Diffusional behavior in B2 intermetallic compounds

The Huntington-McCombie-Elcock (HME) and antistructure bridge (ASB) mechanisms proposed for B2 intermetallic compounds are reviewed. By using a thermodynamic argument, the HME mechanism is shown to be operative near the stoichiometric composition. On the other hand, the ASB mechanism is shown to be important at compositions away from stoichiometry. A combination of these two mechanisms is able to describe the compositional dependences of the experimentally determined self-diffusion coefficients for several B2 intermetallic compounds. By using conclusions drawn from these two mechanisms, it is further shown that the compositional dependences of the activation energies and pre-exponential factors are related to the partial molar enthalpies and entropies of the component elements. The frequently observed decreases in the activation energy and pre-exponential factor with deviations from stoichiometry are in accord with the corresponding decreases in the partial molar enthalpy and entropy. The extensive experimental data for NiAl are used to verify these relations.