors are related to the partial molar enthalpies and entropies of the component elements. The frequently observed decreases in the activatio n energy and pre-exponential factor with deviations from stoichiometry are in accord with the corresponding decreases in the par tial molar enthalpy and entropy. The extensive experimental data for NiAl are use d to verify these relations.