On the occurrence of substitutional and triple defects in intermetallic phases with the B2 structure

Abstract

Theoretical models for the thermodynamic properties of intermetallic B2 phases are employed to explain a priori the empirical observation that B2 phases with a small enthalpy of formation exhibit substitutional defects, whereas B2 phases with a large enthalpy of formation exhibit triple defects. The following expressions for the concentration of anti-structure atoms ( N β A / N) as a function of the enthalpy of formation ΔH ∝ of the stoichiometric compound, the enthalpy of formation of vacancies ΔH v , and the temperature T, are derived: substitutional defects: ( N β A / N) s = 0.5 exp[( ΔH ∝)(2/ RT)] triple defects: (N β A/N) t = 0.315 exp[( 2 3 ΔH ∝ − 1 3 ΔH v)2/RT)] . These equations indicate that the formation of substitutional defects, which depends solely on ΔH ∝, becomes more difficult with increasing ΔH ∝. On the other hand, the formation of triple defects, which depends on both ΔH ∝ and ΔH v , becomes comparatively easier, assuming that ΔH v has approximately the same value for all B2 phases. It is predicted that some B2 phases might exhibit a hybrid behavior, with both defect types occurring in comparable concentrations.