On the composition dependencies of self-diffusion coefficients in B2 intermetallic compounds

Abstract

It is shown that the Huntington-McCombie-Elcock (HME) mechanism proposed for B2 intermetallic compounds is able to account for the compositional dependencies of the self-diffusion coefficients in the compositional vicinity of stoichiometry using thermodynamic arguments in terms of point defects in the lattice. However, this mechanism becomes unimportant at large deviations from stoichiometry. A new mechanism, termed the anti-structure bridge (ASB) mechanism, is proposed. In contrast to the HME mechanism, which requires the diffusion of the component elements to be correlated, the ASB mechanism allows the diffusion of the component elements to be independent. The ASB mechanism is dominant at large deviations from stoichiometry. Using thermodynamic arguments in terms of specific point defects in this class of intermetallic compounds, it was shown that a combination of these two mechanisms is able to account for the compositional dependencies of the experimentally determined self-diffusion coefficients of five B2 intermetallic compounds reported in the literature. The compositional dependencies of the prefactors and activation energies for the self-diffusion coefficients can be properly accounted for by the variation of the partial entropy and partial enthalpy with deviations from stoichiometry.